I've started construction on my second BITX transceiver, this one for 20 and 40 meters. As with the previous rig, I decided to take the VFO first, but this time I pledged not to surrender, not to wimp out with a VXO or (worse!) a DDS. No, this time it would be a real LC VFO.
I took seriously all the admonitions in the tech literature about the fickle permeability of ferrite and iron powder, so this time I used an old-fashioned air-core coil. It is wound around a cardboard tube. The tube was previously the bottom portion of a coat-hanger from the dry cleaner. I wound 40 or so turns on this core, then measured the inductance: 5.6 uH -- that looked about right.
For the oscillator stage I used a Colpitts circuit very similar to the one in the original BITX schematic (but I am hoping I won't need the varactor diode fine tune mechanism). I had on hand a nice Heathkit 19-146 pf variable cap with an internal 4:1 reduction drive. Not wanting to pluck rotor places out of this beautiful part, I had to calculate the series capacitance that would yield a frequency spread of about 175 kHz. It turned out to be 40 pf. Then I had to figure out how much capacitance to put in parallel with the variable. Well, it all ended up like this:
There was a bit of trial and error in the process of getting the VFO to cover the desired range. A big help in all this was an on-line reactance calculator. I found this one to be very useful:
I found that the VFO is more stable if I reduce the voltage from 12 volts down to about 8. Also, I found that when evaluating the stability, it is better (psychologically!) just to use a stable superhet receiver instead of the frequency counter. The counter may appear to be jumping around a lot, especially if the signal you are monitoring is not very strong. Just listening to it at zero beat on the superhet is very reassuring.
I followed the Colpitts oscillator with the FET buffer and two stage BJT amplifiers from page 50 of Doug DeMaw's QRP Notebook (page 50). I now have the requisite 7 dbm signal. And it appears to be quite stable.
I plan using this with an 11 MHz IF, with the VFO running at around 3.875--3.700 for 40 meters and 3.175 -- 3.355 for 20. I plan to use a small relay to switch in some additional capacitance to move the VFO down to the range for 20 meters.
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