Hello all,
I have successfully got the VFO working for the BITX20. However
some questions remain, wondering if I can get some help, comments and
advice from the group:
1. The coil has 115turns. I am using an air variable gang capacitor
which has 2 sections (one larger than the other). I have no idea
about the value but experiments with both of them give about the same
results.
2. With the variable capacitor fully engaged the vfo stops
oscillating. The oscillations start at about 20% from fully engaged
and work all the way to fully open.
3. The frequency coverage is approximately 110kHz. By taking turns
off the coil (from original 125) I was able to get the frequency to
be from 4130 to 4240. The "fine" control pot moves the freq by about
15kHz - quite impressive.
4. Now for my minimal test equipment. I have a homebrew PIC based
frequency meter which is NOT caliberated. So the frequency shown may
not be accurate, however the coverage should be accurate. My goal is
to get the frequency span expanded and then come back to accuracy by
working on the coil and other adjustments (after my freq counter
caliberation of course). RF probe is being built but I have not had
to use it till now since the VFO worked on the first try. A
commercial Digital multimeter completes my test set up.
5. I let the VFO sit for about 5 minutes and was impressed that the
frequency shift was within 3kHz. Unit is built "Manhattan/Ugly" style.
6. I did some experiments with the gang (series and parallel
connections), results were not impressive. Connecting 10, 50, 100pf
in series with the single section gang produced much smaller
frequency spans - in the order to 10 to 20kHz (still within the 4130
to 4240 range).
Now for my questions:
1. What is the frequency span for the VFO design? I think it is 4000
to 4350kHz.
2. How can I increase the span from my current 110kHz to 300 or
350kHz? All tips point to moving the frequency and not changing the
span/range of frequency.
3. How can I adjust values to make the VFO oscillate with my gang
fully engaged. I suspect that the gang is 0-500pf since 20% of 500 is
100pf. The BITX20 spec say 0-365pf so 500 minus 365 is 135pf. So
roughly I need to adjust the design to accommodate a 0to500pf air
variable capacitor. Any thoughts?
Thanks in advance.
Regards,
Jay
