Building XTAL Filters

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From: Edward Pacyna (epacyna@auratek.com)
Date: Tue Jan 25 1994 - 10:03:22 EST


Several folks have asked how I determined the filter response mentioned in
my mail on the NN1G. I use the following setup.

     ................. .............. ....
     . VXO . . . >
     . Buffer/Amp/ ..../\/\/\... Filter . > R l
     . Atten. Pads . Rs . . >
     ................. .............. ....
            | |
           /// ///

The VXO (variable crystal oscillator) uses one of the extra crystals left
over. It's a JFet osc. with a MOSFet buffer amp. and a series of 50 ohm
attenuation pads on it's output (to guarantee a 50 ohm signal source and
allow output level adjustment. Output is from -20dBM to +10dBM.

I take a best guess at the required filter termination R (R = Rl = (Rs+50).
I use my multimeter (low voltage range) and a RF probe (from ARRL handbook)
to monitor the voltage across Rl as I vary the VXO frequency. In effect I
sweep the filter and plot the bandpass on graph paper. The generator freq-
uency is monitored with a frequency counter (or receiver with accurate dial).

If I get the expected response (even bandpass with just a bit of ripple)
my guess was correct. If not, I try another R and repeat. After you do this
a few times you get pretty good at knowing whether to increase or decrease
R based on the response.

73

Ed W1AAZ


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