Saturday, 5 December 2009

Beat Frequency Oscillator

Having completed the audio stages, it is now time to look at the first high-frequency stage - the BFO.

Harry's circuit (edited from the full version on his website) is shown below...

The new challenges in this circuit are the coil and the variable capacitor ...

I found a 75pF variable in the junk box and removed some vanes to make it closer to the specified 10pF. However, the coil was going to be harder...

Harry specifies "205 turns on a 1/4" (6mm) former fitted with a ferrite slug". I thought about how I was going to make the IF transformers - they would need some formers too. Inspiration struck in the form of plastic sewing machine bobbins - my wife has loads of them pre-loaded with different cottons, so I asked her for some. Instead, she stopped by the sewing shop and got 8 for me - at 10 pence a throw they seem a bargain...

I lashed up an impromptu "coil winder" on my lathe and found I could squeeze about 300 turns of #30 enamelled Cu wire on the "former". Rather than take off a hundred turns to get into the right ballpark for the BFO, I went right ahead with the fully-loaded coil (of course I had to change the tuning caps in the (Colpitts) oscillator to get to the required 455 kHz).

You can see the system being tested below...

(couldn't resist turning the counter/timer on and showing 455kHz!)

The output from my BFO is about 1.4V - see the 'scope trace below...

Harry warns that it may be neccessary to reduce the 22k cathode resistor in his schematic to get the oscillator to run - I reduced mine to 6k8 (it probably could have been higher - but I found the 6k8 resistor on the bench and fate intervened).

Now I have the BFO, it is time to turn my attention to turning those sewing bits into IF transformers. But, before I do that, I'm going to make a "Wobbulator" - watch this space!

...-.- de m0xpd

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