The AM Forum

I finally got around to doing some IMD tests on my 24 pill class E PDM rig.  It uses digital drivers and also a homebrew analog VFO with digital converter.  I've been very pleasd with it but never did any serious testing.

IMD = Inter-Modulation Distortion
THD = Total Harmonic Distortion


This is the Class E construction thread back in March, 2010.
http://amfone.net/Amforum/index.php?topic=23354.0

Here's a thread on IMD testing that describes the test set up and shows more sample graphs:
http://amfone.net/Amforum/index.php?topic=33505.0

Pic #1  shows the rig running at 1KW using a 2-tone test. Remember that with AM DSB full carrier, there is a mirror image of IMD peaks. It's like a standard ssb graph but with two sides.  Pic # 1 shows roughly -35 dB 3rd order,(down from the two main audio peaks)  while the higher orders drop off very well. I would rate this IMD slightly better than average compared to most ham signals for cleanliness. Maybe with some custom work it could be improved.

What does a 2-tone test measure?  It checks the linearity of an amplifier. We want the output to be an exact replica of the input. (Or in class C/D/E, the audio to modulate and mix linearly) When we put two tones in, we want ONLY two tones out. But mixing occurs and we end up with something less than perfect.  As long as these mixed tones that march up the band are below an "acceptable" level, then the amplifer is OK.  A standard linear ssb amplifer has to have a linear output as well as a class C or class E stage.  The two tone test does not require audio filtering into the rig... it takes no prisoners.


Voice tests are a little more forgiving than the 2 tone test:

Pic #2 shows voice speech with an "SSS."   I used a 3kc audio rolloff to show what artifacts remained.  I see the first shelf about -60Db down from the carrier peak.  I would rate this good. One could feel confident running this rig at +-3 KHz audio if the band were crowded.


Pic #3 shows an "SSS" with 5 khz of audio bandwidth.  The main point is there are little artifacts and crud as the audio rolls off into the noise.  It has a good slope.

Pic # 4  in the next post, shows a standard voice Yallo.  Up the band 6-8 Khz the sidebands are down 60 to 70 dB, which is good.


In summary, this class E rig is above mainstream "ricebox" average in cleanliness. However, when run without audio bandwidth limiting, it can naturally get very wide, even though the audio may be clean for IMD. A DC-to-light rig like this requires audio bandwidth filtering. In my case I usually run it at 4 to 5KHz using the roll off of the EQ, an adjustable digital filter and a hi-freq cut.   This gives me a sharp slope  as shown in pic #3.   The combination of a sharp audio roll off AND a clean rig for IMD that does not add artifacts can produce a signal without worry of splatter.

The class E rig will never have sharper slopes than an SDR radio with digital filters driving a super clean linear amp, but the huge increase in efficiency along with reasonable IMD of the class E rig is a good trade-off.

T

This is a voice Yallo.  Unscientific, but shows the general trend. Notice 6-8 kHz away the signal is down over -70 dB.

And when you run lots of power, the stuff off to the side needs to be WAY down to not bother other people.
With class E (and other rigs that can run wide) its typical to have strong artifacts out past 20 KHz.
With a100 watt rig that rolls off the high end and just will not pass anything very high its no issue, but people that take up 20 KHz or more are being a little bit of band hogs, no?

Yep.

When we choose to run 10 dB more power than the average low power station,  then our sidebands must be 10 dB cleaner to blend in. So high power gives us a bigger responsibility to run cleaner than the average station.

It CAN be done with good audio filtering AND a clean rig. Without both of these factors under control, we may be wide.

My class E rig can easily pass +- 12Khz, cleanly.   However, as shown in picture #2, if I choose, I could run narrow ... +- 3Khz audio and be down -65 dB  5KHz away either side of center freq.   Once the rig is clean, it's all up to the operator to decide how wide he wants to be.

It's one thing to be intentionally wide with a clean signal on a day when or where the band is not active. No problem.  It's another thing to be wide due to a rig problem on a crowded band.

If by miracle, every station on the air had a -15 dB improvement in IMD on their transmit signals, overnight, QRM from adjacent channel  splatter would mostly disappear.


T

Tom,

Having been responsible, and taken the time and effort to clean your own house first, puts a lot of clout behind the common sense opinions you've expressed here. We can keep hoping for a miracle to occur and perhaps it will, in time, but you've clearly taken the lead in trying to make it actually happen. An admirable effort. And even those who may disagree with you will have to admit that.

Don

Something else that seems to affect how clean a high level modulated transmitter of any type is (class E PWM, class C plate modulated, etc.), is the negative peak limiters that some are running. Those things are fine if you are using it just to keep from ocassionally going over 100% negative, but there are some who feel the need to crank the audio so hard that they are hitting the limiter all the time, and that causes just as much crud as overmodulating without one does.

Thank you, Don.

This IMD journey has been a learning experience for me and I simply want to document it for others to learn.  It's for the people here, the archives and the general readers on the web.  Some of these technical threads have received 20,000++ hits over the years.  They do go mainstream after awhile.   I'll bet these IMD discussions will help some hams. There is precious little easy-to-understand  IMD testing info on the web. Most is chock with formulas and ivory tower jargon.

Even six months ago I didn't have a clue on how to run meaningful IMD tests nor set up a reasonably reliable test set up.   I would just build a rig, look at the scope and hope for the best. Sometimes I'd tune up the band and hear a QSO talking about how my signal was splattering them. Have you ever had that happen? It's the worst. And I didn't know how to fix it, though I tried.  All I had to do is look at some IMD tests and it would have showed the problem - at least a reference  baseline to know if my modifications were working.

My IMD threads are not to point fingers nor make anyone feel bad. It's just to record ideas and sample results - a progression of what I've found works for me.  I'll bet there's not a ham on the air who would not like to make his rig cleaner, if they knew how.   It's like going to a party with a "little" bad breath. We can also get on the air with a "little" splatter. Either way, we are probably going to worry about it. It doesn't have to be that way... ;D

I think in the end, we may hear more talk about IMD and how to make our rigs cleaner. The journey doesn't end when we get a new rig on the air and ask for a report...  

Well, I'm about finished with the tests for now but might add a white noise generator graph that I just built. White noise appears to be a neat way to generate highs, mids and lows all at the same time to look at bandwidth. Doing it by voice is near impossible.   A combination of 2-tone, pulse pecker, white noise and the human voice seems to cover all bases for SDR spectrum tests.

T


BTW, here's a picture of my class E rig - dead carrier.  The SDR dynamic range is only about -75 dB at this high power level.  But I see no signs of spurs or other stuff.  I'll bet it's really down over -90dB.  The analog VFO driving a digital converter board into the digital chip drivers works for me.

First, does that transmitter have a negative peak limiter in it? I'd like to see an A/B comparision, first with it set  so that the audio does not go over 100% negative, therefore not hitting the limiter, and then with it pushed past that point so that the limiter kicks in. I'm curious to see if that makes a huge difference.

Hi Shelby,

I wondered the same thing and actually tested it the other day.  This is a low level diode clipper circuit that comes stock in the QIX PDM generator board.   Into a dummyload, using my voice, I tested the rig at about 90% negative. The positive peaks were about 110%.  The IMD and side products looked normal.  I ran it up to the preset clipper level (about 95% neg) and really hammered the audio to maybe 175% positive.  I mean I was abusing the rig to its limits.  Amazingly I did not see a significant increase in IMD or side products.  The noise floor did come up, but not to the point of generating complaints, in my opinion.  I've had rigs that tore up the band when I did that test, but not in this case.

