[time-nuts] Once again about counter calibration
kb8tq at n1k.org
Sat Apr 25 23:36:17 EDT 2015
OCXO’s come in many different “flavors”. Some older units with rack mount designs may take > 1 day simply to warm up. Older AT based units might take 10 minutes simply to stop pulling maximum power.
Stopping the maximum power pull does *not* indicate they are done drifting. Wit an AT drift in the first 24 hours might be 5x10^-8 relative to the end of the first hour. With a good SC that has only been off power for 48 hours, you might cut that down by 10X.
Note the “48 hours” above. The assumption is that the power is only briefly interrupted after being on for a long period. If the OCXO has been off power for years, time to re-stabalize is anybody’s guess. A good unit might
get to the 1x10^-8 per day level in a few days. Another unit from the exact same production batch might take 5 days to get there.
OCXO’s are designed to be constantly on power. They work best that way. Keep them on power for a while (a couple weeks) before trying to adjust one that has an unknown history.
> On Apr 25, 2015, at 11:12 PM, d0ct0r <time at patoka.org> wrote:
> I tried to use 1PPS as Ext. trigger for the oscilloscope. I was able to "stabilize" signal movement. Then I tried to calibrate OCXO. However its appeared out of range (I reach end of potentiometer limit but counter still shows that OCXO is out of 10Mhz). Which kind of "suspicious".
> Then I decide to disassemble my project to take pure 10Mhz directly from GPSDO to measure OCXO signal. Its read totally different frequency value now. So, using 1PPS didn't work for me. I tried using that GPSDO 10 Mhz as Ext. Trigger. And now I got much better result. I was able to calibrate 5386A to some extent. But my 5386 has TCXO. So, after few minutes its moving out of "perfect" value. May be I need to wait much longer to stabilize oscillator. I am not sure what to expect here.
> Using GPSDO 10 Mhz as REF signal, I was able to calibrate OCXO. And now its potentiometer position was nether at its both extremes. The reading on 5386a (using 10 sec gate) fluctuate from 9.999999993M to 10.000000007M. Again, may be I need to wait much longer when OCXO will be stable.
> So, I think the best approach will be using 10Mhz GPSDO as ref. signal for this counter. In another case, I'll need to wait to warm it up (the manual advised only 30 minutes. But I am not sure). And then re-calibrate it. Its time consuming.
> I am curious, if its practical to calibrate something like Morion MV89A and use it as signal reference for this counter ? Or OCXO still will drift out of desired frequency relatively soon ?
> On , Scott McGrath wrote:
>> Or feed both oscillators into a mixer and extract the difference
>> which will be the offset of your DUT from your standard
>> Content by Scott
>> Typos by Siri
>>> On Apr 24, 2015, at 10:36 PM, Brent Gordon <time-nuts at adobe-labs.com> wrote:
>>> Just feed the PPS into the scope trigger on step 2.
>>> The low repetition rate on the PPS would make this difficult on an analog scope; on a digital scope it is easy. You can use any sub-harmonic that is less than or equal the counter reference frequency (10 MHz).
>>>> On 4/24/2015 3:45 PM, d0ct0r wrote:
>>>> The input: HP 5386A which I would like to calibrate, Well warmed Tremble
>>>> Thunderbolt (1PPS only), 10 Mhz Datum OCXO (unknown accuracy), Rigol
>>>> 1101E Oscilloscope.
>>>> The goal is to calibrate counter to read the Datum OCXO.
>>>> Reading the manual for 5386A, there is simple schema for calibration:
>>>> 1. Connect HP 5386A 10Mhz OUTPUT to Oscilloscope
>>>> 2. Connect Frequency Standard to Ext. Trigger on Oscilloscope
>>>> 3. Adjust the frequency on 5386A TCXO for minimum sideways movement of
>>>> 10 Mhz signal
>>>> However, my Trimble TB 10Mhz output is currently in use. I have only
>>>> 1PPS signal available. The HP manual do not mention what exactly
>>>> frequency needs to be on "Frequency Standard" connected to Ext. Trigger.
>>>> Is there any method/option I could apply to calibrate HP counter ?
>>>> Thanks !
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