The AM Forum

** Update:  Another thread showing class E  AM rig IMD tests:
http://amfone.net/Amforum/index.php?topic=33555.msg259656#msg259656


It's been a long haul...  I've been slaving on a class A linear amplifer project for several months now.  It has been a difficult process, but I'm happy to say the system is even cleaner than I had hoped.

It starts with a low level  FT-1000D (4 mW)  output > into a (1 watt) ZHL-3A lab amplifier > into a homebrew 4CX-350FJ (5 watts) Mr. Clean >  into an 8877 (50 watts), > into a pair of 8877's (1500 watts).

Today, after a lot of tweaking, using a two-tone test, I am seeing -50dB to -55dB 3rd order IMD products.  Using voice LSB at 2.8 KHz bandwidth, I see side crud products down as much as -65 dB within 3khz of center frequency. This is outstanding and blows me away.  Expand the the pics and count the 5dB vertical grids to get an idea of signal to crud (splatter).

These numbers are hard to believe, but the secret is that the complete amplifier chain up until the final stage is running in class A. This is the way to break out above the -40dB 3rd ceiling.

Pictures 1 and 2 are 2-tone tests and pic #3 is my voice doing an atomic Yallo on 3775.  Each grid is 5 dB.  Notice the side frequencies and lack of side crud, especially in pic #3 using voice.  Check out the lower frequency side of pic #3 where the side crud falls into the noise floor at 3772 KHz, 3KHz away from center freq.    This demo is when running about 1.5 KW into a dummy load.  

Each pre-amplifier in the chain is capable of ten times the power it is putting out, thus the extreme cleanliness.

I have roller inductors in both the FJ and 8877 rigs to adjust tank Q.  The trick to getting the last -10Db of good IMD is being able to tune each stage perfectly to feed the next. And it is not necessarily 50 ohms J0 for best IMD.

The noise floor is limited to -75dB because of 1000D synthesizer noise. Never thought that wud ever be a concern to testing... :-)

I'd like to thank Frank/ WA1GFZ and Chuck / K1KW for their help fielding questions via email for this project. They both gave good advice.

As I told Frank, 25 years ago I would have given a left nut for an amplifer chain like this.  This, for me, is the attainment of a ham goal I've sought for a long time, but failed to achieve up to this point...  Yeah, I'm excited.. ;)


T

All pics at 1.5 KW out into a dummyload using a 2.8 KHz LSB signal.  This amplifer will be used on both SSB and AM.  BTW, I can flip a switch and drive this amplifer chain with the low level output of my HPSDR transceiver rather than the 1000D. This opens up a whole new world.

Tom....Way to go!  Your skirts are vertical.....So steep...Congrats...You are to be commended for your dedication toward making this work....
  TMC used 4 or 5 class "A" stages to end up cathode driving a 4CX 5000-A in the Navy FRT 39 (GPT10K) ...I don't think they achieved the results that you have...but I believe their goals  were  similar...
   I wait to see and hear your outfit in the near future...Pictures please...Perhaps we will get some East-West coast QSOs before the bands get too noisy..I'd love to see this TX on my SDR display......Thanks for sharing....Steve

Good Job Tom. The results are impressive.

Here is a way to capture your computer screen and save it or post it here. It's retentively simple and doesn't require any new software if your using a Windows machine. You can grab shots like those you posted and they will look great.

http://windows.microsoft.com/en-us/windows-vista/take-a-screen-capture-print-screen (http://windows.microsoft.com/en-us/windows-vista/take-a-screen-capture-print-screen)

Mike

Wow, it's so slab sided it's like a block of marble.  Congratulations!

Great job man.  Very clean!

C

Very nice.

You are now the standard candle to which we will all aspire.  ..puns on top of puns for the King !

 8)

Heheheh.... thanks for the back slaps, guys.

I look at those IMD pics and still can't believe what I'm seeing. Like a dream come true.  For many, many years it was disappointment after disappointment. I tried every tube and config I cud think of.   Every time the IMD was worse than  -35dB... especially when QRO.