At 150% pos audio and the clipper working, there was barely any difference compared to  running it conservatively  at  110% positive.

So I would say this particular negative peak iimiter works extremely well.   I have run other high level clipper circuits  on the 4X1 plate modulated rig and did not like the results at all.  I am a fan of low level negative peak clipping / limiting.  Maybe a softer limiting would be better, but my results show FB with hard clipping.

In actual use, I usually limit my positive peaks to no more than 125% due to the many diode detectors out there, thus the negative limiter, though working, is barely in use. It is a great safety net and would make life difficult without.

T

Its great to see someone take the time to generate a clean signal.

There are a lot of people on the air who do not have or do not use things to check their frequency, modulation, bandwidth.

Having an SDR can be good and bad, it can show me how my signal is (on frequency, not very wide, enough audio), but you also get to see things that you do not want to see, one guy 2 KHz low, one 1 KHz high, someone 5 KHz below who is 20 KHz wide and over modulating...

The class E stuff is great, once again its the AM guys actually building stuff, and testing it, and using it.
Ham radio is not quite dead yet.

Hi Brett,

Yes, the SDR's can be two-sided.   Just like a dream or nightmare.

I've been having a lot of fun with these projects.  In the beginning it was frustrating and I almost gave up a few times.  But when the good results started, it made all the difference.

Right now I'm a little stuck on how to further improve the class E rig.  Linear tube amps and plate modulated rigs are easier for me.  Frank/ GFZ suggested I disconnect the PDM modulator and test that separately to check its own performance.

Maybe the E Godfather (QIX) will add some input as to what areas can be beefed up, tested or changed to improve IMD in a class E PDM rig.  I am happy with what I have, but there is always room for optimization and improvement.

T

Hi Thom,

I'm glad the class E transmitter has reasonable IMD products.  It should, really.  And, the reason the negative peak limiter doesn't make the rig a lot wider is because the filtering is *after* the N.P.L., which is where it should be!

Over here, the frequency response of the transmitters rolls off reasonably significantly starting just before 6kHz.  I don't think I can actually modulate a 12kHz signal more than a small amount due to the filtering action.  I have measured it, but it's been quite a while!  That's the standard filter in all of the new designs that are published.  The PWM implementations are superior in several respects to any other of the modulators used in the class E rigs.

Over here, I have an 8 pole filter that I can turn on, with a 5kHz sharp rolloff.  It's not a DSP. 

*Nothing* is better than a DSP filter because there is no phase shift, but DSPs introduce their own unique set of issues (like delay).  I'm working on a DSP project over here, and might come out with an adjustable filter for nighttime operation someday, although that is not the goal of the project at the moment  8)

Hi Steve,

OK on the frequency limitation.  I meant 12 Khz based on the high level modulator filters with the low level PDM generator filters set up to cut off higher.   I will check the roll off as a stock radio. I think mine is higher than 6 Khz, however.

Questions:  

1) Besides delay, what problems do you find with digital audio filters?   I have an adjustable audio filter running two cascaded digital chips. I do hear some very slight "cell phone" type ringing, but it is not a problem when running it occasionally when needed.

2) If you were looking to improve IMD on a 24 pill PDM class E rig, where would you start looking?


T

Ringing is always a problem with all filters that have very sharp slopes.  I generally will go with a slightly less steep slope in favor of no ringing.

I really wouldn't bother  ;)  The IMD is quite reasonable as-is, and is far better than pretty much any high level modulated system out there. 

When you start designing super low distortion, perfect audio systems you get need really high switching rates (so the filters don't introduce phase shift in the audio range), then negative feedback (not trivial to impose on a pulse width modulator), remodulation (negative feedback that includes the RF amplifier - demodulated RF is fed back into the modulation), etc. etc.

For ham radio, I don't see the return given the added complexity.

When I was in broadcast engineering, I had occasion to measure the audio performance of many broadcast transmitters.  The IMD figures on your amateur transmitter are better than any broadcast rig I've ever tested.  Keep in mind, I pretty much got out of radio before the newer solid-state transmitters hit the market.  These, no doubt, are much, much better than their predecessors.

Hi Steve,

I'd like to find an audjustable, analog audio filter. A kit form or ready-made wud be FB.  There are plenty of digital ones out there, but no serious analog types I could find.

The analog require precision resistors, so it's not a junk box deal. Also to make it adjustable would require lots of pots.  So maybe a fixed 5 KHz analog filter would be the ticket.  There is probably a good market for this if someone marketed one.


Later today I plan to run some more tests on the PDM rig. I got the white noise generator working so want to see the  bandwidth and general IMD at various modulation levels, straight in, with no external audio filtering..  So far I see pretty consistent performance from low to high modulation levels, probably due to the overkill on the PDM FETS.  The only limit is the PS...

Frank suggested the modulation degradation may show up under 10% where the filter goes discontinuous.

I plan to try loading and other HV levels to see if the IMD changes. Being a fixed digital drive system with no tuning in the lower levels, there's not a lot that can be done for tweaking.  I understand what you are saying about the more complex design if better IMD is desired.

Look for some more test graphs later....

T

Tom:

First of all, wonderful tests on your linear amplification and Class-E. An education for all of us.

I think that as a PDM pulse-width gets very narrow, the linearity would tend to decrease and at some point at say -90% to -100% modulation, the pulse sometimes tends to dissapear. If interested, you may wish to create a resistive attenuator and record a low voltage audio version of the modulated DC at the input to the RF deck. Then you could use PC-based audio analysis to see THD and IM in the PDM modulator. These results may point a finger to any modulated RF distortion.

As far as analog low level audio low-pass filters, I bet you could use a multi-switch to select different capacitors for a 6db or 12db per octave LPF using op-amps. Maybe put a couple in series at 6kc. Here is a good intro on filters here:

http://www.kennethkuhn.com/students/ee431/frequency_filters.pdf

See here (and attached image) for a basic second order low pass filter:

http://www.ti.com/lit/an/snla140a/snla140a.pdf

Walt Jung is one of my favorite tech authors on op-amps. I built an audio console in the 1980s from learning from his book "Op-Amp Applications", found here:

http://waltjung.org/ADI_Books.html

73,
Dan

I did a similar thing Dan.  Walt came to our business and I met him in the early days of my engineering career.  He gave me his book and signed it at the same time.  He is certainly a very talented individual.

Joe, GMS

Thanks for all the info, Dan.

Actually, I have already built up an analog filter using four of the circuits you showed in cascade.  It used eight pots that were preset to various cut off freqs.  It worked rather well, but it was done breadboard and I wish to get a circuit board or equivalent kit to clean it up.


BTW, you have an H-modulated Class E rig, right?  Are you set up to run some two-tone IMD tests with it?  I would be curious what others see for IMD with their E-rigs too.  So far I am impressed with the IMD results and the rig's ability to run very heavily modulated and stay relatively clean.

T

Here's some more results and conclusions testing the 24 pill, class E PDM rig.  These tests are with audio directly into the rig, with no external audio filtering.   My PDM high level filter has a rolloff at about 10 kHz.  Other rigs may vary.

Instead of posting more graphs, I did a bunch of tests and got a general feel for the IMD and audio bandwidth...

First, I swept it with a standard audio oscillator and found the high frequencies start to drop off at about 7khz.  It is pretty flat from 5 Hz to 7 KHz.   There is still considerable energy at 8 kHz and even 9 Khz, but rolls off pretty well by 10 Khz. This is the six-pole 6-7 kHz filter after the NPL having an effect.

