[time-nuts] Vintage Frequency Measurement
kb8tq at n1k.org
Tue Feb 14 06:45:22 EST 2017
That was one of the reasons I was a bit amazed high school students
were doing it as a lab exercise. The presence of high voltage here and
there is something that you simply would not see in a similar school
> On Feb 13, 2017, at 11:19 PM, Alex Pummer <alex at pcscons.com> wrote:
> just be careful, because if you under-heat the cathode you could kill it
> On 2/13/2017 7:11 PM, Bob Camp wrote:
>>> 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).
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