Author Topic: Bench DMMs and GPSDOs  (Read 282 times)

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Offline Electro Fan

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Bench DMMs and GPSDOs
« on: September 23, 2020, 05:27:15 am »
Probably a dumb question, but why do bench DMMs not have a 10 MHz reference input and output?  It seems like if the voltage reference in a DMM is go good that it doesn’t need a ref input then it would be useful to have a ref output, and vice versa.  Plus it would seem that it could be useful to have all the gear in a lab sync’d.  Might not be economical for entry DMMs but for some in the midrange and beyond it might have some utility.  I’m guessing the answer might have something to do with short term clock accuracy vs long term stability but it seems like we are headed toward having both at increasingly affordable costs.  After seeing how a GPSDO output can drive scopes, counters, generators, and spectrum analyzers, why not also bench DMMs?

Offline HKJ

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Re: Bench DMMs and GPSDOs
« Reply #1 on: September 23, 2020, 05:54:58 am »
Probably because they do not really have any use for it. A frequency reference input would not improve voltage, current or resistance readings.
A frequency counter in a DMM would, of course, improve with it, but that function is a very minor function in a DMM.

Offline tkamiya

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Re: Bench DMMs and GPSDOs
« Reply #2 on: September 23, 2020, 06:13:28 am »
There is really not good economical ways to use frequency reference as a voltage reference and vise versa.  I do prefer separate instruments for measurement of separate properties.

Also, GPSDO is NOT that precise short term.  Good ovenized crystal oscillator will be orders of magnitudes better for up to 100 seconds or so.  In order for GPSDO to reach 10^-12 or -13 level, it'll have to be averaged over 24 hours or longer, and also have to have a very good OCXO in it.  So even IF you could use GPSDO as voltage reference (which you cannot), you'll end up measuring for long time to make use of that accuracy.

Offline jpb

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Re: Bench DMMs and GPSDOs
« Reply #3 on: September 23, 2020, 07:35:11 am »
The only reason I can think of is for logging, but timing for that doesn't need to be that accurate as measurements themselves are not that speedy and results can always be time-stamped externally by the PC or whatever else is controlling them.

Some (perhaps many) DMMs have a trigger input for measurements (at least I think they do) so timing aspects can be kept external.

I have just acquired a Gossen Metrahit ISO DMM (hand held rather than bench) which has a real time clock for logging. Looking at the manual I realise this has a down-side in that the thing draws power and drains batteries even when off.

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