[time-nuts] How to measure Allan Deviation?

Dr Bruce Griffiths bruce.griffiths at xtra.co.nz
Mon Oct 23 17:52:21 EDT 2006


Tom Van Baak wrote:
> Didier,
>
> I've been out of town and I see a flurry of postings to
> your original query about Allan deviation. It sounds
> like your goal is to measure the stability of various
> oscillators that you have lying around?
>
> First, your 5370 or any other TIC (Time Interval Counter)
> will be adequate for this. I'd suggest using a 'scope on
> the input channel(s) to make sure your signal and trigger
> levels are what you think they are. Outliers or high 5370
> STDEV is a hint of poor triggering.
>
> It sounds like you have the computer data logging part
> solved. Most ADEV tools take accumulated phase
> difference data; just the data that a 5370 in TI mode
> will generate.
>
> Second, your Jupiter GPS board, or any other sub-100 ns
> GPS timing board, will work fine. Use the 1PPS output.
>
>
> You don't have to build a GPSDO in order to make the
> measurements that you want! In fact, it may only get
> in the way. Instead, consider these two points.
>
> 1) For short-term stability tests, just make a set of
> quick pair-wise measurements between stand-alone
> oscillators at short gate-times. If they are close in
> frequency, or can be made close in frequency, then
> the 5370 built-in STDEV statistic is useful in real-time
> (use an N or 10 or 100). If not, collect raw phase data
> and run it through your favorite ADEV tool.
>
> This method should work fine for fractions of a second
> to 10s or 100s of seconds.
>
> The good and bad pairs will be obvious; in a matter of
> minutes you can tell which one or two oscillators are
> the best, short-term.
>
> Remember that your measurements are the RMS sum
> of both oscillators and the counter itself so there will be
> limits to the resolution, especially visible at short times.
>
> 2) For long-term stability measurements, one at a time,
> compare your oscillators against the raw GPS 1PPS,
> collect phase data, and run it through your ADEV tool.
>
> To avoid phase wrap-around, depending on the frequency
> offsets, you may need to divide the 5/10 MHz oscillator
> down to at least the kHz range, or the obvious 1PPS, to
> get clean phase measurements.
>
> Let the GPS 1PPS be the start channel. For this sort
> of long-term measurement any TTL/CMOS homebrew
> divider will work (the jitter and tempco are in the noise).
>
> It's OK to average the samples down by 100 to 1000
> to reduce the volume of data you collect or process.
> I often use something around 300 (5 minutes).
>
> This will give you frequency offset, mid- to long-term
> stability, and frequency drift information. A couple of
> hours or days of data per oscillator should be sufficient.
>
> Again, you don't need to build a GPSDO for any of this.
>
> /tvb
>
>
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>   
Tom

In comparing 2 oscillators do you mean

1) Connecting one oscillator to the FREQ STD input at the rear of the 
5370A, selecting the external timebase and connecting the other 
oscillator to the Front panel FREQ/PERIOD input and then selecting 
frequency measurement which in effect gives the frequency ratio of the 2 
oscillators?

OR

2) Connecting the 2 oscillators to the LO and RF ports of a mixer, 
lowpass filtering the mixer output and measuring the beat frequency?

If the temperature varies over a large range whilst collecting PPS 
timing data, any old divider will not suffice as particularly with a 
CMOS ripple counter (eg cascaded 74HC390's) the propagation delay 
(tempco ~ 0.4%/K) will vary significantly as will the receiver delay.

Bruce



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