[ARC5] BC453 FET BFO Success!!!

[Bob kb8tq]

Hi

The gotcha with looking at Q only as skin effect is that the dimensions and structure 
of the “coil” can change as you go up in frequency. There are 60 MHz LC oscillators 
out there with > 300 Q coils in them. Your 5 MHz oscillator is unlikely to have a coil in it
with a Q 3,000 (or “even” 1,000). 

Bob

> On Dec 18, 2017, at 8:25 PM, Ian Wilson <ianmwilson73 at gmail.com> wrote:
> 
> 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 <mailto: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 <mailto: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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