Accurately measuring high frequency AC.

BQW · 5 Nov 2019

Hope some one can help with this one, strictly not a DIY job, but I have been asked at work to measure a 8 Khz, 650 Volt RMS AC voltage to a accuracy of plus or minus 5 volts.
Tried several good quality digital meters but they don't appear to respond accurately to AC of that frequency.
Did intend to try a Scope with a special HV attenuation probe, but I don't think a difference of 5 volts will not show up as any measurable size difference on the trace.
Does any one know of a meter or type of meter that would do the job (not yet tried my old AVO

Any comments and help appreciated.

JohnW2 · 5 Nov 2019

BQW said:
... I have been asked at work to measure a 8 Khz, 650 Volt RMS AC voltage to a accuracy of plus or minus 5 volts. ... Tried several good quality digital meters but they don't appear to respond accurately to AC of that frequency.

Even up-market DVMs usually seem specified only up to 500Hz or 1kHz.

BQW said:
Did intend to try a Scope with a special HV attenuation probe, but I don't think a difference of 5 volts will not show up as any measurable size difference on the trace.

I would imagine you're probably right - although it obviously depends upon how much 'offset' ('Y-shift') the scope allows.

BQW said:
Does any one know of a meter or type of meter that would do the job (not yet tried my old AVO

Have you considered doing what your 'old AVO' would do - i.e. rectifying the AC and then using a standard DVM to measure the peak DC? I wouldn't imagine that you would need too much of a capacitor to achieve adequate smoothing at 8kHz.

Kind Regards, John

SFK · 5 Nov 2019

Some ideas on here from the Audio industry:
https://www.diyaudio.com/forums/equipment-and-tools/211547-measuring-voltage-current-frequency.html

and one here that shows how best to address Oscilloscope probe effects:
https://electronics.stackexchange.c...easure-high-voltage-and-high-frequency-ripple

SFK

aptsys · 6 Nov 2019

BQW said:
Hope some one can help with this one, strictly not a DIY job, but I have been asked at work to measure a 8 Khz, 650 Volt RMS AC voltage to a accuracy of plus or minus 5 volts.
Tried several good quality digital meters but they don't appear to respond accurately to AC of that frequency.
Did intend to try a Scope with a special HV attenuation probe, but I don't think a difference of 5 volts will not show up as any measurable size difference on the trace.
Does any one know of a meter or type of meter that would do the job (not yet tried my old AVO
Any comments and help appreciated.

My 34410A is specified to work in that frequency range. Pretty sure the newer 34465A has similar specifications, and if you don't want to buy they can be easily hired from the usual suspects.

BQW · 6 Nov 2019

I have been informed by some one knowledgeable, that the old style AVO 8 Meter with its copper oxide rectifier is accurate up to 100Khz.
So its now time to blow the dust of my old AVO 8.
Thanks for the replies, I did find some suggested circuits and methods using a DMM on the net, but the maths needed to calculate the % of accuracy was beyond me and the small financial recompense I receive.

JohnW2 · 6 Nov 2019

BQW said:
I have been informed by some one knowledgeable, that the old style AVO 8 Meter with its copper oxide rectifier is accurate up to 100Khz.

As Implied, that doesn't surprise me.

However, no matter how theoretically accurate it is, are you not going to have a potential problem with anything that has an analogue display - since, presumably using something like a 1000V range, a difference of 5V is probablyalmost as small as one could hope to distinguish visually, even with the mirrored scale of an AVO (even the pointer is probably about "1 volt wide")? ...

On the other hand, if you 'did the rectifying yourself', you could then use a digital meter as the measuring device, thereby removing that problem.

Kind Regards, John

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Accurately measuring high frequency AC.

BQW

JohnW2

SFK

aptsys

BQW

JohnW2

DIYnot Local

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