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Solder Break

   With everything else that has been going on my radio projects have really been set on the back burner.  Not only have I still not even assembled the Michigan Mighty Mite but I have slacked off on learning the code.  I did finally take a break from the house work and melted some solder.  Some time ago I mentioned a BFO that I built from a design in an old Popular Electronics magazine.  It was from an article by Michael Covington N4TMI in the May 1990 issue titled 'Add CW and SSB to any Shortwave Receiver'. I'm going to be deliberately vague about the actual design because it is not mine to publish here but I will describe it as a simple one transistor Hartley oscillator which uses a 455khz IF can as it's tuned circuit.

   This design was quite simple and used less than a handfull of components.  The basic design was not adjustable but he did mention ways to make it adjustable.  He suggested placing either a low value variable capacitor, a larger one in series with a low fixed value capacitor or a low fixed value capacitor in series with a variable resistor across the if-can tuned circuit.  I opted for the variable resistor because I felt that would be easiest to fit into the space I had available in the radio.  I found that this did work however the shaft seemed to be part of the circuit.  Each time I adjusted the knob it would go out of adjustment as soon as I took my hand away.

   This had really only been a side project anyway.  I had read the article when I was a kid in school and wanted to build it then but didn't get it working so I had built it mostly just because I could. Still I never put that magazine away because I thought I might want to revisit it and fix the hand effect problem.  Later I had also read about Hans Summers G0UPL's experiments with LEDs and rectifier diodes as varicaps. I thought this might be a good way to solve my hand effect problem.  I could keep the diode on the board with the rest of the circuit and all I have to bring to the variable resistor is plain old DC.  That is a pretty nice benefit itself plus variable resistors are somewhat easier to come by than variable good capacitors. I was actually kind of excited about this idea and think I might make it my normal way of doing things for other projects as well. I suppose some power is being wasted through the voltage divider so I will still probably use traditional variable capacitors for small battery powered devices or if I try to delve into making my own components.

   Anyway, last weekend I finally had some spare time for an electronics project.  As I looked around my shack thinking about which one to work on I noticed the magazine.  This would be an easy project to finish quickly and then I can put that magazine away.  It will be one less thing laying around I thought. First I went to Hans Summers website to re-read his article and chose a diode to use.  Now, I could have thrown together a test jig with my capacitance meter and sorted through my collection of diodes looking for one with a nice curve.  I'm still pretty busy and only had so much time so I decided it would be better to look through his tables and just chose one that would give me a range in the 10s of pf at 0-5V. He actually did his testing at 0-12V but I have my bfo running on 5 volts so I only looked at that part of the graph.  I ended up chosing the 1N4007. I actually chose the 1N4007 in error. I was looking at the graph of a forward biased diode which showed about 0-850pf from 0-5V.  According to his data a reverse-biased 1N4007 will only show a range of about 8-20pf over those voltages.

   Hans Summers used a 0.1uf capacitor in series with the diode. I assumed this is meant to block the DC used to 'tune' it so that it doesn't enter circuit being tuned.  At first glance I didn't understand how that could work because I thought the higher value fixed capacitor would mask the lower value variable.  Actually, looking at the formula for series capacitance C=1/(1/C1+1/C2) it turns out that when two capacitors are far from one another in value almost all the influence on the combined value is from the smaller one.  It's kind of counter-intuitive if you ask me but a 0.1uf capacitor in series with one in the 10s or 100s of pf range has almost no influence on the series total. I didn't use the 0.1uf capacitor however.  Originally my tuning adjustment was my variable resistor in series with a 39pf capacitor. I just left the 39pf capacitor in to act as my DC block.  This means that if my 1N4007 behaves the same as the one Hans Summers tested then my total capacitance range is about 1/(1/39 + 1/C) where c goes from 8-20.  That gives me a range of only 6.6 to 13.2pf.  This doesn't sound like much but it works great.  I have plenty of tuning range that I can pretty much always achieve zero beat and yet it is fine enough to be pretty easy.  I'm actually happy with how this turned out.

   I haven't given actual instructions or even a schematic for this bfo because the original design isn't mine.  I suppose that either Michael Covington or whoever owns the assets of Ziff Davis owns it. One could always of course buy or borrow the magazine I found it in.  It might even be available at the local library though I doubt it.  There are many bfo designs on the net however.  I suspect that my same tuning circuit would work with any of them that use an if transformer as the tuned circuit.  Who knows, it may even work with ones that use a ceramic resonator.  Let me know if you try it!

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