Above — Offset mixer schematic. The 20.XXX MHz signal from the VCO offset mixer port gets attenuated 6 dB to establish the correct drive needed to eliminate excessive distortion in the mixer output. The maximum input signal power into the 50 Ω input port = –4 dBm from my experiments.
The mixer output [ 20.XXX MHz signal – 20 MHz crystal frequency ] gets low-pass filtered and then digitized to drive 1 of the the D-FFs in the PFD. In the Jupiter receiver, exact frequencies rank unimportant, so I didn‘t correct the 20 MHz crystal-based oscillator frequency exactly to 20.0 MHz with a trimmer as shown in the inset.
Above — The low-pass filtered offset mixer output measured with a 10X ‘scope probe. A simple low–pass filter seems so trivial compared to the usual triple tuned band-pass filters we must design and employ following mixers in circuitry like transmit mixer chains. Love this. You’ll see simple, pi low-pass filters after offset mixers in old CB radio synthesizers and also in some of Wes’ work.
I studied the PLL circuitry of countless radios + signal generators and continue my reading today.
Above — ‘Scope capture taken on the collector of the BJT switch — ready to drive the CMOS level logic of the PFD.