Sunday, July 6, 2008

Flex 5000 Driver Distortion vs. Final Distortion

Here's another test of TRANSMIT Audio. To listen to it, send an email to jca1955 "at" and ask for the 6 July 08 test file, and I'll send you the mp3 file.

This mp3 again contains two recordings, but this time the source of both recordings is the same 5K (and again, feeding into a second SDR-1K as RF-demodulator/recorder). One recording was made by demodulating the RF received from the 5K's output antenna connector (in other words, the output from the 5K's PA), while the other recording was made by demodulating the RF tapped from the coupling transformer between the Driver and Final stages of the 5k's PA.

In both cases, the 5K was operated at full power (100 watts) and fed into a dummy load. I sampled the Final output by attaching an RF "tap" (essentially a 100:1 voltage divider) between the 5K's antenna connector and my external dummy load. The tap's output was then fed to my "receive" SDR-1K via a big attenuator.

To sample the signal from the 5K's Driver stage (to the Final stage input), I added a single turn link to the transformer between these two stages which I then connected to a BNC. I then disconnected the coax that had been connected to the RF tap (the other end goes to the 1K) and connected it instead to this new BNC at the Driver output. I kept the same big attenuator that I had attached at the input to the 1K receiver. In this configuration signal levels (at the 1K) are essentially equivalent for both configurations when measured with the 5K in TUNE mode.

Again, I used my Heil PR-40 mic as the audio input, and I read the same phrase for each test. TX bandwidth was 100 - 5KHz.

The first recording is from the Driver stage output. The second is from the Final stage (i.e. taken from my external "RF tap" attached to the 5K antenna output). Notice how the Driver stage sounds clean and the output from the PA stage sounds worse? This points to the PA stage (the final two FETs) as being the main source of my Flex 5000 transmit distortion.


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I hear distortion in all 3 recordings. The distortion occurs in different places in each recording but in all cases the deterioration into distortion seems to get worse over time.

Jon K said...


I believe you've nailed it. The third recording is definitive. I don't suppose it's something easy to fix like the bias on the MOSFETs.

Jeff said...

Thanks to all!

Jon - I tried bias recalibrations (and experiments) with my first 5k, all to no avail. But it wouldn't hurt to do a recal with this new radio...

Blair said...

Jeff, I had problem with xmit audio on two separate 5k's. I sent first back, an early unit, for refund. Bought one another around December. I had xmit audio problems too. It seem like some reverb type of distortion would show up an only complete shutdown had any effect on it. I got rid of radio and went back to the 1K. No problem with it.