I think having an SDR with spec anayzer made all the difference to see what was really going on and how each  step affected the overall chain.

The funny part is only a week ago I almost gave up on the whole project.  The drivers were putting out beautiful IMD, but the finals were stuck at -25 to -30 Db 3rd.  WTF??  I tried everything to solve it and finally figgered I had some bad pulls since they were used out of an MRI machine. (bad tube transfer curves)  

In final desperation I added a bigger cathode choke and more bypassing. The IMD got worse to -20 dB 3rd!  I was yallowfied.  Turns out I had a total of 20 ohms in the cathode in the form of big pie-wound BC type RF chokes. This was killing the bias regulation. I ripped out the chokes and used a simple wire thru the ferrite of the bifilar choke. Now it was a standard trifilar.  The IMD dropped like a rock into the -55 dB 3rd area as you see in the pics.  And I was one day before giving up and going with a different config...

Another thang I found is the input tuning to the final 8877's is VERY critical!  I can swing IMD up and down -10 to -15 dB by resonance or non-resonance. In the case of 75M, peak power output corresponds with best IMD, which is good. Without that input tuning the amp wud be back in the low 40's.  It's NOT a simple matter of matching 50 ohms J0.

There's so much more....  I added a 600 uf cap between the cathode choke cold side to the HV power supply capacitor negative. This hopefully brings the total dynamic cathode resistance down to less than an ohm. At 20 ohms it was swinging the bias all over the place.


I used as high a tank Q as I could get away with. Even now, at 1500w, the 3/8" copper tubing coil (only 7 turns) gets too hot to hold after a minute of RF carrier. (Circulating RF current is Q   X   plate current)  The tank Q is up around 35 to 40.  Same Q for the 8877 driver and 4CX-350FJ.  It seems to help bring down the IMD further.

Tank loading (C2) is also important. I find the sweet spot is when there is ample power output AND the grid current JUST starts to flow. No grid current is important to cleanliness for the drivers and especially for the finals. That's what class A1 means in addition to no fluctuation in plate current.

I am using a common 4KV HV supply for the 8877 driver and finals. On Chuck's suggestion, I added a 20H choke from the HV supply to the driver - and a 50 uf filter cap on the 8877 driver side. This reduces any HV ripple caused by the final into the driver HV. The voice syllabic rate ripple will be filtered out giving the 8877 driver half a chance to perform with a stiff HV supply.

There's more... maybe later.

Next, optimize the tuning / parameters on the other bands and see what happens. Then into the antennas. Each antenna is a different load  and requires pre-sets for optimization.  Being sloppy in the tuning can sacrifice -10dB or more of 3rd IMD.

Mike, thanks for the screen shot info. I've done them before but was too lazy this time to remember how to do it.  The next pics will be screen captures.

T

Some more notes for those curious engineers...

It may not be obvious, but much of this chain is devoted to replacing the "dirty" ricebox driver. Normally the average ricebox is lucky to be -30 to -34 dB 3rd order.  The IMD output of an amplifier is always worse than the driver itself. For example a -50db  1 KW amplifier driven by a -30dB ricebox will always be in the high -20's dB 3rd and no better.

The driver should always be at least -5 dB cleaner than the final - and preferably -10 dB cleaner for minimal degradation.

For years I was using the FT-1000D as a driver at 50  to 200 watts. The IMD sucked, so how could I ever expect the final amplifier to be any better?

About a year ago I discovered that the 1000D at 10 watts was about -45 dB 3rd and used a 3-500Z amp as a class A  IPA. This helped.  

Later I found the 1000D low level at 4mW was super clean at about -75dB 3rd.  I drove this into the ZHL-3A which is about -68dB 3rd at 100 mW.  The 4CX-350FJ amplifier is about -63dB 3rd when run at 10 w.  So here we are with a 10W driver at about -63dB 3rd.

Driving an 8877 in class A to 50 watts output is about -59 dB  3rd.  

So, we end up with a ~ 50 watt clean signal approaching  -60dB 3rd to drive whatever we wish.... whew....