A series of voice S's  shows the energy extends out up to 10Khz, confirming the tone test. However, up 10 kHz, the energy is down -40dB.  So if someone where S9 +40, their S's would be about S9 10Khz up the band.

I ran some more 2-tone IMD tests and thought they still looked good. The higher order peaks drop off sharply after 5 Khz, which is what a good linear amp would do using these same tone freqs.   As a confirmation, tuning a second receiver, I hear the tones disappear 5 kHz up the band too.  A good 3-500Z linear amplifer will do about the same.

In summary, the rig is clean for IMD. However, the exisiting internal six-pole 6-7Khz audio filter after the negative peak limiter needs a sharper roll off, perhaps in the form of more poles.  I feel there is still too much energy above 7 Khz, if the goal is to run 7 Khz maximum with the rig stock.

But this is easily fixed...  In my case, I use a very sharp digital audio filter before the rig. This lets this clean rig operate well within the audio bandpass we desire, whether it be 3, 4, 5, 6 or 7 Khz.  I would rather have an analog filter, but that is another subject.

I feel the rig can handle heavy modulation with ease - the negative peak limiter with the filter works superbly... no sign of splatter. It is a great platform for high fidelity, high efficiency AM - it just needs a method of sharper audio filtering, probably in the form of a 6 or 8 pole analog audio filter placed before the audio input.  It appears the internal NPL six-pole analog audio filter is not enough.

In a sense, this is probably nit picking - like saying a dragster needs a governor to keep it from going too fast. But new users need to be aware that there is a way to get full contol of their bandwidth, if they desire.

My hat's off to Steve/ QIX for doing a marvelous engineering design that does everything it should.  

T
 

Hi:

Yes I agree about Steve's designs. Really nice and one of the best things I built. The big turn-around for me is when Steve came out with the push-pull RF deck. I had built a 6-FET single ended deck, and while it worked OK, the deck got more efficient and with a better output waveform when I converted it to push-pull.

GMS: That's cool you got to meet him. I emailed him once, and did some of his circuits, but that is about it.

Tom: I run a PDM class E rig. 6 pill. I should test it, but have been lazy (and scared to see my numbers!). Actually we can test it in three places: after my switched capacitor LP filter (the one you do not like), after the PDM generator (before the switching transistors), then after the PDM LPF. Eh, I am too lazy, but really should do it. It would take me to get the test audio file and an audio recorder set up. I do have an Audio Precision System 1 test set, but the PC has not been fired up in a few years.

Not much time these days either.

So, why not do THD as well as IM?

Thanks for all the good info.

Dan

Cool on the PDM rig, Dan.  Maybe we can do some tune-oriented things to improve the IMD.  I still want to go back and try some stuff now that I'm caught up.

How would you go about testing THD in a linear and class E rig?  I'm game to see how the harmonic distortion looks.   Would THD show something that does not show up in IMD?  IE, if the THD got worse, wouldn't the IMD results get worse too?

T

Oh, I forgot to  mention - the PWM generator board has all of the necessary traces, etc. for an 8 pole filter.  I have schematics for this.  The same board is just configured differently, and poof - it's got an 8 pole filter instead of a 6 pole.

I use the 8 pole configuration in one of my PWM rigs, however it is switchable.  So, I switch in the ADDITIONAL 8 poles of filtering ahead of the 6 pole filter in the main PWM generator board (I use a 2nd, old-rev PWM generator board I had lying around for the 8 pole filter - with the filter only installed and no other components).  Gosh, that's 14 poles of filtering!!

Steve,

Yep, I can tell when you have that second 8 pole filter inline. The highs are attenuated very well and your bandwidth drops to a hard 5-6 Khz, brickwall.  I heard you using it the other night when the band was crowded.  Good show.


Do you have any more of those old-rev PDM boards?   I'd like to build up an 8 pole filter.  Do you have the parts values available for 4.5  to 5 Khz or is it all standard for the 5.7 KHz?    As I mentioned, my rig starts to roll off at about 7 Khz, so maybe I have an older version of the parts from three years ago.  IE, maybe you tightened up the bandwidth to 5.7 Khz since I bought it.


T  

Tom:

THD testing is really a modulator issue. Say you put in a 500kz tone. Look for the presence of 1k, 2k, etc.

I'd like to see the response graph of a 6 or 8 pole filter. Wonder if any peaking or early roll-off occurs.

Dan

Dan,

Here's something maybe you can explain....

I put in a single audio tone of 1 KHz and then another test at 7 KHz.  These are SINGLE tones.  On ssb I get just a single peak.  But on AM I get a spectrum as shown in the two attached pics.

What are we actually seeing here?  Is this a 1-tone test with the tone beating against the carrier or what?


Pic #1 is AM using a 1KhZ tone at 100% modulation.

Pic #2 is using a 7KHz tone at 75% modulation.

T

Tom:

That's weird!

On the 1k tone, when listening to an analog rx in SSB mode on only one sideband, do you hear a single tone, or one every 1 kc as in the image?

Maybe the 1k tones are mixing with each other in the receiver. I do not understand why the 7k tones would not. Just wonder if it is an issue with the SDR rx or software. Try other frequencies to see where things start to go bad.

Anybody else have ideas....?

Dan

Yep. Or harmonic distortion.

On ssb, using low or high power I get a single tone on the SDR screen. It's looks just like an unmodulated carrier as it should.


A single tone on AM produces the spectrum as posted.  

I just tried it with a voice whistle at about 800 Hz. It also shows mutiple spectrum peaks at 20% modulation or higher.

Also, on the FT-1000D in the AM mode I see the same thing, though the higher order peaks are way down.  Of course the FT-1000D low level output is about -70 3rd IMD, so much cleaner than the E rig.  On the 1000D, the higher order spectrum peaks get drastically higher as I go into flat topping, so it does indicate distortion.

This appears to be normal.


Dan, I listened on a separate receiver and the tones are definately there, marching up the band.  They appear sensitive to how hard I hit the audio, just like IMD does when flat topping. Can someone else inject a single tone into their AM rig and see if you get this spectrum too?

It appears a single tone may be a good way to measure AM IMD -  whereas the a two-tone is better suited for ssb.
Maybe this is another example where the carrier comes into play, just like the class C AM biasing thing we talked about recently.

T  

I did exactly this on the NE602 balanced mixer circuit alone and as installed in the FT-102. I got the same spectra use you are getting Tom. The sidebands were -55 dBc and better with the NE602 alone. As installed in the FT-102 they were more than -40 dBc.

That's good to hear, Steve.

How far down is the "3rd order" tone from the carrier peak and the main injected tone when at 100%  or 150% modulation?   What happens when you flat top it?

T

I was in the middle of adding the IMD numbers when you posted. See my previous post.

A-HA!!  We're making progress.

Your AM sidebands are pretty clean as expected.

I just did a 800 Hz single tone test on AM with the FT-1000D 4 mW into the 4CX-350FJ.  I didn't have a lot of signal to work with, but am seeing at least -40 Db 3rd - and about -55 dB down from the carrier.  

The class E rig shows about -25 dB down 3rd order  and  -30dB down from the carrier peak in the same test.  This is a good baseline to try improvements on the E rig later on.

I would expect to see these kinds of differences, but it didn't show clearly until now.  After all, the e-rig is running 1KW and is high level modulated.  That's axing a lot.

T

FT-1000D driving 4CX-350FJ on AM - single 800 Hz tone: (Looks very clean)

To Steve / QIX -

Could you tell me the proper procedure to tune up a class E rig using C1 and C2?  

Does the E/drain waveform need to be looked at for optimization each time or can we do it using the wattmeter and current meters?


