Re: Bitx 20, Help me!

Juanjo Pastor

Hi Chris,

at this moment I spend hours getting the filter working for the IF
frequency of 4.9152 MHz.
Which band are you designing it for? 4.915 is too low for 14MHz SSB and sets
the VFO tuning quite high and prone to QSY/QRH (drifting). As I have read
over there, for SSB IFs over 8MHz are better and set the VFO on lower and
more stable frequencies. Crystals between 8 and 12MHz are easy to get here
in Europe, I can get 8, 8.86, 9.83, 10, 11, 11.059 and 12 for less than 0.75
euros each, there in PA you should be also able to do so...

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?
A way to know the filter shape is to connect a wideband noise generator or a
wobbulator to the filter input, an AM detector (diode and capacitor) to the
output and an oscilloscope (could be a PC audio oscilloscope thru the sound
card) to the AM detector. You'll see the ripple and the bell-like shape of
the filter passband. I have seen images in the net, one of them in the site
of PY2OHH, Miguel, It is in portuguese
but some friends are helping them to translate the pages into english...

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!
Chris, PA3CRX
I am not building the BITX, but I am enjoying this list and learning about
it anyway. All the parts but the IRF510 final amplifier are easy to get here
(and I stock most of them) so it is possible I will build it someday...

73, 72 de Juanjo, EA5CHQ-EC5ACA. EA-QRP #104, G-QRP #9742,
QRP-L #1662.

Juanjo Pastor
C/San Roque, 4-1�
46460 Silla

e-mail: ea5chq@...
web del club:
Tel.: +034 96 120 17 67
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