I know that Frank has been diligently working on his solid state amplifers to push them above -40db 3rd and is making progress. I know he will get some good results in class A.   Though, I've yet to find a HF SS amplifier out there that does better than -40db 3rd when running 50 watts or more.  They exist in the feedforward, pre-distortion software and possibly military world, but I don't see any ham versions or inexpensive ones to buy.

So, I took the hard road using tubes....

More later -

T

I am curious to the power input figures for fun.  Have you ever calculated it?

C

.

Clark,

Interesting question. Well, we won't win any green awards.

For the drivers, I just calculated the efficiency at between 10% to 20% depending on how hard the drivers are pushed...   ;D

This is realistic for class A considering the heavy stage loading and super Hi-Q I am using.  

The plate current stays steady from idle to full signal as well as no grid current flowing.

It's the only way to get into the -50 to -55dB  3rd area for a "brute force" amplifier.  ( No NFB or  predistortion software, etc)

The final amplifier is much more efficient, of course. It idles slightly higher than Eimac recommended specs and is run at only 1/3 of its rated power to achieve its cleanliness. Super high Q tank and input circuits

I'm using the sliding bias circuit on all amps that drops the idle power to about 1/4 during voice pauses, so that helps.

Right now I'm tuning it up for 40M.  So far I've found the proper settings all the way to the 8877 driver output. It shows about the same performance as 75M so far.  Next the final amp and full chain optimization.  I write this all down on a cheat sheet for fast pre-sets later on.

T

Thats not to bad really.. I guess the only real drawback is all the tuning.  But, It is purpose built.  I ran some tests on my rig and simple amp tonight.  I dont have the two tone. I just modulated the rig like you did in the last shot.  Your right, tuning really effects the IMD as does Power output of the rig.

C

What kind of IMD numbers where you seeing when doing the voice tests?

There's a free tone generator to download that will do 2-tones.   Put it on your computer and run an audio cable to the rig.
http://www.dxzone.com/dx17728/sigjenny.html

You will find a good 2 tone test is very unforgiving and takes no prisoners. Kinda depressing at times. Whereas voice transmission looks much better and actually is in the real whirl. We are lucky in that respect.

"Purpose built"  - yep that's a good way to describe it. I will probably park it on certain frequencies for a week at a time. It will take a good three minutes to change bands until I get real familiar with the cheat sheets.


Well, I got the system playing on 40M into a dummy load, though I need to make some C2 mods in the 8877 driver. There is too much fixed capacitance used on 75M and I need to add a relay to toggle it in and out. Always something.

Right now it's not as clean as 75M by -10dB, but I'll get it.

Later -

T

Thanks for the link!

-47 DB for the radio and with the amp on 40.  But its not scientific.  Its just be watching an SDR while using voice.  The rig has a computer on it now. Just need to dig for the cable to go to the input. Then I will run the two tone with and without the amp.  Radio should do -45 easy but the ole SB220 might let me down :(  Eitherway. Its fun to test and to learn about this stuff.

Good.

Yep, get that 2 tone set up and you will have a rock solid standard to compare apples to apples in the shack and with th rest of the whirl.

What kind of radio do you have that will do -45dB 3rd at full power?  That wud be quite good indeed.

The 3-500Z's IMD will depend greatly on where it is idled, how hard you run it and how it's tuned and loaded. Also how good its HV regulation is under load as well as the 240 AC regulation.  I imagine the input tuning is fixed, so that will not help to tweak.  If you get real serious about it you could try a small tuner in between the rig and amp as an experiment to tweak the IMD.

Your radio may not like the varying amplifier input impedance. It pays to sample the driver for IMD when the whole system is running together and see if there is a bottleneck. Maybe custom interstage tuning can help the situation..  I'm finding that good IMD numbers is doing a lot of little things right... :-)

T

Great work Tom but not exactly built for rapid QSY!  Can you even go from 3770 to 3885 without losing a lot of IMD?

Tnx, Carl.

Well, I thought it would be very sharp tuning too.  But the Q, being between 30 to 40, is only 3-4 times sharper than a standard amplifier with a Q of 12.  