Tnx.

T

Tom,

Just out of curiosity, can you do a wide sweep to show the switching frequency sidebands?

I will do, Tom.


The plot thickens....  Frank / GFZ says that the rig will not generate IMD with only one tone unless we are overdriving the audio amplifier into clipping.  He says we are looking at the harmonics of the audio.

I thought I was careful to limit audio clipping on the various rigs, but will look into this today on the e-rig, since it seems the most prominent there. It may be as simple as adjusting the audio input pot on the PDM gen board.

Steve/ HX, you might recheck your audio going into the bal modulater and see what's happening too. But perhaps -40 to -50 Db 3rd IMD is indicative of pretty clean audio to begin with.


Well, at  least this one tone test is another tool for checking the overall health of a rig, since it turned up a possible audio problem.

More later -

T

Update on E-Rig tests:

I checked the audio input with three different audio generators and see the same results.  The input test audio  is clean.

I checked the 187 KHz switching sidebands and see them down about -65dB, but this may be normal  shack overload, since they should be down more. I've had guys check it and heard nothing on the air.

I did tests at  20%, 50% and 100% modulation at 500 w and 1KW.  At 20%, the harmonic distortion is quite good, but once it passes 50% it gets poor. I can hear it up the band.

C1 and C2 tuning had a small effect on the THD, but I need to look into that more. Maybe Steve / QIX will come in and describe the latest C1/C2 tuning. I might even try some different L tank coil values to see if that helps...

T

And, class E rig 100 % modulated with a single 1 Khz tone...

I notice that the harmonic frequencies are exact  X2 X3 X4, etc of the 1 Khz fundamental, which shows they are true audio harmonics.  This may indicate modulator / audio issues to look into.


Still testing.  Any ideas appreciated.

T

Trying to get to the bottom of this...

Here's 20% modulation, 50% and 100% using the FT-1000D driving the 4CX-350FJ amplifier.  I am using the same audio generator and test set up as when testing the class E rig.

We see that the THD is better by quite a bit.  This leads me to believe that I need to find what is going on in my class E rig... 

Can anyone else with a PDM  E-rig run these same tests at full power to compare results?  I'm hoping to see I have a problem in my rig or test set up so it can be corrected.

T

That looks nasty!

It pays to be persistant.... seems like a step in the right direction.

I decided to experiment with tank Q...  I reduced the numbers of inductor turns to 1/2 and added in large padder caps to C1 and C2.   Immediately the THD started to fall.  

I haven't optimized it yet, but at 60% modulation, I am seeing -40 Db  down from the main tone...   and -60 dB down from the carrier peak.   This is really significant when it comes to sideband splatter reduction.

This is the same thing I did with my linear amplifier chain - find the highest Q I could run. It coincided with best IMD.  It may be more about matching the output of a low impedance amplifier rather than Q.  It takes a LOT of tank C1 and C2  to match a big amplifier.

I also measured the HV supply and found it was stiff. This is important for low IMD and THD figures.

So, now I'll bolt in some fixed cap padders and try to make it even better.  I'll post more observations once I get everything dialed in.  I plan to check the e waveform for form and also be sure the efficiency is still up there.  There is also a risk of higher tank voltages that can blow FETS.  But I'll try anything for a cleaner rig.

T

Pic Below:

Class E rig at 60% modulation looking better. Notice the higher order peaks are better behaved. I found my optimized linear chain had the same pattern.. the higher order stuff tended to stay "pushed down" as the power increased. If there is a problem, it tends to climb with power.

Tom,

I noted how far down the (upper sidebands) 2nd, 3rd, 4th and 5th harmonics were relative to the 1 kHz fundamental sideband and plugged those numbers into a THD spreadsheet I did a number of years ago.

Your "original" plot in Reply 36 = 6.8 % THD

Your "Higher-Q tank" plot in Reply 39 = 1.2 % THD.

I do wonder what is going on there though; cud be a number of things.  I would tend to start by running the PDM into a resistor load only and see what distortion levels you get.  

Do you have way to directly get a measurement of THD of the audio from the PDM LPF output?

Thanks for the THD calculations, Tom.

Under 1% would be my goal.

I may try the PDM modulator into a load thing if I don't get good results playing around with the tank.  I'm cornvinced the rig should do well once set up properly.   It's just that I don't have a lot of experience optimizing class E rigs, so it's slow going.

I don't have a method to analyze audio-based THD yet, and will look into it if needed next.

T

There is something wrong there Tom. With the NE602 and the FT-102, the 2nd harmonics were down 40-45 dB. The third was more than -55 and everything else was more than -70 dB.

It's not clear to me how your can put more than one tone (the carrier and the modulation sidebands) into and amp and there not be IMD.

Well, I know that my linear system checks out very well for THD using the same test set up.  So that's the standard.


The e-rig is working OK, it's just that the THD is not quite good enough.  I've found that by setting up the tank for higher Q (less L and more C)  the efficiency goes down, but the THD is improving.  Each e module wants to run at closer to 7 to 8 amps rather than 6 amps to give better THD by about -10 dB. This is almost acceptable.  The e waveform still looks decent.

This would mean setting the trip circuit higher, so there is more risk.

Still trying different stuff.

T

This looks like  A LEAK...

Well, I found the sweet spot and put it to bed.

Once the best L/C ratio was found which gives best efficiency, power out, and best general THD,  C1 and C2 need to be slightly tweaked further for best THD.  When the L/C ratio is right,  C1 and C2 are most sensitive to THD fine-tuning.  I can see the THD go up and down as C1 and C2 are fine-tuned. This affects power output very little.

At 100% modulation I was able to drop the first lower audio peak  (second harmonic) an additional  -10 dB from the main audio peak. This is about -35 to -40 dB down. The higher order harmonics also drop off faster than before.

This is acceptable.  When using voice, the rig seems pretty clean for side products.  

T

Quite interesting. This might explain some things I see and hear on the bands.

Just got back from visiting with Tim and Larry in Maine.

In general, I haven't noticed a lot of difference in the modulation with final tuning at least with my own transmitters, but every RF amplifier is a bit different since they were all built by different people.

It seems as if you've got it figured out, in any event.

I made some distortion tests way back when I first designed the PWM modulator.  I was consistently able to get less than 1% distortion which I found perfectly acceptable for the application.  Unfortunately, I don't have access to the distortion analyzer anymore, else I would run a couple of more tests  :P

Hi Steve,

Yep, it's all sorted out for now. Was on 75M with the e-rig for a few hours today and the bamdwidth reports were good... just what I see on the SDR monitor in the shack.

In summary, after finding the best L/C tank ratio and tweaking C1 and C2 for best THD, I found that running a FAT carrier as high as possible and limiting modulation percentage to about 80% negative (using external processor limiting, not the internal NPL) produced the cleanest signal.   For my voice, 80% negative equates to about 100% positive.   Coupled with the external 5 kHZ audio filter, it's hard to beat.  It also seems the higher the current per stage, the better the THD.   IE, at the same power output, the THD was poorer at 5 Amps vs: 7 Amps per stage. (better THD when tuned for lower efficiency)   So I run the current up a little, have lower efficiency on purpose and see cleaner performance.  Something like idling a linear amplifer higher maybe.

Sure, we could run the modulation much higher and get no complaints, but this is the sweet spot where it runs the cleanest for me.  This may or may not apply to other rigs. We will just have to see if others try these tests.

T

It just dawned on me..

When modulation gets above about 90% negative, 110% positive, the THD starts to take off. I wonder if this is the internal negative peak limiter clipping action?   The IMD stays reasonably low, but that THD soars.  I'll check this out tmw.

