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Author Topic: Transceiver Intermodulation Distortion ( IMD )  (Read 1007 times)

VE7DQ

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Re: Transceiver Intermodulation Distortion ( IMD )
« Reply #15 on: September 20, 2022, 03:39:36 PM »

Hamhock75,

<quote> I plan to test the FLDX400 for IMD in the near future. The topology of amplifiers may favor tubes especially as power goes down. The push pull design has to deal with cross over distortion that does not happen in single ended tube designs <unquote>

That paragraph inferred that the '400 has push-pull output, but, of course, you are correct; it does not.  It would have been more appropriate to say "A push pull design has to..."

I rather enjoy these discussions, so carry on!
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KT4WO

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Re: Transceiver Intermodulation Distortion ( IMD )
« Reply #16 on: September 22, 2022, 07:10:07 AM »

And it's about time that the ARRL reviews started talking about this.

From the FT-891 review(sidebar), and I paraphrase -
"Please don't hook this radio to a amp or even a good antenna"
At least that is what I got...haha
That's probably the best we can get from the ARRL at this point.

I was looking at that rig for a dedicated HF packet station, I'm glad I
saw that review before they were back in stock!!
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N0GV

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Re: Transceiver Intermodulation Distortion ( IMD )
« Reply #17 on: October 07, 2022, 03:40:00 PM »

Transmitter IMD isn't just limited to the final stage. It can be generated at the modulator, at the upconverter/downconverter mixers and at any driver stage before the finals and after the the up/down sidebar filtering stage as well.

One thing in the new LDMOS amps that isn't being used as much as it should be is feedback for linearization. With gains of ~30dB putting in negative feedback to limit the gain to 15-20 dB you can really clean up a lot of harmonics and IMD. This helps the transient response as.well....

Grover
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HAMHOCK75

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Re: Transceiver Intermodulation Distortion ( IMD )
« Reply #18 on: January 19, 2023, 03:34:52 PM »

I had a chance to study the Yaesu FT1000MP Mark V amplifier recently. It is a standard push pull configuration with two FET's ( BLF147's ). The only change from class AB to class A is that the bias current for each device increases from 1A in class AB to 5A in class A.

That improves the IMD from -31 to -50 dB typically. This seems to suggest that crossover distortion is the main determinant of IMD.
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K6AER

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Re: Transceiver Intermodulation Distortion ( IMD )
« Reply #19 on: January 19, 2023, 07:10:16 PM »

The single improvement in IMD would be to have the radios shipped with lock-tight on the mic. gain shaft.
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