I would like to downconvert a signal at 169.55MHz (10khz bandwidth) into an IF of 20.4MHz. For that purpose, I need a local oscillator of 149.15MHz or 189.95MHz.
But... of course, I cannot find a crystal at these frequencies (at least I think I don't have the budget for a custom Xtal. But I might be wrong on that point. I have no idea.) and I have to find another way.
I decided to try to generate one of my LO frequencies from a PLL. But most of the PLL have a CMOS or other kind of logic output, not sine waves. This is especially true when the output of the PLL IC is post scaled. This approach is convenient, because it's easy and low cost to design such a PLL with an integrated, high speed, VCO (such as the ADF4351). That VCO runs at frequencies around 1GHz, is divided and output using a logic driver (here differential at 0dBm).
My question is: can I inject the logic signal from the PLL at the input of my mixer? That signal has a lot of harmonics (only the odd harmonics, in theory.. right?) Or do I need to low pass filter that LO at the input of the mixer?
I have computed that none of the LO harmonics, mixed with the input signal (or harmonics of it) falls into my 20.4 IF band pass filter. But is it enough? Do I have to expect another issue related to this?
I would feel very confident to inject a "as clean as possible" sine into my mixer, but not a square wave...