Easy solution to fix if so.

T

If changing the voltage/current ratio of the RF amplifier changes the audio performance, it may not have anything to do with the RF stage, but rather, may have something to do with your PWM filter.

If the input inductor isn't correct, the PWM waveform will distort, particularly at high levels of negative modulation, where there is little energy, and the pulse width is small.  Anyway, just a thought.  Back in the tube days of PWM over here at QIX radio, the input inductor value was very critical to creating a low distortion system.

What is your input inductor value?

I was thinking the RF final needs to have good, matched load, thus the L/C experiments.


Anyway, my high level PDM filter was designed by Jay / W1VD and called an "infinite impedance filter."  The three coils are air-core wound with #6 wire on 4.5" PVC plastic pipe.

L1 = 28.8 uh      C1= 10.83 uf     L2= 28.8 uh    C2= 7.55uf    L3 = 14.1 uh     (actual values)

I have some more bypassing to ground at the output of the filter.

Would you suggest some changes?


I hope to try some NPL tests later today.  

T      

I was just reading some of the earlier posts on this subject.
I work for a company that makes audio frequency filters (.1 Hz to 100Khz), we make LP,HP, BP, BR.
We also have some custom DSP filters.
Take a look on the web for Frequency Devices inc.
I'm sure that any kind of filter could be designed for whatever frequency range you need.
Just trying to help.
Don W9BHI

Hi Don:

I looked at your web site and you have some good filters to offer. Bear was mentioning a long time ago about making a passive LC low level filter. Found this calculator on the web:

http://www.calculatoredge.com/electronics/ch%20pi%20low%20pass.htm

Maybe this can be put in the middle of two op-amps that control source and load Z.

Tom:

As you are finding more distortion around -90%, this could be due to a changing pulse waveform, which as Steve said could be due to filter or load issues. A few years ago I came across this paper which describes some issues to PDM performance, and an interesting "tail biter" circuit (see figure 6), of which one person on this board who designs Class E transmitters is intimately familiar with:

http://www.analfatecnicos.net/archivos/73.AM-HDRadio-DRM-Harris.pdf

Always thought of trying it out.

I have been extremely busy and am considering doing some tests on my E-rig. I have two high priority projects on the bench right now, so it may take some weeks to get to. If I test, I would use a PC with sound card and editing software. My Adobe Audition can do a spectrum display of a recorded file. I would try to break up the test into two parts, 1: the PDM generator, where I would use a simple RC filter at the output of the generator to verify this simple circuit is doing well, then 2: at the modulated DC in to the RF deck, to verify my modulator frequency response and linearity. I know I will find filter issues as my filter is not optimized.

73,
Dan

I'm sure the filter is fine.  I use a completely different filter here - it's a standard Butterworth.  The input inductor is 11uH.

Redoing the filter is a big deal, and I wouldn't bother.  By the way, I have a 1.7 ohm terminating resistance (the RF amplifier).

Thanks for the info Dan. I will read it over and see if I can try any of it.

For AM, I am hoping to use the class E rig exclusively instead of the class A linear system.  Heat is the major reason. So I am determined to get it running as cleanly as possible.

Here's what I'd like to do...  It's very easy to run THD tests on the air to see what some of the other guy's e-rigs look like. Many of the guys on 3875 are running e-rigs and last night seemed very interested in the tests I've run. All we have to do is put a 1 KC audio tone into a class e rig and run the modulation up to -99% negative and watch the SDR spectrum on the receiving end.  I'll bet there are different PDM filters being used so it will help to screen out the filter issue, if it exists.   If some look better than mine, then I know I can improve things here.

Last night one of the guys did a whistle, which is a pure sine wave.  At low modulation levels, I saw just the 1 KC tone. But at high modulation levels I could see the "xmas tree on its side" of X2 X3 X4, etc., THD peaks - just like my rig here. A "perfect" transmitter would put out just one 1 KC peak. But nothing is perfect, of course, nor should we expect it with any AM rig. It's just a matter of testing and trying to get things set up the best we can and then forget about it...

I'll see if some of the guys will run a simple signal gen 1 Khz single tone on-air test tonight or tmw.


Don: Appreciate the filter info too. As hams we would probably like to find an inexpensive active 12 pole R/C analog kit for $29... ;)

Steve:  OK on the filter.  I'l continue on as described above... thanks for the input.  BTW, how did you calculate terminating resistance for the final... is it a simple total drain current / drain voltage or is there a factor involved?


T

Tom:

I have a couple of clean audio test oscillators. If I am around this weekend and you are interested, maybe I could insert tone. My rig should be the reference for the most distorted! :-)

Dan

Sounds good Dan!  At this point I have no data from others' rigs and desperately need it to continue on.


NPL Test Update:

Nope, it's not the internal negative peak limiter doing it. I adjusted the NPL completely out and the THD peak levels  grew similarily as they approached -99%.  However, at -100% negative, the NPL does have an effect mostly on the higher harmonics, causing them to increase slightly.  

The NPL is still a good safety net in case of stray peaks.  I wouldn't lean against it all the time, however.



T

I got some on-air test results from two of the class E guys tonight on 3875.   They both injected a 1Khz tone directly into their H-modulator class E transmitters.  These results were generally confirmed by Al/ W1VTP watching on his SDR receiver too.

These tests were done at about 95% modulation without activating the negative peak limiter.

They pretty well match my own e-rig results when hitting the audio near 95% modulation. The difference is that I'm running a PDM modulator.

At 20% to 50% modulation, (not pictured here) the THD was excellent - down 40 Db.. which is about 1% THD.  
Still trying different things...

T

You just did!  N1GTU and W1LLY are both running class H modulators.

How 'bout that.  I corrected it above, tnx.  


Since both the H amd PDM modulators show the same performance, I wonder if adding more ferrite cores to the output combiner might be the next logical test?  Possible saturation at higher modulation levels?  I have eight cores for each group of six Mosfets.  They do get warm after a transmission.



T

Funny, I did tests on 3 different transmitters today, I fed in a 604 Hz signal into the rigs and watched the spectrum, and on all three, it was clean up to about 50% modulation, above that, I started getting the additional spikes.
Plate modulated rigs all, with all different tube types, and I adjusted bias, grid leak, screen voltages, tuning and loading, and nothing made a bit of difference.

Could this be an sdr thing?
 

No, it's the reality of what's been put out on the air all these years. Most of us never had equipment to take such a look at our transmitters. I've done it with a traditional swept tuned spectrum analyzer and saw the same sorts of spectra. It's not unique to the SDR.

Hi Brett,

Thanks for taking the time to give us some more data.


Usually, the harmonic spikes are always there, even at 10% modulation, but way down, maybe below the noise floor. They will increase with modulation and sometimes hit a point when the growth accelerates due to some non-linearity problem being hit.  I haven't tested any tube transmitters yet. Maybe some more guys will.

Next, try a two tone test and see if you can improve the IMD using the same things you tried already for THD. Maybe the THD is "built into" the modulator system with little adjustment available - and you can have better influence improving IMD overall, dunno.  Mod iron and tubes in general can be one reason for higher THD.  Just like any tube audio amplifier used for music...


Here's what keeps me from questioning the SDR results...

1) I ran these tests with my new class A linear system (4CX-350FJ) at 100% modulation and see excellent THD results, below 1%.  Steve/ K4HX did the same on his FT-102 and saw the same good results.  So it CAN be done.

