Bert, WF7I, was recently struggling to get his BITX 20 going and he asked some good questions about carrier suppression. I realized that I hadn't really paid much attention to this. Perhaps as a result of my long experience with DSB, I was happy as long as I was able to null out MOST of the carrier.
I fired up the scope and took a look at the output from the BITX 2040 on 40 meters. Here's the test setup: Coax from the antenna terminal to a 50 ohm resistive load at the Rigol O'scope probe. Just keying the transmitter (no mic connected), carrier was at 980 millivolts rms or about 19 milliwatts. I then connected an AF sig generator into the mic in connector and pumped in some 1000 Hz sine wave. Peak output was 20.7 volts rms, or about 8.6 watts. That puts the carrier about 27 db down. I felt I should be doing better.
I took a look at the shape of my crystal filter and the frequency placement of my carrier oscillator. I noticed that the carrier oscillator freq was fairly close to the bandpass portion of the crystal filter -- fairly high up the skirt, only about 9 db below the passband level. I figured that if I just moved that carrier oscillator up around 300 Hz, I would get around 10 db of additional carrier suppression.
Sure enough, with the carrier moved a mere 300 Hz further away from the passband, the residual carrier dropped to 346 millivolts rms, or about 2.4 milliwatts. Now peak output was 20.9 volts rms, or 8.7 watts. 36 db of carrier suppression.
I guess I could do better if I moved the carrier up another little bit, but I like the sound of it now. I may have been able to better if I'd fiddled with the balanced modulator diodes a bit more. But what do you guys think? Should I worry about 2 milliwatts of residual carrier? Heck I once ran a CW rig (W1VD' VXO 6 watter) that kept the oscillator running on key up, producing about 15 mw of "backwave." No damage was done, few noticed, no one complained.
Oh yea, is this the way to measure carrier suppression?
While doing all this, I pulled out my trusty copy of EMRFD. The index led me to the balanced modulator section on page 6.56. There I spotted a familiar call: W6JFR!!! That's Pete Juliano, N6QW! Pete is credited with a mod to the SBL-1 mixer that adds a balance control pot to the device. Wow, actually being IN EMRFD fully confirms Pete's homebrew guru status.
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