20/40 BITX Build Update #1: VFO Success

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:

  
http://www.1728.org/resfreq.htm

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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