[ARC5] BC453 FET BFO Success!!!

[Ian Wilson]

Les,
I wonder if that means that the basic figure of merit for an LC tank is the
Q of the inductor. The Q of the capacitor is huge at these frequencies, so
can be ignored.

This would also be consistent with skin effect being the major mechanism.

73, Ian K3IMW


On Dec 18, 2017 9:49 AM, "Leslie Smith" <vk2bcu at operamail.com> 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!
>
> 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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