[time-nuts] Vintage Frequency Measurement

[Bob Camp]

Hi


> On Feb 13, 2017, at 8:15 PM, Scott Stobbe <scott.j.stobbe at gmail.com> wrote:
> 
> On Mon, Feb 13, 2017 at 6:41 PM, Bob Camp <kb8tq at n1k.org> wrote:
> 
>> 
>> 
>> I think what you would find is that it *is* a fairly normal AT cut and the
>> data book
>> that came with the instrument plotted out the data for the specific
>> crystal in
>> the device. The usable temperature range was fairly small, so the plot will
>> be pretty linear.
>> 
> 
> Attached is a plot of crystal calibrators temperature stability. Span is
> roughly 65 degC.

Which eyeballs out to be pretty close to an AT. Without knowing the PPM 
scale there isn’t much way to be sure. 

> 
> One of the other aspects I think is intriguing is the DC PSRR of a vacuum
> tube crystal oscillator. In the case of a bjt based oscillator you have the
> C-V relation for depletion capacitance and the base-emitter dynamic
> capacitance as a function of collector current. I would suspect that for a
> one active device oscillator, tube vs bjt, a tube crystal oscillator would
> be less sensitive to small power supply variations (+- 10% ).

Except you *do* have miller effect which pretty much messes things up
for a triode. A pentode is a bit less sensitive, but you still have issues.

> Which is a
> convenient attribute for a poorly/unregulated battery supply in the vacuum
> tube case. Unless filament current has an appreciable impact on frequency,
> I wouldn't think so…

Umm… errrr …. check it out :)
Oddly enough, I remember a high school physics lab where they had us plot
the effect of filament voltage on plate current and gain. Seemed like a weird
thing to do to me at the time. Turns out the teacher grew up with microwave tubes
that were tuned by varying the filament. Who knew ???  Pretty strange stuff if 
you ask me. 

The bigger issue is the tubes get hot. The heat varies with supply voltage. 
Temperature change is the result. That temperature change messes up 
oscillator stability. You pretty much have to wait for things to hit equilibrium 
before you do useful stuff ( = let it warm up for an hour or four).

Bob



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