[time-nuts] 10MHz Rubidium reference source for frequency counter

Karen Tadevosyan ra3apw at mail.ru
Tue Nov 4 03:57:12 EST 2014


Hi Tom,
Thank you for the comments. The results are better than my expectation. 
Please find the new result with two Morion's OCXO: the first is an external reference for CNT-91 and second - DUT.

https://dl.dropboxusercontent.com/u/21338179/hamradio/OCXO_Adev/morion%20ocxo%20vs%20morion%20reference%20adev%20freq%20mode%20smartfreq%20sample_1s%2020141104.jpg

Blue line - Frequency mode / Sample interval 1s / Smart Freq OFF / CNT-91 reference source - internal OCXO
Crimson line - Frequency mode / Sample interval 1s / Smart Freq ON / CNT-91 reference source - internal OCXO
Green line - Frequency mode / Sample interval 1s / Smart Freq ON / CNT-91 reference source - external Morion OCXO

Regarding CNT-91 Max Measurement Rate (p.8-10 of CNT-91's User manual Rev.18):
1) via GPIB - 2000 readings/s (block) / 350 readings/s (individual). Hope it's OK for my Prologic USB-GPIB interface ...
2) To internal memory - 250 k readings/s

> One other thing to try is the ADEV function on the front panel. The CNT-91 is
> one of the few counters that calculates Allan deviation, not just mean and
> standard deviation. With this, you do not get a fancy log-log ADEV(tau) plot
> but the number reported will be valid for the gate time (tau) that you have
> selected. The advantage of this is that you don't need any GPIB adapter or
> computer connection or CNT-91-compatible PC software. So maybe try tau
> 1, 5, 10, 20, 50 and write down what it reports.

Well, I use this feature (STAT/ADEV) also but only for the first time look. 
IMHO, if someone has GPIB interface TimeLab solution is more powerful and provides much more opportunities ...
It also makes possible to document the results for further use.
I didn't find how to change tau value for this mode and it seems by default tau is 1 sec.

> As I understand it, his project is to
> use the high frequency output of a ublox NEO-7M to discipline a MV89 with a
> James Miller-style analog PLL.

This is a beginning of the project to get a compact and non-expensive GPSDO solution on base of uBlox GPS.
The links are:
http://www.ra3apw.ru/proekty/ublox-neo-7m/
http://www.ra3apw.ru/ublox-neo-7m-ocxo-gpsdo/

Karen 
 
> Date: Mon, 3 Nov 2014 06:43:06 -0800
> From: "Tom Van Baak" <tvb at LeapSecond.com>
> 
> Hi Karen,
> 
> Ok, you are making very good progress. Thanks for the update.
> 
> Your new ADEV plot, "Morion OCXO ADEV Freq mode SmartFreq Sample_1s
> InterpCalib_On.jpg" is now looking more like I would expect; it is showing the
> expected trade-off in measurement speed and measurement resolution. I
> see you are getting below 5e-12, past about tau 10 seconds. There are some
> periodics in the measurement which might go away with different trigger
> levels or better isolation or much larger frequency delta between the
> external ref OCXO and external input OCXO.
> 
> Attached is CNT91-error.gif -- the uncertainty calculation as found in chapter
> 8 of the CNT-91 manual. It is worth reading the details of this chapter to gain
> further understanding of the measurement process.
> 
> The single-shot spec for this counter is 50 ps. But from the equation you can
> see why they are able to claim 1 ps rms resolution for sufficient averaging
> (up to 1000 measurements per second). This works because the CNT-91 is an
> extremely quick continuous time-stamping counter with massive internal
> memory (something like 750,000 readings in internal RAM). And it has true
> back-to-back (zero deadtime) period / frequency measurements, which are
> available over GPIB.
> 
> The question is if these GPIB capabilities are compatible with TimeLab, which
> may just treat this as "dumb" frequency counter instead of a high speed
> timestamping counter. I will continue to test my own CNT-91 here to see if I
> can improve on the noise floor. I will loan John my CNT-91 if it looks like a
> change to TimeLab is necessary; to make best use of raw high-speed
> timestamp-based measurements.
> 
> One other thing to try is the ADEV function on the front panel. The CNT-91 is
> one of the few counters that calculates Allan deviation, not just mean and
> standard deviation. With this, you do not get a fancy log-log ADEV(tau) plot
> but the number reported will be valid for the gate time (tau) that you have
> selected. The advantage of this is that you don't need any GPIB adapter or
> computer connection or CNT-91-compatible PC software. So maybe try tau
> 1, 5, 10, 20, 50 and write down what it reports.
> 
> The other question is the validity of the ADEV calculation when the counter's
> "smart" features are turned on. The good news is that this should not impact
> your goal to compare multiple oscillators against each other; since that is
> essentially a pair-wise, relative comparison. But it may affect the absolute
> accuracy of the calculations themselves. Its a subtle point, which you are
> welcome to ignore for now.
> 
> Yes, it is possible that some of the OCXO that you are testing are near or
> even better than the short-term measurement ability of the CNT-91. In this
> case either you can be very happy to have such good OCXO, or you can
> develop a new method to compare oscillators at high resolution at short
> times.
> 
> Time-nuts -- Have a look at Karen's intro page
> (http://www.qsl.net/ra3apw/ra3apw/). As I understand it, his project is to
> use the high frequency output of a ublox NEO-7M to discipline a MV89 with a
> James Miller-style analog PLL. Some links:
> 
> http://www.jrmiller.demon.co.uk/projects/ministd/frqstd0.htm
> https://sites.google.com/site/g4zfqradio/u-blox_neo-6-7
> http://www.ra3apw.ru/proekty/ublox-neo-7m/
> (perhaps someone can post an English translation for us)
> 
> Thanks,
> /tvb
> 



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