The PSSST-20 Band Pass Filter
Within this transceiver there are several signal mixing processes with some being the ones we want and others are "out of band". Thus there is a need for filtering to pass only the signals we want while rejecting the undesirable signals.
Based on our topology we are able to use a single Band Pass Filter (BPF) on the Receive side so we only hear the signals in the band we are operating within. That same BPF works on the Transmit side to only pass the desired transmitted signal. A cousin to the BPF is the Low Pass Filter following the transmit stage so that no harmonics are passed.
The following filter has been used in many of my transciever projects and is adequate enough to satisfy the FCC. It is not a brick wall filter; but satisfactory for this application. The last photo shows a layout for the PC Board you can etch in your kitchen sink. The pad size is 3/4 inch by 3/4 inch.
Many would be itching to whip out their Nano VNA and take measurements. Go right ahead. But for me I won't! I have two Nano VNA's and trust neither of them! For one with the same Device Under Investigation you don't get the same answer with the two units. The one I think may be working does not give repeatable results. Test and get one set of values. Come back later and nothing changed and not the same result. Yes I did the calibration process and had everything terminated properly. I think the Nano VNA is overhyped and purely a cute toy. You should be suspicious of anything you do with your Nano VNA.
But old school is to use a signal generator and an Oscilloscope. That is what I did and have some photos of the readings. My DSO readings correlate to the LT Spice Simulation. Now there is two independent tests and that I Trust!