[time-nuts] Ancient OCXO in scope calibrator.

[David C. Partridge]

Agree that the tank slug can also affect the frequency slightly.

I've tried a number of 7587s including what I was fairly sure to be NOS
parts - at least they came in the correct box.

I suspect I can pull the frequency a bit by paralleling another 5 or 6pF
across the trimmer (why has the parts box never got what you are looking
for? - this feels like a corollary to Murphy).

The main issue is how unstable it is - maybe I just stop chasing this and
write it down to a very old crystal that's aged out of spec, and just
replace it with a regular 20ppm room temperature part.

Cheers
Dave
-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of Bruce Griffiths
Sent: 08 September 2008 08:41
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] Ancient OCXO in scope calibrator.

Magnus Danielson wrote:
> Dave, Bruce,
>
>   
>> Need a few more circuit details.
>> The crystal current wont be very high unless their are other 
>> components connected to G1.
>>     
>
> As expected, BAMA can assist:
> http://bama.edebris.com/manuals/tek/184/
>
> I am thinking about undamped overtones/other modes of the crystal and 
> similar.
>
> Cheers,
> Magnus
>   
The crystal oscillator is in effect a miller oscillator using the screen
grid as the oscillator anode/plate see:
http://www.ieee-uffc.org/freqcontrol/tutorials/s6310508.pdf

The oscillator plate/anode (screen grid is grounded) in this circuit.
The tube plate/anode forms a well isolated auxiliary output tuned to the 5th
harmonic of the crystal.

If the tube emission has dropped the gm may have fallen rather too close to
the critical value required for oscillation making the circuit sensitive to
loading at the grid.
Both the trimmer and the cathode tank slug affect the frequency.


Bruce


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