2)  Using a single tone, push any transmitter close to 100% modulation and listen on a separate receiver as we tune up the band. The worse the THD, the louder the harmonic tones will be as they parade up the band.  The cleaner the transmitter system, the weaker the tones.  I hear this on my rigs which confirms the SDR results.

Obviously, getting a transmitter to modulate 100% at high power with THD below 2% is no easy feat. But we are all learning.

Distortion is quite normal for any power modulation system. It's just a question of degree.

T

4CX-350FJ at 100% AM modulation:

Just to prove to myself that there can be the "perfect" AM signal, I tried the test using my HPSDR low level transmitter. This is the SDR going into a MiniLabs ZHL-3A 1 watt lab amplifier.

A "perfect" amplifer would do ONE tone in... ONE tone out (one per sideband)

The pic below shows a 1 Khz tone at 140%+ modulation.  If I had more signal to work with above the noise floor, we would likely see the peaks down over -65 Db.  But as-is, this shows that an ideal AM signal has very little harmonic X2, X3, etc.  activity.

Of course it's easy to get a low level signal to be this clean - but a bear to do it with a high level modulation system and good efficiency.   The next step up is a class A linear amplifier, but lots of heat.  (Or heavy NFB or predistortion software techniques)

T

Well, it seems to me that it's "good enough", as they say  ;)

On a practical level, you have to be generating a LOT of distortion for anyone to notice the signal is "wider" - and in a good system, that means serious clipping and/or nothing to keep the high frequencies in line at all, and probably some high boost to boot.

I think if this were a problem, we would have been hearing unusually wide signals for many, many years.  There have traditionally been a lot of high power transmitters around for a long time and I don't remember anyone being wide due to a few percent (and that's probably being generous - most are much worse) harmonic distortion unless they were overmodulating or "flat topping" in some way.  And you're well below that.

Yep, as I said back on the first page of this thread after the initial IMD tests, a good working class E rig is better than average for IMD.  When run with voice modulation, the sideband crud is way down. It's a beautiful working design....  :)

As for the THD numbers, I honestly thought they would test out better at high modulation levels. That's why I've been trying to determine if I had a problem with mine or not.  I'm the kinda guy who could never be satisfied if I at least didn't try to improve things.

I doubt any improvements could make any noticable difference in less bandwidth at 80% modulation or below - or even improve voice quality on freq - but when hitting 150% positive using the NPL, I think we might hear some extra crud on my e-rig due to THD.  That's why I plan to keep it down around 100% positive most of the time - unless I can eventually come up with better THD numbers when hitting it hard.

That being said, I'll bet if some of the guys do THD tests on their high level class C tube rigs, we will see similar or even poorer THD numbers.   With iron that cud saturate and tubes, run a big tube rig at 150% positive and we might be surprised by the THD numbers, I dunno.  I'd like to see some tests. Then again,  ones with modulator negative feedback may really shine here.

I'd be willing to capture a screen spectrum shot for anyone who wants to put through a 1 KHz tone directly into the audio at 95% negative modulation or whatever. (with no processing or EQ's in the way)  Let's test  a 4X1 X 833A's, a stock ricebox driving a linear, a DX-100,  Flex driving a linear , etc.   I'm really curious.
 

T

** BTW, yesterday Clark/ KB1NFS, said he was able to improve his  e-rig THD by optimizing his tank L/C ratio. He ended up with more turns on L1 and used a distortion analyzer to monitor the improvements.  I also ended up with MORE turns for L for some THD improvement. Higher Q did not work in this case.

Tom:

How about later Saturday afternoon, maybe evening?

I will be doing some testing at W1ZZZ's, and can bring my oscillator over to test his D-rig.

Maybe later we can do my E-rig? I do not have a mod-monitor, so can you talk us in to various %'s of modulation? Can you record an audio *.wav file?

If good on all, please PM me your phone number.

Dan

Sure, no problem, Dan.  We can do it Saturday.  

Maybe we can find the "sweet spot"  settings, if any.   Be prepared to try a few things.
I haven't tried a .wav file recording on the SDR yet. Should be easy enuff to do....

I'll send ya a PM.


BTW, I read your polyphase info.  It looks impressive and complex -  though I don't think I am quite THAT motivated to pursue it right now ... :-)

T

Tom:

Maybe if you can record a wav file, then we can look at the spectrum in an audio editor application and see if the harmonics of the test tone are high at all. This might eliminate the SDR display as any potential issue.

Thanks,
Dan

Sure.  I'll get set up for recording the .mpg.   In fact, I can audio record THD of my own e-rig signal and get it to you soon.  

I would guess there is little chance of the SDR display being off, but it's worth a look to verify it in your audio editor.   I'll bet it will give us real % THD numbers too?


T

Tom:

I was thinking of importing a wav audio file into Adobe Audition, then looking at spectral view, so no analysis. I do have an AP System 1, but it has not been run in years. It would take some effort to get 'er going.

Dan

K1JJ Plot from Reply # 64:
"HPSDR near perfect" USB only, 0.91 % THD.

Today Rob/ W1AEX wanted a THD test. The band was quiet and stable. Using his Flex 5000 SDR and pair of 3-500Z's, he tested at better than  1.8%  THD at 100% modulation.   He has that system really well tuned and optimized.


T

Rob / W1AEX's   100% modulation  -   1.8%  THD pic:

I thought it was odd that I got the exact same results on various rigs:
4x150a modulated by 811's, the 3x4D32 modulated by the same 811's, a pair of 813's modulated by a pair of 4cx250b (ab1) or push pull 100TH's.

All the rigs showed the same clean below 50% modulation, all got nasty above that, and went crazy past 100% neg modulation.

Maybe I will run the tests again and actually note just how far down the spikes are.

I am also building a push pull 812a rig to see how the triodes take modulation since there is no screen to screw around with the modulation.

I used to enjoy the test suite in the flex 5000, you could actually transmit a square wave and it was square.

Disappointed? Where do you find this stuff Clark? So you do not like software defined transmitters, I get it. Your facts are specious. They sound good but are not based on facts.

Directly sponsor  any "Published" data, or for that matter, published "second hand reviews" right here in this thread. I want to see the "data" or "reviews" your quoting that Flex Radio's "Buckshot The Band" Clark!

Clark,

Well, first of all, this is not a 2-tone test - it's a one tone THD test for AM.  (Total Harmonic Distortion) There's a difference.

The center peak is the carrier. Ignore it.    The second peak (- 1KHZ) on the left is the main 1 khz audio tone.  Count down in db to the third peak. This is the second harmonic. (-2 KHz)   The second harmonic is down about -35 dB from the audio peak.  -40 Db equals 1% THD.     Rob is seeing excellent results at -35dB = 1.8% THD.

Two other guys running SDR's saw the same readings today when this pic was taken. I actually asked each one what they counted for dB before reporting my reading. We all agreed.  All SDR's can't be wrong, OM.  We then all tested a second W2 station with agreed results. (not pictured)

To the contrary, I find the Flex 5000 to be a pretty clean transmitter/ exciter as shown by the results above.  The IMD, a different measurement, is at least -31db 3rd, which is reasonable in the ham market.

Many of us are using SDR's effectively and they can be very accurate. I'm sorry you are having so much trouble trusting yours.  BTW, I am not using a Flex, rather an HPSDR.  Similar software but different hardware for a 160 - 6M transmitter/ receiver.


T

Here are the THD % vs: dB reference numbers for an AM full carrier transmitter, based on the distance in dB between the main audio tone to the lower second harmonic tone.   ** Normally THD calculations are composed of several harmonics factored in. We are using just one harmonic, for a simple, easy-to-do estimate. Hopefully harmonic 3 and 4, etc. keep decending sharply into the noise, otherwise the THD is really worse than what we are estimating here.  It's easy enough to eyeball estimate the overall quality of the rig by how the higher order peaks fall off.

