Hello,
I have a problem with understanding some designs in phase locked loops. I want to stabilize signal from some kind of electooptical oscillator; it works like a normal VCO, with frequency about 87MHz and Kvco~=50Hz/V. This signal may be mixed in DBM/phase detector with reference signal and output after filtering in low pass filter is used to tune/detune VCO. This can be done also on higher harmonic (and we get lower phase noise).
In some publications I saw a little bit advanced scheme: 10th harmonic from electooptical oscillator (870MHz) is mixed in DBM with high frequency signal from synthesizer (about 880MHz), difference signal is filtered in low pass filter and then is mixed with reference
10MHz signal in phase detector. And another scenario: signal from electooptical oscillator (120 MHz) is mixed with 1GHz then filtered with band-pass filter for 1120MHz, divided by 256 in prescaler and mixed in digital phase detector with reference frequency. Why signal is mixed with high frequency, but detection in phase detector is made on low frequency? Because diode phase detectors don't work on high frequencies?E.C.