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Old 6th Nov 2019, 7:40 am   #1
its ur aerial
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Default AVO Meter, frequency response.

Does any one know, if the old style AVO 8 OR 9 meters will accurately measure a AC voltage with a frequency of 7.5 to 8 Khz.
I need to very accurately measure 650 volts AC at about 7.5 Khz, with a rating of about 80 Amps..! ( so need to be a bit careful with the probs ).
I didn't realise modern DMM`s are only good to about 100Hz.
Information most welcome.

Ken G6HZG
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Old 6th Nov 2019, 7:59 am   #2
Craig Sawyers
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Default Re: AVO Meter, frequency response.

I measured my AVO8's on my HP AC voltage calibrator, and much to my surprise they are accurate to the maximum frequency of that rig - 100kHz.

I suspect that measurement is lurking somewhere on this list.

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Old 6th Nov 2019, 8:10 am   #3
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Default Re: AVO Meter, frequency response.

Thought so. Thread title "Analog avometer to measure AC on 60Hz". I measured at 10V and the accuracy was within a meter needle width at 100kHz. That was a MkIII with the old copper oxide rectifier.

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Old 6th Nov 2019, 8:21 am   #4
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Default Re: AVO Meter, frequency response.

Craig, thanks for your prompt reply, really appreciated, 7.5Kcs will be no probs then.
Ken, G6HZG.
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Old 6th Nov 2019, 8:54 am   #5
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Default Re: AVO Meter, frequency response.

Note I have not checked the response on anything other than the 10V range. It might be different on higher voltage ranges. But I can easily measure that.

Just to be clear, are trying to measure 650V rms? And for what AVO? Most of mine are II and III models.

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Old 6th Nov 2019, 9:25 am   #6
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Default Re: AVO Meter, frequency response.

OK it is different on a MkIII when on the 1000V range. At your test frequency of 7.5kHz it measures high. So 500V at 1kHz measures 500V. At 7.5kHz it measures 600V. So +20%

A Mk 6 has an even greater error - at 7.5kHz it measures 666V. So +33%

Quite different to the 10V range, presumably because of inductive and capacitive effects in the high value dropping resistors on the higher voltage range.

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Old 6th Nov 2019, 9:35 am   #7
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Default Re: AVO Meter, frequency response.

You are right - the spec of current production Fluke DVM's says "unspecified" at frequencies above 1kHz in the higher voltage ranges, but 20kHz on lower voltage ranges. Presumably because of similar effects to the AVO's on the high voltage ranges.

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Old 6th Nov 2019, 10:06 am   #8
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Default Re: AVO Meter, frequency response.

With this doubt of error, you may be better off with a scope.
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Old 6th Nov 2019, 10:30 am   #9
Craig Sawyers
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Default Re: AVO Meter, frequency response.

The old 1960's Fluke differential voltmeters had far better AC accuracy than their modern DVM's.

As an example, the 1967 vintage 887A on the 500V to 1000V range is specified as:

30Hz to 10kHz +/-0.1% and 10kHz to 20kHz +/-0.15% +0.01% of range.

Looking at the schematic, there are compensation capacitor on the input attenuator and throughout the front end.

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Old 6th Nov 2019, 10:57 am   #10
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Default Re: AVO Meter, frequency response.

I measured an EA113 avometer on a Datron standards lab calibrator some years ago. The -3dB point was about 200kHz on the 1-3-10 v ranges I checked. I didn't try higher ranges over frequency, but I think the range dividers may be better compensated than the model 8.

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Old 6th Nov 2019, 12:26 pm   #11
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Default Re: AVO Meter, frequency response.

Thinking of Datron, their all singing all dancing 19" system DVM's (like the 1071 and 1081 of the early 80's)are highly accurate 7 1/2 digit true RMS measurement at up to 100kHz on all ranges up to 1000V.

These still sell for good money. I paid around 200 for mine about ten years ago.

They were so good that Fluke bought them for the technology, and they have taken it further to 8 1/2 digit ridiculous stability and accuracy house mortgage priced instruments. https://us.flukecal.com/products/ele...nce-multimeter

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Old 6th Nov 2019, 2:09 pm   #12
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Default Re: AVO Meter, frequency response.

I have been asked to measure the out put voltage of a Induction Generator at work, its out put voltage is 650 Volts at approx 7.5Khz, with a current rating in excess of 80 Amps, ( Elf & Safety, so will need caution ) this voltage they want measured within + or - 5 Volts, hence I don`t think 5v difference will be measurable and visible on the Scope display.
Tried it with a couple of Fluke meters, not sure of the Model, buts its the type were you can detach the display from the Meter Body, but the readings were inaccurate by about 40%.
So its time to get out the old AVO 8 and I have a Mk 9.

Ken, G6HZG.
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Old 6th Nov 2019, 2:41 pm   #13
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Default Re: AVO Meter, frequency response.

Do you have a scope with 'measurement' functions?
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Old 6th Nov 2019, 3:37 pm   #14
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Default Re: AVO Meter, frequency response.

That's a lot of Watts! I would be interested to know how you get on.

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Old 6th Nov 2019, 7:25 pm   #15
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Default Re: AVO Meter, frequency response.

How about using a Potential Divider?

You could use 100:1 Divider and measure across the :1 which would be around 6.5V.
You just need to know the Ratios accurately, not the actual Resistances used, which should not pose any major issue.


Just a thought.



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Old 6th Nov 2019, 7:48 pm   #16
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Default Re: AVO Meter, frequency response.

I'm just imagining what the "line whistle" would be like....
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Old 7th Nov 2019, 12:29 am   #17
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Default Re: AVO Meter, frequency response.

The whistle would most likely sound like a professional solar panel array.
We have some a mile or so away and they sound like 7 to 10 KHZ with 100 HZ over it.
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