Rob French (KC4UPR)
For USB/LSB on the uBITX, what is a reasonable carrier suppression? (and how is that properly measured, i.e. I assume it's carrier relative to peak?)
Background: I should have left well enough alone, but... I've been using my uBITX for FT8 and PSK31, and had directly connected my uBITX mic input to my USB soundcard's headphone output, and my uBITX speaker output to my USB soundcard's mic input. No special adapter circuits or anything. Worked fine, bunch of good QSOs. Fast forward, I was reading about interface circuits, and saw someone said that I MUST put a blocking capacitor between the respective inputs/outputs. Well, I didn't do that. Also read about someone saying that a few volts into the mic input could kill diodes in the balanced modulator, leading to carrier leak through. So then I got paranoid and figured I should check my signal to make sure I hadn't done something like that. Which brings us to now...
Test Setup: uBITX v5 transmitting into a dummy load. RTL-SDR dongle plugged into my PC, operating in direct mode. "Tone generator": me whistling into the mic. GQRX running on my PC to view the spectrum. Just eyeballing the spectrum display.
(1) with background noise only ... noise floor ~ -75 to -80 dB on GQRX readout. With PTT on, no tone, desired sideband ~ -75 to -70 db, carrier ~ -75 to -70 db (visibly distinct from desired sideband), opposite sideband absent.
(2) with a tone ... noise floor ~ -75 to -80 dB on GQRX readout. With PTT on, "whistle" tone, desired sideband ~ -35 dB (peak), carrier seems to be unchanged (-70 to -75 or so), opposite sideband still absent (carrier kind of gets lost in the spectrum from my "whistle" at this point, but as I increase the volume from nothing, the carrier appears to be staying the same).
Does this all seem reasonable?
(PS - As an aside, I checked the DC bias on the USB sound card mic in and headphone out... mic in ~ 2V, which I assume it to drive electrets, and headphone out is 0V)