71. The VFO is completed – ta-raah!
23rd January 2015. The finished VFO board. Yes, it is ugly, isn’t it? And yet, thanks to the short, vibration-free leads, the general inherent rigidity of the small board, and mainly because we decided to make it operate on 1.75 MHz rather than 3.5 MHz, it’s works quite well. That is, works well for something I have made! 8^) At ‘a’ there is a 2N5458 FET, which is the oscillator itself, flanked by the two large silver mica capacitors. It is a Colpitts oscillator. My favourite is the Clapp (or Clapp-Gouriet, or Series-tuned Colpitts – whatever you want to call it; after 60-odd years they’re still discussing the correct name for it. See Wikipedia!), but for 1.75 Mhz we just followed the circuit. The coil, or should I say inductor? No; it’s a coil as far as I’m concerned, and is wound round two stacked Amidon T-50-2 toroids in order to get the higher inductance needed for 1.75 MHz. At ‘B’ is another 2N5458 FET, a buffer to stop frequency pulling when power is drawn from the oscillator. ‘C’ is a plain old 2N3904 to act as an amplifier, for the oscillator circuit is working on only 5 Volts, so doesn’t give much output. The black toroid next to it is an FT-50-43 as a choke. At ‘d’ are seen two varicap diodes, which will give us our tuning range, when a suitable voltage between 0 and 12 V is applied to them. The board as a whole is supplied with 12 Volts.
This disgracefully out-of-focus shot is the little frequency doubler board. This was necessary because we want our transmitter to work on 3.5 MHz. Having the VFO on 1.75 MHz helps us to make a stable circuit, and it is easy to double it to 3.5 MHz. The transistor is another common-or-garden 2N3904, and in its collector, is a tuned circuit made up of the yellow T-50-6 toroid, and a 100 pF polystyrene capacitor – underneath it – plus a small 100 pF variable capacitor to adjust the resonant frequency of this ‘tank’ circuit to 3.5 MHz. Why they call it a ‘tank circuit’ I have no idea. But I know how it works. The oscillator at 1.75 MHz will produce a strong second harmonic – twice the fundamental, or 3.5 MHz – and a circuit resonant at 3.5 MHz offers a high impedance (or opposition) to that frequency. So the 3.5 MHz component cannot ‘escape’ to ground through the tank circuit; it can only pass through the upside-down 0.1 µF capacitor to the right of the 100 pF trimmer, and be led away through the pink wire. In other words, it’s got us exactly where we want it. <8^)
Well then. It was simply a matter of drilling a few holes in the magnificent (and not really terribly expensive) die-cast aluminium box we ordered a couple of days ago, and mounting all inside. It is always a temptation to stick the thing ‘as-is’, in a plastic box, or even leave it en plein air. To be sure, I have often done so myself. But there is little else more satisfying, than to have your frequency as stable as possible. And I remember once, I upset a cup of coffee – or it might even have been a glass of beer – into the open-plan VFO, and all Hell broke loose. There are only four externals here: 1. A phono socket to take out the Radio Frequency product of our unit to the power amplifier; 2. A knob (you can’t see it, because it’s black) which turns the 47 KΩ pot that varies the voltage to the varicap diodes, giving us our tuning range; 3. An offset switch. When down, we get our operating frequency. When up, it puts a 10 KΩ resistor in series with the pot, shifting the frequency about 10 KHz away, so that it won’t swamp the incoming signals in receive mode; 4. A phono socket for the 12 V power for the VFO. It’s true that QRPers (which is what I basically am) are much wont to use phono sockets for power input. They are cheap, and perfectly OK for the low current we tend to use. Downside of course, is that if you miss the socket with the plug, you are highly likely to short out your power supply. Also, if you leave a phono lead with a live centre pin lying around on your bench, a considerable number of undesired consequences may occur. I only write this to make plain that I am aware of these things, and even though I do it, I do not really condone it! 8^)
And here, at last, is the finished product. A stark, plain box with a knob and a switch on it. It works from 3.5 MHz to ~3.62 MHz. Our own operating window is most likely to be the narrow one of 3.558 MHz (the FISTS calling frequency) and 3.560 (the QRP calling frequency). We were tempted to slug the pot so that it would give just a tiny range of say 3.550 to 3.570; but left it as is. You never know if you might hear an old mate calling CQ on 3.530 – then you wouldn’t be able to call him & say ‘Aa-doo’, as we have it in this locality! As to the stability of the device, which has four little self-adhesive rubber feet on the bottom, just right-click on the link below & open in a new tab, and you will hear the VFO idling on about 3.550 MHz, before being bashed pretty hard with my fist. Even I was surprised – and gratified – by its steely resolve.
Don’t miss the next thrilling episode. Will the PA ever work? Can we get rid of the chirp? Why is there a chirp anyway? Will we ever make a contact?