| Hey folks, I was wondering if anyone better versed in analog
| electronics could help me out with an idea. I've seen various
| projects on the internet where people have repurposed old radio
| equipment for other uses, like using a CB to receive amateur radio
| frequencies. So stuff like that got me wondering. I have an old
| crystal-based scanner that belonged to my dad. It has a range of
| 30-50 and 148-174mhz. While police/fire/rescue have all moved to
| digital systems, I know that the city's public works still uses
| frequencies in the latter range. I'd like to be able to receive
| those, without buying several crystals. So I'm wondering if it's
| possible to create some kind of oscillator (LC-based or whatever) to
| simulate a crystal for this type of device. It wouldn't be nearly as
| accurate/stable of course, but I'm curious if it's possible.

Almost certainly, yes. The details would depend on just how the
circuitry is designed.

You would probably want to completely bypass the existing crystal-
oscillator circuit, and inject the appropriate local-oscillator
signal (created by your new tunable oscillator) near the point where
the crystal-oscillator signal is fed to the "mixer".

As to the type of oscillator you could use... you could build
an LC circuit, of course. If you want something a bit more
off-the-shelf, you could buy a commercial VCO (voltage controlled
oscillator) module from a company such as Mini-Circuits... feed
this a very-well-regulated control voltage, through a multi-turn
potentiometer, and feed the VCO's output to the scanner's mixer.

The scanner documentation probably tells you what crystal
frequencies correspond to which over-the-air frequencies
(i.e. what the "offset" is). If it's a single-conversion
receiver, the offset would probably be equal to the operaring
frequency of the radio's IF strip and filters. You would need
to build (or buy) a variable oscillator circuit, which would
generate frequencies covering the correct offset range.

--
Dave Platt AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
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