[time-nuts] Measuring 10 MHz accurately: correction

Tom Duckworth tomduck at comcast.net
Thu May 8 03:14:02 EDT 2008

To all, regarding measurement a 10 MHz signal to parts in E-12.

Here is one way which I believe I had sent a while ago:

You will need the following test equipment:
Traceable 10 MHz Frequency Standard (GPS disciplined Rubidium, Cesium, etc.)
40 GHz Synthesized Signal Generator
40 GHz Frequency Counter with 12 digits of resolution at 40 GHz (1/10 Hz

Setup. Connect the equipment as follows. The 10 MHz signal from the
traceable Frequency Standard is connected to the Synthesizer's external
reference input. Set the output of the Synthesizer to 40 GHz (or the highest
frequency that the test counter can count). Connect the Synthesizer output
signal to the microwave input of the test counter. Connect the '10 MHz Out'
signal from the DUT to the 'External Reference Oscillator Input' on the test
counter. Set the resolution on the test counter to display the greatest
resolution possible.

This method effectively multiplies-up the traceable Frequency Standard's 10
MHz output to a traceable 40 GHz signal into the test counter's microwave
input (with the accuracy of the frequency standard). The accuracy of the
test counter's readout of this 40 GHz signal is now dependant on the
accuracy of the 10 MHz in the DUT.

To interpret the counter's readout of the 10 MHz DUT, a reading of:
40.000 000 000 1 GHz is equivalent to 2.5 parts in 10E-12
40.000 000 003 GHz is equivalent to 7.5 parts in 10E-11
40.000 000 023 GHz is equivalent to 5.8 parts in 10E-10
40.000 000 040 GHz is equivalent to 1 part in 10E-9
40.000 000 800 GHz is equivalent to 2 parts in 10E-8
40.000 006 000 GHz is equivalent to 1.5 parts in 10E-7; etc.

Tom Duckworth
-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of Bruce Griffiths
Sent: Wednesday, May 07, 2008 5:13 PM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] Measuring 10 MHz accurately: correction

Bruce Griffiths wrote:
> 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
> Martyn
> To achieve this you need:
> 1) A frequency standard that has an ADEV < 1E-13 for Tau = 1 sec.
> Such sources tend to be rare and expensive. (eg hydrogen maser, BVA 
> OCXO etc).
> The frequency of the standard also has to be known to better than 1E-13.
> 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 0.005C.
>    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 0.001C 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.
> Bruce

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