Sideband Selection – QSD Receiver

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I had about 15 minutes to spend on the project today, so I got the sideband selection function working. Since it’s just a matter of swapping the I and Q LO phases, and those phases are defined in the Si5351 software, it was merely a matter of a few lines of code. With a signal tuned in, touching the Mode button on the touch display makes the signal go away and appear again when you tune to the other sideband. It’s an interesting way to qualitatively assess the opposite sideband suppression.

If you come from a crystal filter radio world, you’re probably used to the character of the noise changing when you switch from USB to LSB. This is because the crystal filter bandwidth is not symmetrical and you hear more noise when the BFO is tuned on the lower  skirt than on the upper skirt, or vice versa. With the phasing mode of demodulation, that doesn’t happen and the noise is absolutely the same no matter the mode. Neat!

Now I have to decide which sideband is appropriate for receiving CW signals.

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