I just tried moving 20 kHz on 40M and saw no noticable difference in IMD or in the power peak.  I'll try some bigger excusions later.

I think a Q up around 100 or more is when we would start to see extreme bandwidth QSY problems, akin to peaking a Hi-Q preselector in a receiver.

T

Update:

I just did a sweep test on 40M.   Into the dummyload (as I always use for tests)  I swung the VFO from 7150 up to 7200 and down to 7100.   This was at 1.5 KW output.   The power output changed about 10% as expected.  

However, the 3rd thru 7th IMD peaks simply tracked the main power tones and stayed rock solid in relative amplitude.  So it appears the IMD tweaking, once set up, is not going to be a bother.

One point:  At the present time the Q on the driver or finals is not as high as on 75M, maybe 20 - 30. Still trying to find the best optimization points.  Though I now have the 40M IMD performance within 3-4 dB of the 75M settings.  Getting there.

Update:  Just got there.  40M 3rd IMD now sitting at about -55dB !!

Tried an antenna on 40M for the first time - slightly different load.  The IMD held exactly the same as the dummy load. This is a good sign.

T

What are you using to couple your tx signal to the SDR RX/ analyzer?

Hi Steve,

There's several ways to do it, but, I use a two-tier sampling method.  In an aluminum box I have a toroid wound with 40 turns. It is slipped over the inner conductor of the coax inside the box.  The 40 T winding connects to a coax which runs to a relay box that feeds the SDR reciever antenna input.   I use an additional  60 dB pad there consisting of   50 ohms to ground, 5K in series and 50 ohms to ground. (Or something like that - use an online pad calculator)    This connects to the SDR ant input.


For TX / RX transfer, in the PTT RX position, the relay connects the normal antenna to the SDR.  In the PTT TX position, the relay connects to the 60 dB pad.  The HPSDR is in DUPLEX mode, so I see the actual transmitted signal coming out of the sampled final amplifier.

Be careful with exposed leads and connections in both boxes. Just a small snub can pick up lots of signal and make the attenuator useless.

Use good quality coax too - that doesn't leak due to poor shield.   
T

Cant get Sig Geny to run on windows 7 64bit or windows 8 64bit. Looking for a new way to do the two tone.  I was using the Clean RF sample box with an attenuator that dennis gave me a few years ago.. I could adjust midrange to the SDR.

C

Hmmm... maybe they need to update it.

Do a Google on 2-tone or audio signal generator software  and see if there are more out there. Shud be.

I ended up buying an Elecraft 2T, 2 tone kit board.  It works very FB.  I have it on a switch that can choose my voice audio, the pulse pecker or the 2 tone for fast use.

BTW, the pecker is a great way to test peak power with 10% load on the system. Very little heat.  But the 2-tone is a power hog, so be quick with the tests.

** Two-tone tests can be used on AM just as well as SSB.   The waveform will look somewhat different, but it will still show meaningful dB improvements and comparisons.  I plan to do some tests on my class E  AM rig for IMD, when I get caught up.  There may be some ways to improve on that through optimum loading, drive, better HV regulation, etc - who knows?



T

Try this one:

http://www.dr-jordan-design.de/Download/SigGen.zip

For two tone, run it twice...works here in W7...but 32 bit machine..YMMV

Jim
WD5JKO

Got 40M working.    I did a screen capture and expanded the screen to show farther out. This shows the noise floor a little better.   All pics at 1.5 KW output.

The first shot is a 2-tone test.   I believe I can tune the 5th to 7th order peaks down a little farther when I get back to it.


The second and third are VOICE yallos.  

** For some fun, start at the top of the peak in picture #3.  Count down in  5dB grid lines until you hit the crud level.   (Elevator going down!)  This is a perfect example of how friendly an amplifier can be to the average human voice vs: the harsh reality of a two tone test.  For this we are all fortunate.  Though, if we try, there ARE some voice noises that will produce wider crud.


I'm moving on to 20M now.

T
 

40 Meters:
 

The forth is the pulse pecker. This is a nasty pulse rich in harmonics that is about 10% duty cycle at 1.5KW peak.

