[ARC5] BC453 FET BFO Success!!!

[Kenneth G. Gordon]

On 19 Dec 2017 at 4:49, Leslie Smith wrote:

>   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!

That is 0.028 Hz/hour, gentlemen. If this bears out in actual practice, then Mac's "apparent" 
drift is very, very excessive, and there must be other issues at play there. We need details.

Even including the drift of the HFO combined with the drift of the BFO, the drift, on average, 
should be something less than 0.06 Hz/hour.

We need to test this. However, in my case, I sincerely doubt that my equipment can reliably 
show such minimal drift.

> I've seen others use a linear relationship for drift and frequency.  If that holds (in my book it's 
> wrong)

Correct. Again, in actual practice, that curve is, at the very least, highly irregular. As I 
mentioned, there was at least one, and probably several, articles which were somewhat 
recently printed in Electric Radio Magazine which detailed such drift in LC oscillators of 
several designs.

> 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.

Likewise here.

> I'll watch your result with interest.  Of course at 85kHz an RC oscillator may prove a second 
> option.

Yes. With its own, somewhat different, drift characteristics.

Ken W7EKB

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