This morning I breadboarded the 10 MHz BFO for the BITX20 transceiver.  I’m glad I did too, rather than just begin building.  This allowed me to make a couple of mistakes, like putting the transistors in backward, that could be easily corrected on the breadboard.  Now I have some confidence with the circuit and can construct.

I found some ECG123A transistors in the junk box.  These are pretty much the same as the metal can 2N2222’s that I have on order.  Those should be here this week.

The Ver 3 schematic calls for a 22pf variable cap in series with inductor L5.  That inductor is described as “20 turns 45 SWG on dumbell core from 10mm transformer”.  I don’t know if I understand what that is and, if I do understand it correctly, I doubt I have those materials.  This was a good chance to play with the LC meter I got on ebay and brush the dust off of the basic algebra I haven’t used since school.  The ARRL handbook helped refresh my memory on resonant circuits and the formulas needed.

I have a cap that will adjust from about 9pf through about 60pf. To pick a value somewhere in the middle let’s call it 30pf, that way we have wiggle room on either side of the cap.  The math, if I’ve done it right, tells me I want around 8.4uh of inductance.

I had some #26 enamel wire and a FT37-43 toroid core right there handy.  Online calculators tell me that 5 turns would get me where I wanted to be.  Being a firm believer that it’s easier to remove material than to add, I went long and wound 8 turns and fired it up.  I got oscillation but the bottom of the wave was distorted.  I went through a process of removing a turn and then trying it again until I got a wave that looks clean.  Right now I’m at six turns and it measures about 14uh.  I did try another one with 5 turns that gave me about 9uh.  Both are clean.  I’m going to wire in the six turn coil for now.

I don’t have a frequency counter here at the moment.  Adjusting the cap seems to adjust the voltage of the wave but isn’t doing much for the frequency, at least as far as I can read it on the scope.  I won’t get too lost in that for the moment.

Ok, time to build it up for real and get it off the breadboard.

An update before calling it a day.  I finished construction of the actual BFO.  Darn glad I did the breadboard first and worked out the coil.  It’s all built on perf board and takes up about 1″ x 0.75″ of space, not that I was trying to make it small.  It powered up the first time with a nice clean wave.  Looks like roughly +8 dBm output.

I even wound up the transformer for the mixer stage that follows the BFO.  I cut three 12″ lengths of #30 enamelled wire and wound them together using the drill so there are about a dozen turns per inch.  Then I took that wire bundle and wound 8 turns on a FT37-43 core.  So that’s ready to go.  I also sifted through the diode bin and matched up a pair of 1N4148’s that have identical voltage drop so those will hopefully work well in the mixer.  I’m still waiting on my 100 ohm trimmer pots to come from Thailand, hopefully they will be here tomorrow as I need one for this next stage.