I’m trying to make a simple wireless alarm link. The transmitter will be in an unheated garage, so the only way to easily get it to stay within the -+ 10kHz channel will be to use a XO (crystal oscillator).
But for the receiver, I figured the best way to go would be to put a bandpass filter right after the antenna to keep the high power CB stations from wiping out my signal. I figured a Pi type bandpass filter coupled by a low PF capacitor between the two LC sections. Or perhaps I should use a link coupling so I can move the link away from the resonant winding to get the passband to be narrow. This is what I need to experiment with.
After that a LO and mixer to downconvert the signal. The reason I built the tunable Vackar Oscillator is for tweaking the bandpass filter. I don’t have a decent RF signal gen for this freq. I have an old tube signal generator, a Realistic 22-040 clunker from the old days when they still called them Megacycles. And I have a Heathkit Dipmeter, which I find very useful.
I have some old aluminum IF cans from tube radios that I can use to hold the filter and jacks, and connect stuff together with some RG-174 coax. I need to make the bandpass filter narrow band. I thought about using some crystals to get a narrow bandwidth. I had a low profile crystal for 27 MHz, and I checked it with my crystal checker, and it checked intermittently then died. I think the crystal element was damaged by too much power. It has to be small to fit in the small package, and it most likely was broken by the power from the crystal checker.
I have looked at several downconverter designs and I liked one because it used a crystal that was half the frequency. The converter used two 1N4148 diodes in anti-parallel, so it rectified both halves of the sine wave, thus doubling the frequency. I also got some SA602 chips for downconverting RF. There are several designs using this chip when I do a search.