The fifth is a two-tone using a narrower screen bandwidth.  I believe we are seeing the slope of the 1000D synthesizer noise, but not sure until I runs some different tests.  It may just be normal low level signal crud.

T

Amazing. Looks like the output of a signal generator it's so clean!

yeah Steve .... I believe you could call Tom's rig a signal generator .... nice work Mr. Vu !

Thanks for the kudos, guys.  It really makes it all worthwhile.  Other than using the project, half the fun is showing the results off to your peers. Who else would give a crap about it anyway, the dog or your XYL?  :-)


Last night I got half way thru the 20M tuneup.  The 4CX-350J had a small glitch that required re-neutralizing it. It runs at 30 dB of stage gain.   Finally, the output of the 8877 driver showed good IMD, like the other bands.
Other than that, today I will try the whole chain on 20M. And go from there.


All of the drivers also work on 160M, but I think the final amp (pair of 8877's) need some mods to cover 160M.


I also changed the final's input circuit from a L network to a pi-network. Seems to work better that way and give me more versatility to compromise loading  vs: best IMD.

It really a juggling act now, but I hope to carve these settings into stone and easily go from band to band later on.


T

This 1.5 KW "yallo" voice print from 40M still blows me away as I look at it this morning. I still wonder if the slopes down -70dB (starting at -90 dB on the graph) are from the 1000D's synthesizer noise -  as the real signal drops into the noise floor.  :

Tom,  Which lower marker are you using?   The little right side pip at about 91db or the "mass" below that around 80?  I have the same thing here.   Not sure which point to use. 

Maybe you are using the left slab down to the first point?

I got some work to do here with my test setup.. I am seeing more signal on the left side harmonic than the right... That makes no sense to me right now...  It should be close to equal on both sides??

C

Clark,

I usually do not see a perfectly symetrical SSB signal. Remember that we are attenuating the upper side on LSB and it shows.. the signal is centered on 7135 and it drops like a rock above 7135, but it extends down ~ 3 KHz into the lower side.  So some of this non-symmetry is the ssb filter - and could also show poor opposite sideband suppression. It depends how far down it is and what the ssb driver TX specs are..

In a 2-tone test, we use the first significant peak after the signal drops off. This is 3rd order. This freq can be easily calculated using the tone freqs.  The next peak is 5th order, etc.   Tuning can affect the symmetry of the tones too. I often see the upper side tone vary from the lower side by 5 dB, especially the 3rd order.  The two main tones should alway be set up equally, however.

For voice tests, we just eyeball it and pretend we are listening up or down away from center freq and hear the crud down so many dB. The 2-tone is scientific while the voice test is more subjective, but still a real inducation.

We want the amplifier output signal to be as close to the original input as possible. As a check, look at the input signal and see how close it looks to the output.

Also, some cud be artifacts or "humps" as Frank calls it, generated by who knows what in the test setup - overload, signal leakage, etc.

The point is to get a reasonable comparison and baseline so you can make improvements and know when you are making headway.  We will never get perfect lab-type results, but we don't really need it at the extreme low levels near the noise floor.

As long as you are not overloading the SDR's ADC, you should get very useable graphs.

Post some of your results once you get a handle on it via screen captures.


****  BTW, with a 2-tone test an easy way to indentify the 3rd, 5th, etc and weaker tones is to intentionally overdrive the amplifier. Those higher order tones will jump right out.  The higher order ones can sometimes hide, but not when overdriven.

Voice audio analysis will not be as clear since your voice is always changing.  Also, using a white noise or pulse pecker into the audio can help to standardize your subjective "voice" results for comparison.

T

I am getting false results here.  Here are some pictures of the radio alone at 100 watts.  It shows peak at -38 to -40 DB.  The left shelf is -105 DB.  This is not possible.  The rig is on 17 meters.  I just worked TX5K on 5 bands and 17 was the last one I used so I left it there. Rig is into bird 500 watt dummy load.