Thanks to Al, W1VTP for sending me his custom spreadsheet that calculates these numbers.


dB DOWN           Percentage of THD -Total Harmonic Distortion  (Not to be confused with a 2-tone IMD linearity test)

-5 Db                56%
-10                    31%
-15                    17.8%
-20                     10%
-25                      5.6 %
-30                      3.1%
-35                      1.8%
-40                       1%
-45                       0.56%
-50                       0.32%
-55                        0.17%
-60                        0.1%

As you can see, once we are down about -30 dB, which is 3.1% THD, we start to run into diminishing returns that few ears can ever discern.  A -40Db down second harmonic is really FB at 1% THD.

Here's how to test and then estimate your THD:  (for full carrier AM transmitter)

Put a CLEAN single 1 Khz sinewave tone (or any tone freq within the transmitter's bandpass including second harmonic) directly into the transmitter audio input. No processing or EQ's, etc. Turn off RF processing in riceboxes.

Run the modulation up to the test level, 20%, 50% 80%, 100%, etc.  The higher the modulation level, the worse the THD generally gets.  Look at the spectrum analyzer display for three peaks and some lower ones.  The center peak is the carrier. Ignore it.    The second peak (- 1KHZ) on the left is the main 1 khz audio tone.  Count down in db to the third lower peak. This is the second harmonic. (-2 KHz)   For example, if the second harmonic peak is down -30dB from the audio peak, then 30dB  roughly equals 3.1% THD.    (Peak of tone #2 minus  peak of tone #3 = dB)


T

Great info.
For people with plate modulated rigs, and rigs into amplifiers, the distortion all adds up right?
Modulator, RF deck, amplifier....

If I was to hook the secondary of the mod transformer into a dummy load and look at the audio spectrum, would I see a 2nd harmonic?

I never thought much about class E, but you are dumping audio on an RF power amp, right?
Is it like tubes where the output signal might not follow the input signal because of the way devices react to the modulation?

In the tube world, you can run out of emission, the final might not react like it should with the changing plate voltage, screen grids hose things up, etc.

No one wants to hear bad news. There have been plenty of wide signals over the years. Some have been consistently wide for years. It wasn't always due to clipping. The spectral displays are eye opening - kinda like moving the stove in the kitchen after 20 years and finding a bunch of crud behind it.  ;D

I did some of these same tests over 10 years ago with a high-end FFT based analyzer. I also looked at on air signals. I did similar tests with a swept-tuned spectrum analyzer back in the 1980s. It wasn't always pretty then and it still isn't.

Look at it this way. If you are 40-over-9 on frequency and your distortion products are -40 dBc, that means they are still S9 off frequency. The bigger your signal, the more concerned you should be with a clean signal. Those who are 10 dB louder than others on the band better be 10 db cleaner than everyone else. Otherwise, they will stand out like a sore thumb.   :'(

Just a short note:  THD by implication assumes accounting for several orders of harmonics, ie: 2nd, 3rd, 4th etc.  The math for THD involves the "[square] root of the sum of the squares or RSS" of these harmonics to arrive at the final figure - we need not get into this point here.  What we are assuming from practical experience with these measurements is that the 3rd, 4th etc harmonics due to descending values from a carefulluy adjusted transmitter contribute little to THD. A poorly adjusted transmitter is quite another matter.  For example, improperly loading a linear and overdriving it can really throw out a nasty totel harmonic distortion number.

Brett - yes you would see some harmonic content.  But that's not the final result.  As I stated above there can be several contributing factors to what an AM transmitter can put out for THD of the modulation component.

I have found this whole thread stimulating and provides extra motivation for carefully adjusting my transmitter.

Al

Don't get wrapped around the axle on THD, IMD and any other distortions. The reality is that if you put a 3 kHz tone into your TX and there is a spectral line on the output at 12 kHz and it's -40 dBc, you may have a problem. From my last post:

If you are 40-over-9 on frequency and your distortion products are -40 dBc, that means they are still S9 off frequency.

How much this this "4th harmonic" contributes to the THD is of little concern to the guy receiving interference 12 kHz up the band.

Extremely interesting thread!   

For me after a rig is built, frequency response checks are done along with looking at the quality of the waveform.  Now I will add THD and IMD as standard tests.  Fortunately I have a HP Spectrum Analyzer that is up to the task.  I also have an HP Distortion Analyzer. 

Any form of distortion can potentially add to ones bandwidth.   Saying that, I would think that some forms of distortion would be worse than others.  Those on the East Coast may remember Irb, W2VJZ running his DX-100 that was typically 30 KHz wide!  This problem can happen with old and new technologies.   

Some transmitter / modulators make it very easy to transmit an abnormally wide signal whereas other older topologies have built in frequency response limitations but can easily produce distortion if driven to hard.   Saying that, each topology when run properly will produce the appropriate sidebands based on a cleanly modulated signal.

I have heard a lot of the K7DYY rigs on the air lately and none that I have listened to were "excessively wide".  They also all sounded very good.  I can't say that for some of the other DC to light topologies, but again its not the topology but how its set up and run.It would be interesting to see some THD figures on the K7DYY boxes and compare its numbers with those topologies already categorized. 

Joe, W3GMS 

Great post, Joe. You have hit the spirit of this perfectly...

Yes, having a series of tests like you described is the best way to quickly identify problems. I know from experience that we can be lulled into complacency by just looking at an o'scope waveform thinking it looks good  - and then go into denial if we get bad repeorts.  The tone tests take no prisoners and are very harsh with the truth.

Steve / HX said it right - that we need to look for "BIG" problems, not nit pick over the last few dB of cleanliness.  If the IMD and THD tones show the rig to be generally clean, then we can play around a little more and then forget about it. Maybe do a periodic check.

Look at how a doctor operates. We go in there for an exam and he has a fast set of checks he makes on us. If something jumps out, he sends us out for more detailed tests.  But everyone is different, just like rigs. As long as the results come in around average, everything is FB.

The cool thing about running a clean rig is we can then open up the audio bandwidth wider than we can with a dirty rig.  Highs extending out 6 Khz that are clean are like wispy snowflakes that do no harm. They actually sound kinda cool to me.  But if the same 6 Khz is mixed with harsh IMD /THD buckshot crud, mixing frequencies make it extend even farther out,  and this usually offends other operators.  

Good show, OM.

T

What is the best way to test for overall cleanness of the signal/audio?

SDR with a two tone signal into the transmitter at max power?

As I said, I put a single tone into my rigs and above 50% mod had harmonics increase, but no adjustments changed anything.
Grid drive and tuning, grid leak (bias voltage adjust), screen voltage, current, resistance, I did not see any real change unless I really went far from normal settings.

I have equipment to measure intermod, par, but if I put the signals into the transmitter I have to use a receiver to measure the result which has its own unknown issues.

2 tones at say 75% mod and tune for best (least) harmonics?
 

Hi Brett,

I would divide the tests into two steps. One is the THD test looking for modulator distortion - and an IMD test looking for RF final non-linear mixing.  They are related and some problems can cause both IMD and THD to be poor.

Since you already ran a one tone THD test and could not improve things, perhaps the problem is something you cannot help, like iron saturation, tube characteristics or whatever.  

So I would run a series of IMD 2-tone tests looking to improve the linearity of the overall transmitter. Run the rig at several modulation % levels and see if any of the things you already tried help. I would be surprised if nothing improves your IMD numbers.

