[time-nuts] Would this work as a frequency standard? Would it damage the Stanford Research function generator?

Alan Melia alan.melia at btinternet.com
Mon Jul 20 13:03:54 EDT 2015


Hi David I live a little to the North of you and I have used R4 a number of 
years ago...In fact I spotted and reported to NPL  "breathing" or hunting on 
the output compared to my two Austron 1250As the period was about 100secs. I 
was emailled a couple of months later to say the exciter had failed.....not 
the Rb but something in the chain and the contractor didn't even have an 
off-air standard receiver. It did not affect the long term average in parts 
to 10^12 but it made the short term accuracy very poor ......up to 1.5 in 
10^8 over 10secs

I gave up monitoring it when I got regular wideband interference which 
temporaly unlocked my off-air standard so check you can get a clean 
continuous signal. Daytime performance is not too bad but after-dark is not 
so good.

Some earlier EG&G Lock-in amps have a PLL function, outputting a DC signal 
that is the phase difference.

Alan
G3NYK

----- Original Message ----- 
From: "Dr. David Kirkby (Kirkby Microwave Ltd)" 
<drkirkby at kirkbymicrowave.co.uk>
To: "Discussion of precise time and frequency measurement" 
<time-nuts at febo.com>
Sent: Sunday, July 19, 2015 3:49 PM
Subject: [time-nuts] Would this work as a frequency standard? Would it 
damage the Stanford Research function generator?


>I had a plan to construct an "off-air" frequency 10 MHz standard based on
> the 198 kHz from the Droitwich radio 4 transmitter, just by hooking up
> various bits of test kit and writing a computer program to control the
> equipment via GPIB. No electronics, apart from an antenna, would need to 
> be
> built at all.  My thoughts were the following - hopefully it makes sense,
> if not I could add a drawing.
>
> 1) Connect an antenna suitable for 198 kHz into the input terminal of an
> EG&G 7260 dual-phase lock-in amplifier.
>
> http://www.signalrecovery.com/download/190163-A-MNL-D.pdf
>
> This is not a very nice lock-in to use interactively, but is okay via 
> GPIB.
>
> 2) Set the EG&G lock-in amplifier to use an external reference.
>
> 3) Feed the reference input of the lock-in amplifier from a Stanford
> Research DS345 30 MHz function generator
>
> http://www.thinksrs.com/products/DS345.htm
>
> which is set to output a 198 kHz square (or sine?) wave. The function
> generator has a frequency resolution of 1 uHz, and 12 bits of resolution 
> on
> the DAC. This can produce arbitrary waveforms.
>
> This function generator has a 40 MHz crystal, the frequency of which can 
> be
> set by adjusting the Calbytes via GPIB - similar to the SR620 
> time-interval
> counter. Obviously if the frequency of the 40 MHz oscillator is high, so
> the output frequency will be above 198 kHz. The specification of the
> oscillator are
>
> Frequency: 40 MHz (according to the manual, unless a typo)
> Type: Ovenized AT-cut oscillator
> Stability: < 0.01ppm, 20 - 60°C
> Aging: < 0.001ppm/day
> Short Term: < 5 x 10-11 1s Allan Variance
>
> 4) Use the lock-in as a phase detector, measuring the difference in phase
> between the function generator and the 198 kHz from Droitwich. The
> sensitivity of the lock-in, as well as the time-constant of the filters,
> can be adjusted via GPIB.
>
> 5) Write a computer program that tries to set the oscillator in the
> function generator to exactly 40 MHz via GPIB, based on the rate of change
> of phase between Droitwich and the function generator.
>
> 6) Since the function generator has a 10 MHz reference output, which is
> derived from the 40 MHz crystal, It should give me a solderless 10 MHz
> frequency standard.
>
> Does this seem viable, or have I overlooked something? I'm not looking to
> build a lab standard - just have a bit of fun.
>
> Does anyone know if regularly writing to the calbytes of the function
> generator would damage it?
>
> Dave
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