I dont understand what I am doing wrong.  The rig is rated at -45 to -55 depending on whos numbers you want to go with. But still, This is not possible at 65..  I cannot trust the SDR in tests like this.  I need to get Spec AN running in shack to confirm.  The data seems good but the PEAK is way to high on the screen. So maybe SDR has flaw in software. There is no way radio is a full 10 to 20 DB better than the published and private tests. 

This is the latest board (they had six major revisions over the years) which has not been tested anywhere, with the latest software which has not been tested. However, I think its a false test.

C

Is it possible you are keying the SDR and seeing it's own low level internal TX signal? Or is this a receiver only?

Is this a hard sample from the ricebox output back into the SDR receiver?


The voice test is hard to tell until you get a real 2 tone generator going.


The acid test is to use a regular second receiver and use a 6" stub wire for an antenna off the back.  Then adjust the stub length until your 100 watt signal into a dummy load is S9 +40.  Then tune the receiver and see how far down the side crud level is 3, 4, 5 khz away from center. You will hear the raspy noise of IMD once tuned away from the real signal. The s meter may not be perfect, but you will get a rough idea how much drop off there is. Then you can look again at the test set up and figgger out what's going on.

T

 

The SDR has no TX.   I will try to use a piece of wire for an antenna next.  The SDR is hooked to a sample box I use for an output to my O-scope. Its possible its just to much signal period.

Thanks

C

Use a second RX, like a tube RX for the ant stub test so you can eliminate the SDR set up and software possibility problem.

Actually too much signal will usually give you poorer IMD results from RX overload, etc.  

Don't feel bad - it took me a long time to get MOST of the bugs out of my own high power IMD testing set up. It is very sensitive to stray and overload. Getting down to the noise floor with reasoanably accurate results is not easy. One step at a time.

T

20 Meter AM


Here's some IMD shots of the amplifer chain running AM on 20M using my voice.   A combination of audio roll off and the lack of side splatter makes for sharp skirts.  Using 1500 watts output for all pics.

I show both +- 4 KC  and +- 5KC  audio examples, which is where I intend to run it most of the time.  

The first two shots are using the FT-1000D.


The third shot uses the HPSDR xcvr. Notice how sharp the skirts roll off with digital filtering. Of course, the amplifier must be clean to support that kind of roll off without side crud.

The third pic shows an "SSSS" which fills out the AM bandwidth.


I meant to run thru some  AM 2-tone tests, but forgot.  Next time.


T

And, some 1500 watt SSB  USB IMD pics from 20M.  

#1 is 2-tone and #2 is voice.
  

Now on to 160M and then printed cheat sheets to get fast repeatability when changing bands.


T

Congraturations Mr Tommy Vu, you have vely cleen USB siganel.  Yur low frequencie energy is vely high.  Yu must have big BAs.

Open S.P.S.T. switch my IMD drops below grass.

Tom, it's tough uttering highs and lows at the same time... ;D

I'm gonna build up a white noise generator. That oughta be a good way to fill out the audio bandwidth for tests.
(or as Tron calls it, a "Brown noise generator")


Dave, I figgered your comment would be, "Now that you got this thing working, are you finally gonna get on the air?"  Or, "It doesn't matter how clean it is if ya never hook up the antenna and push the PTT switch... "  

T

Tom, The Dr. Jordon PC based Audio Generator has the "White Noise" feature enabled in the free version. I linked to this earlier..Just in case you want to try a PC based audio source..

Jim
WD5JKO

Thanks for the info, Jim.

I think I will stay with a board version so I can put in on the same switch as my pulse pecker and 2 tone generators. This way there's a fast choice of any three.

Here's a simple two transistor version I will build.

T


White Noise Generator:

I cant get good results with the SDR.  Its showing me a false positive.  Not sure why. Its not possible for my rig and 2x3-500 amp to show -65 db.  Should be -35 to -45.  I have yet to figure out why. 

One thing I would like you to do Tom is to run the test but go to the second harmonic Frequency on the SDR.  I did and the PIP was right there showing -43 db. That is believeable for my little amp and radio.

C

I usually save that for internet warriors  ::)

When the wx warms up you might not want all that dummy load heat around....

