Author Topic: 'Netting' frequency for calibration  (Read 1771 times)

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Offline CJay

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'Netting' frequency for calibration
« on: June 03, 2015, 11:36:47 pm »
It's late here in the UK and my brain might not be working correctly so I may be about to make a fool of myself but...


Two frequency counters with an internal 10MHz reference that's brought out to a BNC on the rear panel (Racal-Dana 1998 for instance).

Connect them together so counter A's reference is displayed by counter B.

Add another cable so counter B's reference is displayed by counter A.

Adjust A and B reference oscillators until both displayed frequencies coincide.

Result, both reference oscillators are the same frequency and should be extremely close to 10MHz?

Or am I missing something subtle?

Or worse, something blindingly obvious?

FWIW, I've just bought a pair of 'faulty' 1998s, both now repaired but in need of calibration. I'll be buying a Rubidium oscillator as soon as funds allow so should be able to calibrate them against that next month but as a thought exercise the above occurred to me.
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Offline Ian.M

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Re: 'Netting' frequency for calibration
« Reply #1 on: June 04, 2015, 12:25:06 am »
That doesn't do you any good.   

Lets suppose A's reference oscillator is 0.01% too fast.  All its gate times will be 0.01% too short so an input signal from a known good frequency standard will read 0.01% low.      If B's oscillator is also exactly 0.01% fast,  both will read 10.000 MHz even though the actual frequency is 10.001MHz.   This means that as you adjust them to coincide there is a significant risk that you will increase the error.   

Its unfortunate that the Beeb no longer transmit analog TV as the colour subcarrier was closely controlled and analog colour sets had an oscillator locked to it that could be picked up with a tuned sniffer coil.

Take a look at http://www.npl.co.uk/science-technology/time-frequency/products-and-services/time/msf-radio-time-signal.  Its a 60KHz caesium atomic clock controlled carrier with CW modulation.  You can use a regenerative TRF receiver to get the carrier, detect it and put it through a 550ms non retriggerable monostable to get a one pulse per second signal that you can use to externally arm the 1998 which should be set up for 100ms gate. You can then get a stable measurement of the 60KHz carrier frequency even though its CW modulated as the 1998 will only be counting when the carrier is on.    That should give you a four digit check.  For a better calibration, you'd need to lock a stable local oscillator to the carrier, and frankly it isn't worth the effort to haywire something that complex if you are planning to get a rubidium oscillator anyway.
« Last Edit: June 04, 2015, 12:47:03 am by Ian.M »
 

Offline Solder_Junkie

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Re: 'Netting' frequency for calibration
« Reply #2 on: June 04, 2015, 07:09:06 am »
Unfortunately a rubidium oscillator drifts and needs netting to an off air standard... So you will eventually end up begging, buying or making an off air standard.

There are some useful articles on making both MSF (60 KHz) and GPS locked standards at http://www.g4jnt.com/freqlock.htm
I used to use a JNT MK2 MSF standard before getting a GPS derived one.

Some of the 60 KHz standards such as a Quartzlock 2A and the G4JNT MK 2 (as per the link above) have a short term instability of the order of 1 part in 10^8. However they are good enough for most purposes.

You set the Racal internal oscillators by comparing the phase difference between them and an off air standard, an ordinary dual channel oscilloscope will be good enough to show the difference and will also show the drift of a rubidium over a period of 2 or 3 weeks.
 

Offline CJay

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Re: 'Netting' frequency for calibration
« Reply #3 on: June 04, 2015, 07:14:07 am »
Thank you, I knew there had to be a failing in my thinking but for the life of me I couldn't put my finger on it last night. (never afraid to be proven wrong because that way I can learn)

The method would be ideal if all I wanted was a consistent indication but I might as well just pipe the reference oscillator of one counter to the external reference input of the other if I want that.


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Offline CJay

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Re: 'Netting' frequency for calibration
« Reply #4 on: June 04, 2015, 07:23:48 am »
Unfortunately a rubidium oscillator drifts and needs netting to an off air standard... So you will eventually end up begging, buying or making an off air standard.

There are some useful articles on making both MSF (60 KHz) and GPS locked standards at http://www.g4jnt.com/freqlock.htm
I used to use a JNT MK2 MSF standard before getting a GPS derived one.

Some of the 60 KHz standards such as a Quartzlock 2A and the G4JNT MK 2 (as per the link above) have a short term instability of the order of 1 part in 10^8. However they are good enough for most purposes.

You set the Racal internal oscillators by comparing the phase difference between them and an off air standard, an ordinary dual channel oscilloscope will be good enough to show the difference and will also show the drift of a rubidium over a period of 2 or 3 weeks.

I've seen the phase difference 'scope method, was going to use that with the rubidium and I've also seen Andy's site, his code was very helpful understanding the AD devices when I wrote the firmware for my DDS Sig Gen. Will take a look at his off air standard later today.

There was a very simple GPS DO in GQRP by G4OEP, I've got a small pile of U-Blox GPS modules which offer PPS output and a couple of active antennae in the parts bin so I might breadboard that later and see how I get on with it while funds accrue for the rubidium oscillator.
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