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)"
<[email protected]>
To: "Discussion of precise time and frequency measurement"
<[email protected]>
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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