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building a better mouse trap

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laroche73

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Hi folks, I'm looking for some suggestions for a simple circuit to compare the resonant frequency of a crystal (10 - 20 Mhz) with that of a LC circuit.
The LC is tuned by laser trimming a capacitor bank on an ASIC, and the whole process is currently based on a rather complex DDS system.

I'm thinking of using some variation of a Wheatstone bridge to compare reactance, any other ideas? Thanks - Claude
 
laroche73 said:
Hi folks, I'm looking for some suggestions for a simple circuit to compare the resonant frequency of a crystal (10 - 20 Mhz) with that of a LC circuit.
The LC is tuned by laser trimming a capacitor bank on an ASIC, and the whole process is currently based on a rather complex DDS system.

I'm thinking of using some variation of a Wheatstone bridge to compare reactance, any other ideas? Thanks - Claude

I don't think a Wheatsone bridge could be used to do that?.

Simply build an oscillator around each of the circuits (the crystal and the LC), feed them both into a double balanced mixer and check the output frequencies. They should be sum and difference, as they get closer together the difference will get closer to zero!. Obviously you can filter the sum with a low-pass filter to get rid of that, leaving just the difference you're looking for.
 
mixer or bridge?

Thanks for the suggestion, Nigel (why didn't I think of that first...?).
any suggestions for a mixer IC? I seem to recall some analog PLLs (565?) use a doubly balanced mixer in the frequency/phase comparison stage.

BTW, I got the idea for using some sort of bridge from the following articles. Near the end of the second one there's a comment:

Some impedance bridge circuits are frequency-sensitive while others are not. The frequency-sensitive types may be used as frequency measurement devices if all component values are accurately known.

**broken link removed**
 
Re: mixer or bridge?

laroche73 said:
Thanks for the suggestion, Nigel (why didn't I think of that first...?).
any suggestions for a mixer IC? I seem to recall some analog PLLs (565?) use a doubly balanced mixer in the frequency/phase comparison stage.

You could probably use a non-balanced mixer anyway, after all the only product likely to be low is the difference one?, so the low-pass filter will remove the original frequencies as well.

Assuming two 10MHz oscillators:

Xtal 10.000MHz
LC 10.001MHz

Mixer outputs

10.000MHz
10.001MHz
20.001MHz
00.001MHz (1KHz)

I think a low-pass filter should pass 1KHz and reject the others quite satisfactorily :?:
 
You could use an SA602, its a double balanced mixer, and it will work at the same time as a LC or crystal oscillator, so then you would only need one other circuit.
 
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