[time-nuts] How to measure Allan Deviation?

Magnus Danielson cfmd at bredband.net
Mon Oct 23 18:22:35 EDT 2006

From: Dr Bruce Griffiths <bruce.griffiths at xtra.co.nz>
Subject: Re: [time-nuts] How to measure Allan Deviation?
Date: Tue, 24 Oct 2006 10:52:21 +1300
Message-ID: <453D3995.8080004 at xtra.co.nz>

> Tom

Hi Bruce,

> 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?

I have advocated this approach, but nobody seemed to care.

> 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?

Could potentially cause very long cycles which could mean unknown amounts of
wrap in the counters time-counter. Unless you ensure them to be sufficiently
offtuned so it is a very small risk.

Actually, I think he meant:

0) Connect the two oscillators to start and stop channels and measure the TI

> 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.

You will handle that to a high degree by a synchronising D-flip-flop clocked
with the input clock and taking the divided PPS for input. Then most effort is
on that one alone.


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