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Sunday, 28 November 2010

RadCom Dec 2010 SSB Exciter

I saw the SSB exciter in the Hombrew column of RadCom Dec 2010 and thought I'd have a go at building it. I had a rummage in my components and found a 9MHz Crystal Filter and two suitable oscilator crystals; 8.9985 and 9.0015, so this seemed like a good starting point.

The first item I built was the Mic amplifier:

I built this using vero board; the output seems quite clean and tidy, and I make the gain about 18dB:

I've built the main part of the exciter and I'm seeing OK carrier suppression through the balanced modulator, however I think this can be better. I've used some BAT85 diodes in the modulator, but I didn't match them first, so I think I need to match some more and see if I can suppress the carrier some more. This image is taken from the spectrum analyser with a 1.5KHz tone injected into the mic amplifier. We should expect to see the carrier plus a signal at +1.5KHz and -1.5KHz (they are the upper and lower sidebands). The balanced modulator should be adjusted for minimum carrier at the output:

The Upper and Lower sidebands aren't quite the same amplitude for some reason, but the carrier is some 30dB down in the noise, so that will do for now. The output of the Crystal filter looks like this with USB selected:

And like this with LSB selected:

So, I need to try some different diodes in the balanced modulator, then I need to try a range of audio frequencies to try and figure out where the skirt of the crystal filter is to ensure that the carrier and the unwanted sideband are set in the correct place for maximum rejection.

Here's the board as-is:

So once I'm happy with the SSB exciter I need to mix this RF output with a VFO and try and get a signal on the Air!

I'll post more when I've figured out the best band to aim at.