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PDM Class E IMD AM Tests




 
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Mike/W8BAC
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« Reply #75 on: March 12, 2013, 07:54:39 PM »

Quote
Typical is only -14 to -17 DB on two tones. It truly is one of the worst.  Search the net and read published and second hand reviews on the subject of disapointed hams. They buckshot the bands.  Take a listen sometime..

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!
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« Reply #76 on: March 12, 2013, 08:02:33 PM »

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

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« Reply #77 on: March 12, 2013, 10:08:07 PM »

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
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« Reply #78 on: March 12, 2013, 10:32:00 PM »

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.
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« Reply #79 on: March 12, 2013, 10:46:51 PM »

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.  Grin

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.   Cry
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« Reply #80 on: March 12, 2013, 10:51:04 PM »

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
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« Reply #81 on: March 12, 2013, 11:31:16 PM »

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.
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« Reply #82 on: March 13, 2013, 10:34:35 AM »

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 
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« Reply #83 on: March 13, 2013, 12:45:52 PM »

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
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« Reply #84 on: March 13, 2013, 09:44:16 PM »

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?
 
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« Reply #85 on: March 13, 2013, 11:24:55 PM »

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


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


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« Reply #86 on: March 15, 2013, 11:19:39 PM »

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...
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« Reply #87 on: March 16, 2013, 09:29:27 AM »


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.

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.
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« Reply #88 on: March 16, 2013, 06:16:00 PM »

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...

 
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« Reply #89 on: March 16, 2013, 07:04:33 PM »

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
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« Reply #90 on: March 16, 2013, 07:11:17 PM »

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:


* w1zzz 80%.png (223.2 KB, 1280x800 - viewed 822 times.)

* w1zzz w7dyy 100%.png (223.66 KB, 1280x800 - viewed 837 times.)
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« Reply #91 on: March 16, 2013, 08:23:44 PM »

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.





Brett,

Good job.

You 4X-150 rig is very clean at all power levels. To stay below 4% is quite 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
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« Reply #92 on: March 17, 2013, 05:01:43 PM »

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


* W1DAN 60%.png (230.77 KB, 1280x800 - viewed 827 times.)

* W1DAN -- 100% mod THD test.png (268.5 KB, 1280x800 - viewed 813 times.)
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« Reply #93 on: March 17, 2013, 09:48:15 PM »

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
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« Reply #94 on: March 17, 2013, 10:49:13 PM »

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

* W1DAN THD - 60% & 100%.pdf (136.7 KB - downloaded 363 times.)
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« Reply #95 on: March 17, 2013, 11:50:32 PM »

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
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« Reply #96 on: March 18, 2013, 10:51:56 AM »

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


* 60.jpg (79.13 KB, 1280x720 - viewed 836 times.)

* 80.jpg (79.56 KB, 1280x720 - viewed 794 times.)

* 100.jpg (80.22 KB, 1280x720 - viewed 806 times.)
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« Reply #97 on: March 18, 2013, 06:26:34 PM »

This is where Steve will jump in and say he was running QRP...

Heck with QRP - that was the driver only into the dummy load   Grin Grin Grin Grin Grin Grin Grin Grin Grin Grin Grin Grin Grin
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« Reply #98 on: March 19, 2013, 12:28:39 PM »

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

You know Steve... it did have the sound of a couple of class E MPF-102 FETs straining into a rubber duckie!    :O)
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One thing I'm certain of is that there is too much certainty in the world.
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