Starting a new home made V.F.O.


vfo bench copy


3rd January 2015.


Prodnose: What is a V.F.O?


Myself: It’s an abbreviation for ‘Variable Frequency Oscillator’.


Prodnose: What is a Variable Frequency Oscillator?


Myself: A V.F.O. is a device for generating radio frequency oscillations. This frequency is variable within a certain, adjustable, range.


Prodnose: What is the utility of such a device?


Myself: There is a great number of uses for such a device. But the one with which I am concerned, is to facilitate the transmission of low-power radio signals on the 3.5 MHz amateur radio band.


Prodnose: Ah – you mean you will talk with other, like-minded radio amateurs?


Myself: Er, talk; yes, but not as such. I will communicate with them by means of the Morse code.


Prodnose: I thought the Morse code was obsolete?


Myself: That may be the case for most commercial purposes; but there are hundreds of thousands of radio amateurs around the world who still use the Morse code.


Prodnose: Why on earth do they do that? Surely talking is quicker & more efficient?


Myself: Yes; usually, it is. BUT: it is possible to make contacts over very long distances, using very small power, with Morse code, that could not be made using speech. So that, in such cases, Morse code may be said to be more efficient than speech, albeit slower.


Prodnose: H’mm. There may be something in what you say. Anyway, you may proceed – at least for the time being – with your explanation of this ‘V.F.O.’


Myself: Thank you. Here is a close-up of the prototype of the VFO.


vfo copy


Prodnose: It looks like an awful lash-up to me.


Myself: Well, it’s just a prototype yet. I mean, you have to try things out to see if they work. This one is actually working quite well!


Prodnose: Was it not said by a distinguished engineer, that every prototype should be made to the same exceedingly high standards, as if it were the Finished Product, in order to ensure reliable development?


Myself (laughing): Well, yes; that was said. And it is perfectly true, in its own way. Given infinite time and funding, that ‘Counsel of Perfection’ would be the best way forward. But it is not usually practicable. For example, this is the original circuit we copied down from the ’net:




Circuit borrowed from with grateful acknowledgements.


We built it up, and it worked fine. But we changed it a little, and it ended up like this:


vfo modified 3


Prodnose: If it worked OK, why did you change it?


Myself: Because I only need a very small variation in frequency. As long as I can cover say 3550 to 3570 KHz, that will do fine. So we can dispense with a variable capacitor and just use varactor diodes – the MV2109s – instead, and vary their capacitance with a very ordinary & cheap 47K potentiometer. Varactor diodes are stable in use, far cheaper than a good quality variable capacitor, much smaller & easy to get firmly mounted on the circuit board for frequency stability.


Prodnose: I will confess I am slightly impressed by all this. I take it you will now be building this successful prototype in a proper fashion?


Myself: No.


Prodnose: WHAT? But you said…


Myself (imperturbably): While checking out a few more things on-line, I found another very similar circuit, but which was slightly different, and I thought it would be best to try that one out also, as it might prove better.


Prodnose: But all your initial work will be wasted!


Myself: Perhaps it will. But far less work will be wasted, than had I built this first prototype as a finished product, in accordance with what you said above.


Prodnose (mutters darkly): Grrh! Grumph… yes; well. H’mm. Very well then – but get on with it; I don’t like the suspense!






Page written 3rd January 2015.

Re-formatted 19th December 2015.