Chris van den Berg
at this moment I spend hours getting the filter working for the IF
frequency of 4.9152 MHz.
It is importand that measurements are done with the circuits
connected, it seems that the load determines the passband a lot.
To have an idea about the functioning of the filter, I connected my
LF generator to the microphone connection. Measuring the output of
Q12 (with an osciloscoop or a mw meter), you see variation of the
output level while variating the LF frequency.
For the filter I made, it learned me that my filter had some peaks
that were not close together. Playing with different capacitor
values took a lot of time but with capacitor values of 68pF, 82pF,
68pF I have peaks at 1052Hz, 1538Hz, 2000Hz (after I also corrected
the coil in the BFO to get the peaks on the right places). With a
mircophone connected, I see output while talking in the microphone.
Between the peaks, the output is not completely down but I think the
filter shape is not flat enough. This weekend, I will play with some
additional serie coils, may be the impedance of the filter is much
higher then the circuit. (I hope the peaks do not shift again ;-( ).
Anyone else suggestions?
By the way, for the ones that want to see it with an analyser: may
be the BFO signal can be connected to both mixers resulting in audio
frequency mix up, filtered and down to LF again. Then a spectrum
analyser can be used with the PC sound card? (pink noise to the
microphone terminal, output of the mixer to the LF sound card input).
Just a thought, not tried.
Lets experiment further!
IPlease I do not have the 10MHz crystals to make the filter, but
possible tohave 5 pieces of 8MHz crystals with this 8MHz crystals is
3) To test my crystal filter I built the BFO circuit from themake the filter with new values of resistor and capacitor?>
BITX20 design on a separate of circuit board and added the inductorpassband
to determine the shape factor. This signal was injected into thethat
was being output by my test oscillator. As the frequency wasvaried I
was able to plot the output voltage on a sheet of grid paper. Iwould
assume that you might do something similar in building your filter