[time-nuts] Allan Variance with an HP53132a counter
Tom Van Baak
tvb at LeapSecond.com
Tue Jan 24 11:56:19 EST 2017
> I developed a home-brewed OCXO crystal oscillator disciplined with a long wave radio
> station signal (162 kHz). Its working quite well with a long term stability that is
> « probably » better than 10E-8 which I am actually looking forward to better understand.
> I am considering using an HP53132 counter for this purpose, although I understand that>
> it may not be the most recommended approach….
Hi Gilles,
1)
The HP53132 is fine for this application. 1e-8? No problem at all.
For distortion-free, long-term, automated measurements use the counter in TI mode instead of FREQ or PER mode. This is very simple: just divide your 162 kHz signal down to, say, 1 Hz and then compare that against a reference 1 Hz, such as a GPS 1PPS. That way you get a clean [phase difference] time series of your clock against GPS.
Your raw data is directly importable into TimeLab [1] and from that you will get correct phase / frequency / stability plots. The jitter in the GPS/1PPS will "drop out" quickly, especially if your OCXO is only 1e-8.
2)
In this case you only need a plain GPS board with 1PPS [2] [3]. Lots of these on eBay in the $20 to $40 range as well [4]. In other words, you do not need a GPSDO; at this stage of your being a time nut that only adds new layers of complication. So just compare phase and keep it simple.
3)
If you want lower cost, or more resolution than a HP53132, consider John's new TICC board [5], which will match and even out-perform the 53132. Both the 53132 and TICC are directly usable into TimeLab.
/tvb
[1] http://www.ke5fx.com/timelab/readme.htm
[2] https://www.adafruit.com/product/746
[3] https://www.parallax.com/product/28509
[4] for example, search eBay for LEA-6T and search the web for LEA-6T threads
[5] https://www.tapr.org/kits_ticc.html
----- Original Message -----
From: "Gilles Clement" <clemgill at club-internet.fr>
To: <time-nuts at febo.com>
Cc: "Gilles Clement FREE" <clemgill at club-internet.fr>
Sent: Sunday, January 22, 2017 6:52 AM
Subject: [time-nuts] Allan Variance with an HP53132a counter
Hi,
I am new to this list and have been following it for a while with great interest. I am afraid I am getting contaminated with the "TN" virus… !
I developed a home-brewed OCXO crystal oscillator disciplined with a long wave radio station signal (162 kHz). Its working quite well with a long term stability that is « probably » better than 10E-8 which I am actually looking forward to better understand. I am considering using an HP53132 counter for this purpose, although I understand that it may not be the most recommended approach….
Please find hereafter the rationale:
- The HP53132 implements a triangular averaging algorithm (Delta averaging) in its standard frequency or period measurement mode of operation
- This feature provides a pretty high resolution (up to 0.5*10E-12 if I am correct) which should be appropriate at least to get a first idea of the oscillator performance
- But it comes with the drawback that an Allan Variance computed directly from such frequency (or period) readings, would be distorted with respect to the "standard" Allan Variance (which assumes a PI averaging instead of a Delta averaging algorithm to compute the fractional frequencies)
- However, from the Australian paper titled : « Considerations on the Measurement of the Stability of Oscillators with Frequency Counters » I noticed that:
When I look at OCXO crystals oscillator stability discussions in the literature, White Frequency, Flicker Frequency and Random Walk Frequency seem to be the dominant noise factors considered (ex: in phase noise spectral density graphics)
The distortion from Allan Variance to Delta Variance is limited for White Frequency, Flicker Frequency and Random Walk Frequency noises (though it can be several orders of magnitudes for White Phase and Flicker Phase noises)
Moreover formulas are provided in the paper to evaluate the amount of such distortions. The impact appears to be a simple multiplier of 1.33, 1.30 and 1.15 for the three corresponding noises, the slopes of the curves are not impacted (i.e: tauE-1 ; tauE+0 and tauE+1)
- So the idea is the following: start with a simple bench test, reading the standard mode of Periods measurement, calculate the "naive" Triangular Variance from these data, identifying the noise types (according to the slope of the curve sections), and applying the corresponding offset correction, hopefully leading to an estimate of the "standard" Allan Variance.
- This approach would have the great advantage to be simple and cost effective, as one can find nowadays second hand 53132’s at reasonable cost even with the (mandatory) OCXO option.
Comments welcomed,
Best,
Gilles.
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