[ARC5] BC453 FET BFO Success!!!

[Leslie Smith]

  Jim,
  The general rule about LC oscillator stability is this:
  Stability at a higher frequency decreases with the ratios of square of
  frequency.  As an example an oscillator at 4000kHz will drift 4x more than the
  same oscillator at 2000kHz.  (F2/F1)^2.

  Inverting this formula, I get stability at a lower frequency increases
  with the reciprocal of the square of the two ratios.
  This, of course, is a very general statement and assumes a lot about
  the contribution to drift by coil and capacitor, but speaking
  generally I have found the formula to be generally correct.
Taking this as a rule, and knowing that a well built oscillator at 5MHz
may drift 100Hz in one hour then your circuit should have a stability of
100 * (85/5000)^2.  That's my prediction.This is about 28mHz.  milli-Hertz, not Mega-Hertz!

I've seen others use a linear relationship for drift and frequency.  If
that holds (in my book it's wrong) then your circuit will drift not more
than 2Hz.  I predict the drift will be less than I can measure with my
(very-ordinary/old) Fluke counter.
I'll watch your result with interest.  Of course at 85kHz an RC
oscillator may prove a second option.
   73 de Les Smith
   vk2bcu at operamail.com



On Tue, Dec 19, 2017, at 02:19, J Mcvey via ARC5 wrote:
> I have succeeded in implementing a FET driven BFO in the BC453 using
> the the original BFO coil and a minimum of wiring changes.> It uses the original BFO cut-off circuit too!
> Total parts count = 6
> Basically the mod disconnects the 12SR7 triode and replaces it
> with the FET> The BFO supply is clamped to 24V with a zener.
> 
> There are some bias and stability parts that are added to make the FET
> happy. It's not a "direct changeover".> The "grid leak" setup on the triode is not used with the FET
> 
> Looks to be very stable .
> 
> I'll write this up sometime in the next couple of weeks if there is
> any interest.> 
> JIm
> 
>  
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> 
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