25 November 2013

Monitoring transverter output

Without a correspondant to try a QSO on the air, I had to test my 70 MHz transverter output with my own equipment.

First of all I used the RTLSDR dongle to record my output, so that I could easily play it back afterwards for a self-audio-quality check. Easily done, with the transverter transmitting into a dummy load in the shack.

Then I wanted to check for harmonics at 2x, 3x and so on: the RTLSDR tunes much higher than that. So I moved the output to the 70 MHz dipole and started calling CQ, while RTLSDR receiver was running on the computer screen. Look at what appeared:

That's the fundamental and two noticeable splatters about +/- 200 kHz away, about 30 dB below the center frequency. What the ...?!??

Checking at 140 MHz and 210 MHz the remaining signal was not too strong, spurs obviously following. If I transmitted in FM there was no splatter whatsoever, nowhere.

That's when I remembered an article by SM5BSZ about the (mis)use of ALC in the FT817 (search for "The abominable ALC"), which creates heavy splatters, especially at lowest power settings. While they are barely noticeable in the HF noise, they pop up in VHF and do annoy neighbouring stations!

So I checked the FT817 output at 144 MHz USB, 0.5 W output, and spurs were there. No surprise. My RTLSDR does not tune down to 29 MHz, my 4 m transverter IF output, but I bet the situation is not different.

Since you always have to doubt the bounty of your test equipment, and a 15 USD TV dongle should not be over estimated, I cross-checked with a true receiver: splatters are generated for real.

A quick check at 5 W output has shown that the FT817 doesn't behave acceptably better. One solution, that would improve my '817 in any case, is to change the ALC timing with a hardware mod. Otherwise I will have to drive the transverter with the IC706MKiiG, but SM5BSZ pages contain a warning about a full-power spike when PTT is pressed...