I have not seen your THD graph, but maybe you can estimate how far down the peaks are in dB and compare it to the post that shows THD %. If it's below 10% THD at 100% modulation, you are doing well for a tube rig.  Though, I have checked only two tube transmitters and both were around 6-7% THD.

A few of us have gotten quite good at it on the air. Today  Rob had a guy hold his whistle at 100% modulation and immediately told him his THD was good. We could see  two clean, tall peaks and little side spectral peaks. It can be as easy as that. We have also seen a transmitter that when whistled, showed higher order harmonic peaks up 15 Khz.  

But for IMD we need two steady tones.   If you want to get on tomorrow and try it with us, do so. We can at least confirm if your own testing is telling you the truth.

Anyway, for your rig, next do a 2-tone IMD test. If you cannot improve your IMD by changing loading, drive, modulator idling bias, exciter tuning, better regulated modulator screens and grids,  newer tubes, reducing power to 1/2 max, etc., I will be very surprised.  Also, make sure no RF is getting into the audio and you have no distortion on your test tones.

An old HP audio oscillator works beautifully. Use two for a two tone with two resisistors to mix the tones in a summing pad for the audio input. Or use a computer with signal gen software off the web.

T

I downloaded NCH tone generator and plan on running some tests tomorrow.

I built a push pull triode RF deck (812a's) to test triodes modulated by triodes but it does not work worth a crap, unlike the one I built years ago...
I cant get it to neutralize, maybe its the hand made coils...

When using an SDR receiver with a panadapter, you can see the modulated noise floor (MNF new test parameter?) bouncing around the signal.  I've noticed this with both AM and SSB signals. As Steve mentioned, if a station is 40 over 9 this phenomenon is easy to see.

I ran some tests, and will run some more later.
I had to change things to get enough signal into the sdr by tapping off the mod monitor feed.

4x150 rig:
-40 carrier, db below the single tone (not below the carrier)...

20% mod, -37 db
40% -30 3rd harmonic (2nd was -35)
60 -30 db
80 -28
100 -28
I noticed this transmitter does not hit the baseline, no matter how much audio I dump into it.


2X 813 rig (4cx250b modulator), -40db carrier.
From the tone, not the carrier...
20% modulation -30 db
40% modulation -35 db
60% -20
80% -20
100% -20

On both transmitters, I noticed a higher screen voltage helps improve things, as does more grid drive.

Next up is the 3X4D32.
I also ran the 2 tone test, which seemed to look better then the single tone, more but lower harmonics...

 

Brett,

Good job.

Your 4X-150 rig is very clean at all power levels. To stay below 4% THD for a plate modulated tube rig is an accomplishement.

The 813 rig is very clean at < 60% modulation and just average above 60% modulation.  

If you run it that cleanly on the air, you should be FB.


The two-tone should show additional mixing frequencies that tell their own story.  Use the F1-F2, etc., IMD formula to identify them.  There will be some X2, X3, etc., harmonics mixed in too.

T

Dan/ W1DAN and Peter/ W1ZZZ requested some THD tests today for their new K7DYY class D AM rig.

Dan ran several full power modulation levels from 20% to 100% modulation.

Posted below are the 80% and 100% modulation SDR spectral graphs.  I received them over the air in the afternoon when the band was reasonably stable for tests.

I thought the rig was very clean at 100% negative modulation, being at about -26dB down from fundamental tone to the second harmonic peak. (-30 dB on the lower side)   The THD difference between 80% mod and 100% mod was very little, which is good. The THD % (for second harmonic test) was as good as 3% THD at 100% modulation.  The rig stays well behaved at higher power levels. The signal was not strong enough to see the weaker third harmonic - but that is a good sign. I was impressed.

The overall THD is below 5% at worst - and better at lower modulation levels. Nice clean rig for THD.

T

Notice how the main audio tone peaks stand high and proud above the crud:

I thought they were lousy, but i guess in the big picture, its not bad.
I have no idea how clean the lap top is as a signal generator, or the other equipment I have in line, my symetrix audio amp is under powered at 20 watts (but its small).

I did the 3x4D32 rig, -40 carrier, harmonics DB down from tone...uses the same modulator as the 4x150 rig, 811A's at 1200 volts.

20% -32
40% -28
60% -28
80% -25
100%-23

2 tone was -25db for all modulation levels.
On the scope, with one tone, as I get close to 100% neg mod the waveform looks odd, I might remove the NCL system and re run the test.

Here's a THD test of Dan / W1DAN's class E PDM rig.

Video made by Rob, W1AEX:

http://www.youtube.com/watch?v=Vueob5SV5us&feature=youtu.be


His rig looks clean at about < 3%  THD below 60% modulation - and up to about 5.5% THD at 100% modulation.
The main audio tones were only about 30 dB above the noise, so I could not see the higher harmonics or the lower level stuff when modulating below 60%.

Overall, it looked good.

T

Rob and Tom:

Thank you so much for testing with me, recording and posting this! It seems Rob can see a bit more than at Tom's QTH. Nice and steady sigs at your rx, and smooth base noise.

So I see my transmitter is clean through 80% modulation. At 100% (and I guessed this level using a scope), I see the 2nd, 3rd and 4th harmonics of the 500hz sinewave, starting at about 20db below the fundamental tone. Wonder if I was over 100% negative?

It would be interesting to see your (or someone's) transmitter on your system with an RF tap off of your transmitter. Here we would not have to fight atmospheric noise.

Good info, and thanks again Tom and Rob. Good education.

73,
Dan
W1DAN

Dan

I did some work on those pics.  Take a look at my PDF upload.  At 60% the 2nd and 2rd order harmonics are barely out of the noise.  Now do a document page down (not the page down key) you will see the 100% perfectly aligned where the 60% image was - you can toggle back and forth.  The rise of the 2nd and 3rd does not follow at all.  I think this may prove your point that you were, as you suggest, going past the 100% modulation cutoff point.  That would create the rise in those harmonic components as shown.  Too bad I wasn't there with my samler, load and SA.  Bet you could have witnessed the sudden rise in those harmonic components and used that as a "calibration" of 100% modulation.

Thoughts?

Al

Hi Al,

I see what you're saying about the higher harmonics not following..  lot's of noise in there.

We should run the test another time when the signals are big and we can see way down into the weaker harmonics.

With high level modulated rigs, I usually see accelerated THD increases as 90 to 100% is approached.

T

It's difficult to say Al but it does look like things changed rapidly between 80% (~35 dB) and 100% (~25 dB). At 80% I can just see the 2nd harmonic poking up out of the noise. It would be interesting to see that again Dan to make sure you hadn't hit the baseline. The three attached pics show 60%, 80%, and 100% as I was receiving them.

Dan, I think Tom's location is around 10 dB quieter than mine (-122 dB vs. -112 dB) but your signal was about -15 dBm stronger here so it was my lucky day to see a bigger signal from you! Your E rig sounds great by the way.

Tom, that 100 foot high dipole is a killer. As I was shutting down last night, you were making a transmission and I heard another signal come up on frequency with the usual 75 meter challenge of, "Am I strapping this guy?" It sounded fairly PW under your signal but what was astonishing is that it was Steve QIX! You crushed him like a little bug... This is where Steve will jump in and say he was running QRP...   :O)

73,

Rob

Heck with QRP - that was the driver only into the dummy load   ;D ;D ;D ;D ;D ;D ;D ;D ;D ;D ;D ;D ;D

You know Steve... it did have the sound of a couple of class E MPF-102 FETs straining into a rubber duckie!    :O)