Doesn't need to move that far. ?Typically it only needs to be shifted approx 6 kHz to the other side of the crystal filter's bandpass.?
Eg LSB (as it is now) requires the filter's bandpass to be below the carrier crystal frequency. ?
USB requires the filter's bandpass to be above the carrier crystal frequency.?
Since it's easier to change the frequency of a single carrier crystal rather than the centre frequency of a ladder crystal filter that's what we do.?
So to get USB we need to move the carrier crystal DOWN by approx 6kHz. ?Fortunately shifting a 12 MHz crystal down this amount is fairly easy and you won't necessarily need a crystal of another frequency. ?You just put inductance in series with it to provide a VXO. ?Adding a second crystal in parallel makes shifting down easier but is probably not required. ?
73, Peter VK3YE