Clark,

First step is to tune yourself in on a second tube receiver and determine how far down the crud is. Look at my post a few back.  This will ball park it.

Then get a real 2-tone generator going to get a reference that is standardized.

I'll take a look at my 2-tone results on the second harmonic today. I would expect it to be a duplicate of the fundamental, though way down in amplitude and possibly distorted from low level test set-up artifacts.

* BTW, looking again at your picture... is there a chance that the grid divisions are 2 dB per rather than 5 Db per?  If so, I see about -42 dB to the crud level, which is what you are expecting.  You don't have the scale numbers showing in the pic, so I can't tell.  Though you did say the lower shelf is -105 dB, so maybe it is 5 dB per.


Dave, if I can just get out of this building phase, you might hear more of me than you can take over the next few months...  ;D

T

The divisions are normal.   105 for the crud and -40 for the peak.  I cant get good results with the SDR this way.


C

   Clark,   Maybe you have too much attenuation at the SDR receiver input. If you were to get the peak signal up 50-60 db above the noise, then you'd be good so long as the SDR system has the dynamic range. If the dynamic range isn't there, then the SDR will create its own IM crud. I believe you own a spectrum analyzer also;if it has a resolution bandwidth down to 300 hz or lower, then that would be an option as well. What Tom is doing is tough, and that makes the test setup even tougher to get useful and believable data from....

Jim
WD5JKO

It's akin to tuning a Yagi antenna for -45 dB front to back.  Any wind or weather will detune it back to -25dB.

Holding -50dB 3rd or better in a "brute-force" amplifier without NFB or predistortion is difficult.  All the tuning has to be right on.  In fact when I switch bands and then go back to preset band settings, I'm finding the physical markings are not good enuff and my IMD is off by -10 dB. I have to put thru a 2-tone and re-optimize for the full -50 to -55 dB 3rd.  I'm getting better at it though.


The strange thing about Clark's issue is that usually the test setup gives poorer IMD results due to overload, artifacts, etc. But he is seeing results that are -60 dB better than what is expected, so I can understand  why he doesn't trust what he is seeing.


Well, I finished with the 75 - 20M  IMD band preset settings and now moving on to other things.  Built a new 8877 final input tuning system yesterday that switches between pi-network and T.  I'm finding I get the best IMD on 75M with a pi input - but best on 20, 40M using a T.  It's quite dramatic.  At least it gives me more versatility to handle all situations..   This is like tuning a dragster... ;)

T

After all this work, I wondered to myself... "Why you go thru so much trouble Mr. Vu?    Did you waste plecious time? Are you clazy?"


I decided to see if there were any real improvements from the old driver system to the new...

So I hooked up the old FT-1000D driver and ran it at 100 watts as a stand-alone for an IMD test. This is a 200 w rig. Pic #1. At 100 w out, the IMD 3rd is barely -28 dB 3rd.

Then I used the 1000D at 15 watts to directly drive the 8877 amplifer chain.   This is pic #2.      When used as a driver at about 15 watts, it is slightly better at about -31 dB 3rd.  IMD generally sucks, but this is about average for a ricebox.  This is how I used to run my rig...


The new setup:

The 3rd pic is using the new driver setup: The FT-1000D low level 4 mW  > Mini Circuits 1 watt amp >  4CX-350FJ driver into the 8877 amp system.  Loaded up to 1500 watts. The difference is stunning.   :o   The IMD is about -22dB better.... to -55 dB 3rd.  That is like night and day when it comes to splatter.

It goes to show if we have a clean final amplifier, it pays to be careful in our driver (ricebox) selection - or consider alternatives.


 Tom Vu feel better now... ;)

T

Yes, My tests are showing better results than what I can believe and as a result, I just dont trust SDR in this kind of test.  Its showing what I believe is 10 to 15DB false positive. Unless this latest board is really that clean.  But I need to use the Spec AN to know for sure.

