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Author Topic: Real IMD Results for the New RF Linear Amplifier Chain - Pictures  (Read 32876 times)
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K1JJ
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"Let's go kayaking, Tommy!" - Yaz


« Reply #50 on: March 05, 2013, 11:57:08 PM »

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
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Use an "AM Courtesy Filter" to limit transmit audio bandwidth  +-4.5 KHz, +-6.0 KHz or +-8.0 KHz when needed.  Easily done in DSP.

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There's nothing like an old dog.
ssbothwell KJ6RSG
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« Reply #51 on: March 06, 2013, 12:14:41 PM »

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?


* P1030769.JPG (2309.61 KB, 2816x2112 - viewed 503 times.)
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K1JJ
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« Reply #52 on: March 06, 2013, 12:32:29 PM »

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  
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Use an "AM Courtesy Filter" to limit transmit audio bandwidth  +-4.5 KHz, +-6.0 KHz or +-8.0 KHz when needed.  Easily done in DSP.

Wise Words : "I'm as old as I've ever been... and I'm as young as I'll ever be."

There's nothing like an old dog.
ssbothwell KJ6RSG
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« Reply #53 on: March 06, 2013, 12:42:22 PM »

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.
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