[time-nuts] Measuring 10 MHz accurately.

Bruce Griffiths bruce.griffiths at xtra.co.nz
Wed May 7 18:09:22 EDT 2008

Martyn Smith wrote:
> Hi,
> Two questions for all of you today.
> 1st Question
> I can measure 10 MHz frequency to an accuracy of about 2 parts in 10E-12 in 
> a one second gate time.
> I use a SR620 time interval counter and make a timeAB measurement.
> I make two measurements of phase (between my UUT and my reference)  and then 
> calculate the frequency.
> I have some nice software that controls the SR620 and does all the math.
> I'm sure most of you understand my measurement technique, since it's as old 
> as the hills.
> Does anyone have a good application note explain this standard procedure? 
> I'm trying to explain it to a friend, and can't find a nice application note 
> with some diagrams, formulas etc.  I know HP did one and Standard did as 
> well, but can't find any quickly.
> 2nd Question.
> I really need to measure 10 MHz to an accuracy of 1 part in 10E-13 / second 
> (ignoring the accuracy of my workshop standard).  I've tried multiplying the 
> 10 MHz to the GHz frequencies, but never manage to improve on the technique 
> I mention in my first question.
> Any ideas for a relatively cheap way of doing this.  I actually represent a 
> company that can measure to parts in 10E-15 in one second, but their boxes 
> costs $50k.
> Best Regards
> Martyn

To achieve this you need:

1) A frequency standard that has an ADEV < 1x10E-12 for Tau = 1 sec.
Such sources tend to be rare and expensive. (eg hydrogen maser, BVA OCXO 
The frequency of the standard also has to be known to better than 1x10E-12.

2) Heterodyne techniques can easily achieve the required resolution but 
achieving the low noise and drift isnt easy:
    a) The zerocrossing detector shaping the mixer beat frequency output 
has to be designed to amplify the slope whilst minimising the noise.
    The naive design approaches often advocated are inadequate.
    Low frequency ground loops can easily corrupt the measurements.
    b) The temperature of the mixer has to be held constant to better 
than 0.05C.

    c) The zero crossing detector temperature has to be held to constant 
to better than 1C.
       A naive zero crossing detector design may require temperature 
control to 0.01C or better.

    d) A low noise stable offset frequency source of accurately known 
frequency is required

Even when all of the above conditions are met the measurement range may 
only be around 0.1Hz or so.


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