C

The display on the Flex SDR is amazing! The dynamic range exceeds what many of the analyzers that are out there in the industry today. Having used various spectrum and sideband analyzers over the years find it almost too good to be true the quality of the display that the SDR produces. Maybe it’s just like when most people migrated to DVM instead of analog meters, where people use to round numbers off with analog with digital they can go out one or two decimal points? And the SDR radio and software now makes this possible. Have to say however you do this by SDR or analyzer that’s one clean amp you got there for that power level.

Hi Ray,

Thanks.

I'm using an HPSDR radio for spec display.   These are semi-kit boards using a Mercury and Penny to produce a transmitter/ receiver combo that covers 160 - 6M. It uses Flex software, so has the same spectrum analyzer layout.  I have it in the "duplex" mode so that I can sample an external signal and feed it into the receiver while transmitting.  Most SDR's (Flex) are set up to monitor their own internal, low level signal, which always looks perfect... :-)  Using a second Flex RX or a duplex option is required to see the high level final TX signal.

Later, I may use the low level signal from the HPSDR to drive this new amplifer chain that I've built up and have been testing.

The HPSDR spectrum display noise floor is quite deep when running 100 watts or so, but when up at 1500 watts, I can see some noise coming in at the floor.  I think it is standard leakage thru coax, etc. It doesn't hurt the readings I am interested in since -70 dB is plenty of dynamic range.

The good thing is if the IMD is poor, it certainly shows up as it did in pic #1 of the barefoot FT-1000D ricebox.  I used to think the 1000D was a pretty clean radio when running at reduced power, but it's no different than the average ricebox.  The technology is available to produce riceboxes that are -40dB 3rd and much higher, but there's maybe one or two available that cost over $6K.   I don't understand it.

As you said, these software spectrum analyzers are the best tool we've had as hams in a long time.  I appreciate it and it gets a lot of use, now that the test set up is reliable.


I plan to run some IMD tests on my 24 pill class E  AM rig soon. That should be very interesting to see.  Stay tuned.

T

wow all these IMD tests have really got me excited. i recently got an FFT module for my oscilloscope and have been messing around with two tone tests on home made linear amps. i'm not really sure if i am doing the tests correctly but i'm attaching a photo of my analyzer output. i inserted two 50mW signals (roughly 1.5mc and 1.6mc) into the amplifier via a home made 6db hybrid combiner and got -40dB 3rd order imd. i arbitrarily chose those frequencies, how close together should i have the test signals in a proper imd testing setup?

Pretty cool, Solomon!

You might experiment with the tone spacing to see.  But spacing is not at all critical, as long as it's within the bandwidth of the amplifier's tuned output. (Q)  (For tube amps)   The IMD peaks should not change based on the frequencies unless they get too far away out of the bandpass.  Same for a simple 2-tone audio signal. As long as it's within the transmitter's audio bandpass and one of the main peaks do not shrink, it is a valid test.

However, for broadband solid state amps, if the tones are too wide spaced, we start to run into changes in IMD that may appear across the wider spectrum of the amp and dilute our test data for one particular band area.


What kind of linear is it and what power is it running at?   I'd be curious to see full power, 1/2 power and 1/4 power graphs.

Yes, this is exciting stuff.

I plan to test my class E rig later today.  Heck, anyone could test their DX-100, Ranger, Apache, homebrew 4-1000A X 833A's, broadcash rig, etc., using a 2-tone IMD test.  It's not limited to ssb or linear amps.  It would be fun to see some of these popular AM rigs up on the table.

Heck, if I had a Ranger, etc., I'd have it on the IMD table and try all kinds of stuff... drive levels, different screen resistors, voltages, loading, input tuning, bias, modulator resting current, NFB, better regulation, etc... Try anything to improve its IMD data.

T  

thanks tom.

its a pair of irf510s in push/pull. i think the amp was putting out 1 or 2 watts when i ran that test, i forget. i tried various amounts of bias (between 20mA and 60mA per FET) but didn't notice any significant changes in IMD.

the largest power supply i have available is 30V/1.5A so i cant do anything really exciting yet. but i have all the parts, other then the enclosure, to assemble a 30V/3A supply. when i get that supply running i will do a more methodical experiment with various drive levels.