A Homebrew 455 kHz BFO

This video describes my construction of a 455 kHz Beat Frequency Oscillator (BFO) to resolve SSB and CW signals on an old AM radio receiver. I undertook this project at the requesy of an amateur radio friend ; Charlie (EI2EM). The range or swing of the BFO is about 6 kHZ. Instead of using an expensive variable tuning capacitor for adjustment by the user, I took advantage of some varicap diodes which I had in my junk box, using a 100K linear potentiometer.
I had to make some minor changes to John Fuhring's (WA6TLP) original citcuit to enable the BFO to oscillate reliably. I am grateful to John for the design idea.John's wonderful vintage wireless website can be found at
www.geojohn.org...
Best 73 de Tony, EI5EM

[@acestudioscouk-Ace-G0ACE]

Thanks for posting this, I'm very much a beginner and this helped my understand a little about how BFOs work.

[@EI5EM]

Thanks for taking the trouble to comment. i am glad that the clip shed a little light on how BFOs work. 73 de Tony EI5EM

[@edbeckerich3737]

Nice build, oscillators are unwieldy beasts to tame, good job!!

[@EI5EM]

Ed they sure are wild animals.Frustrating too but rewarding also. Thanks for feedback.

[@edbeckerich3737]

@@EI5EM i would have bought a crystal or a ceramic resonater, made it easier, maybe..

[@EI5EM]

I work with what I have in junk box Ed.

[@UpcomingJedi]

OK! So i tried another home made bfo before tinkering with this and i have a 65-to 298 pf air variable cap laying around. My calculations indicate that i needed around 963 pf +- 25pf to get 452 - 457 khz to get a good range of adjustment so i tried using 4 200 pf ceramic caps in parallel with it which added to 783pf plus the variable and i must admit, im very impressed it actually worked! So what i think the problem is with THIS model i plan to use in hallicrafters radios is that the 1000pf cap is at or beyond the limit to get 455 with the coil i constructed due to manufacturing tolerances so i probably get around 432hkz output or so since i dont have a frequency counter. This may be why it does nothing useful if it works so im going to try a lower value capacitor and make the adjustments again using a larger variable than these plans call for. I think that will then work. Or ill go with the MPF102 bfo again. Thanks for the encouraging ideas!

[@EI5EM]

I had trouble with my first build and had to mess around with different values of capacitor. I used the toroid for the inductor instead of the one specified and it fired staraight off first time. I just had to fine tune and then used varicap diodes to swing both sides for fine tuning. I have a frequency counter which helps. Best of luck with it Garras.

[@AlexBurtonMusic]

What does 455 kHz represent? The tonal range that the oscillator can produce? More precisely, I wonder whether a single BFO will produce tones for all CW signals received in the radio frequency range. I can't say that I understand this.

[@itukuppoo4280]

Did you change the schematic or was the c change just to the toroid? Im looking to make this for my radios

[@EI5EM]

Tony Breathnach, EI5EM
1 second ago
The schematic is the same. However, adjusting the trimmer capacitor didn't give the required frequency ao I had to experiment with that value and ended up with adding a 120pF in parallel with the 1000pF and adjusting the trimmer again. Once you get the correct frequency, the unit should work ok. However, oscillators can drift due to temperature, humidity etc. To prevent having to adjust the capacitor constantly to bring it back into range I added the varicap diodes. As an alternative, the trimmer could be replaced by a panel-mounted variable capacitor. If you need any further info you can email me direct at [e-mail]. I hope this helps. Note that I used a toroid coil instead of what was described.

[@itukuppoo4280]

Thanks! Ive tried builing this originally but it didnt seem to work for me either so ill try these changes.