[time-nuts] Re: CTI OSC5A2B02 OXCO testing
Tom Van Baak
tvb at LeapSecond.com
Mon Jun 5 19:42:49 UTC 2023
Reg,
Ah, it wasn't clear the 10 readings were from 10 different oscillators.
Thanks for the clarification. Ok, what you see then is simply that all
ten oscillators can be set on-frequency and still be within your desired
EFC tuning range. That's a good result.
The next step is to determine what your accuracy or stability
requirements are. There's no point in spending extra money for an ultra
stable Vref if all you need is just 7 or 8 digits of accuracy, or if
that's all your 5386A counter can even measure.
I don't quite understand your comment about expensive resistors. Most
instruments with OCXO have an EFC adjustment, usually implemented as a
single 10T or 20T panel- or PCB- mounted pot. To first order tempco
doesn't matter because the pot is being used as a *divider* across Vref.
Yes, a temperature coefficient in the pot changes the resistance of each
leg but it doesn't change the ratio.
It's a cheap experiment for you to try. Surely you have a ~10k
multi-turn pot lying around. Measure the frequency periodically over a
day and also record temperature. As a baseline do the same with EFC
shorted. The difference between the two sets of data tells you how much
of the tempco you see is in the OCXO and how much is in the Vref /
divider / EFC.
You mentioned you have a hp 34401A as well. You could use that to
directly monitor the EFC voltage and add that to your plots. Best to
automate the data collection; do you have a GPIB adapter for your DMM
and your frequency counter?
/tvb
On 6/4/2023 11:51 AM, Reginald Beardsley via time-nuts wrote:
> Tom,
>
> Thanks. Typo is correct. I didn't describe the data properly.
>
> Those are a single measurement on 10 OXCOs to determine the 10 MHz bias voltage requirement for each. I was adjusting the PSU voltage which is rather coarse, to get close to 10 MHz (within 1 Hz) and measuring the actual voltage with a 34401A.
>
> The purpose is to calculate what precision, low temp co resistors I need to set the Vref for each oscillator. With particular care to try to make it neutral at a convenient temperature. Those resistors are seriously not cheap. The Vishay 50 ppm/C precision foil trimmers ar $25.
>
> The 5386A was running on the internal XOCO as I simply wanted order of magnitude values for each OXCO. So an accuracy of ~2 ppb (5386A and DUT) for time and <50 ppm for voltage.
>
> I'll run the same series with a better reference for Vref so all are tested for frequency at 2 V input to Vref.
>
> Reg
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