Crystal Filter
The crystal filter appears quite simple and easy to build. Testing it to verify performance though is a bit
more complex. To determine the passband characteristic for your crystal filter it should be tested in the
circuit where it will be operated. In the BITX designs this means that you need to have the IF
amplifiers on each side of the filter wired and working properly.
Insert a low level signal at the input of the receiver 1
st
IF amplifier. Monitor the output at the top of
R64 which is located at the output side of the 2
nd
receive IF amplifier. Decrease input signal until the
output decreases as your decrease input (this avoids working through a saturated amplifier).
As you tune very slowly across the IF passband, note and record R64 signal levels at every 200 Hz
frequency step. For a rather crude looking chart you can simply plot the voltage readings on graph
paper to determine your filter passband, but the graph will not look like the classic ones in radio
handbooks. To improve your chart's appearance and to look like the commercial diagrams, you need to
convert each reading into decibels and then plot the response with DB as the vertical axis of your graph.
Mark the 6 db points (or halfvoltage points if you are plotting raw voltage readings) on upper and
lower skirts of the filter response curve. The frequency range between these points is your filter 6db
bandwidth.
Note the points of 12 and 20 db down the lower sideband skirt of filter response. Somewhere between
these points is where you should set the BFO frequency for best audio and good LSB and carrier
suppression.
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