How to measure AC level of 100 KHz

Thread Starter

Wendy

Joined Mar 24, 2008
21,914
Most DVMs are rated 40-400 Hz, Any suggestions beside an oscope how to measure this frequency?
 

Reloadron

Joined Jan 15, 2015
4,749
You can look for an RF Voltmeter or would a scope work? If you have a clean sine wave I would use a scope and look at the peak to peak and do the math. As to an actual RF Voltmeter names like HP and Boonton come to mind.

Ron
 

Thread Starter

Wendy

Joined Mar 24, 2008
21,914
I was thinking about a precision diode. With a Hi freq op amp I'll post the circuit later anyone know of a AC voltmeter that goes into the ultrasonic range?
 

strantor

Joined Oct 3, 2010
5,178
Would an old school analog VOM work?
I don't see why an analog meter movement would care about frequency.
Simpson 260 datasheet doesn't give any frequency stipulation for voltage measurement.


EDIT: actually in the owners manual it gives frequency info:

upload_2017-12-16_23-40-32.png
 

Wuerstchenhund

Joined Aug 31, 2017
187
Most DVMs are rated 40-400 Hz, Any suggestions beside an oscope how to measure this frequency?
What voltage levels are we talking about? And what precision/resolution do you need?

In general, a DMM like the Keysight U1252B which is spec'd for frequencies up to 100kHz in some of its voltage ranges can be used, although there will be a certain amount of error. A RF voltmeter could be another option, provided its frequency range goes down to 100kHz.

But I still think a good DSO (set to average/HiRes/ERES) is probably the best way to measure the AC voltage. Especially if the signal isn't a sine wave.
 

OBW0549

Joined Mar 2, 2015
3,105
I have an oscope I won' to able to access for years (most likely) due to my stroke. I'm thinking in terms of a low budget hobbyist bench.
There's low budget, and then there's LOW budget. Not sure which you're referring to.

I have what I consider a rather middling (some would call it low-end) bench DVM, a B&K Precision Model 5491B, that gives true RMS AC voltage measurements up to 100 kHz. It's not quite what I would call "professional" equipment, but it serves me well now that I'm retired. And at ≈$400 it fit my budget at the time.

If you don't want to spend that kind of money, perhaps the best thing would be to put together a precision AC-to-DC converter circuit and measure its output with a DC DVM. This version is from NatSemi appnote AN-31:

Untitled.png

If you want to measure to 100 kHz, replace the LM101 with a modern, fast op amp such as an LM6171 to maintain accuracy (eliminate C1 and C3, of course).
 

Thread Starter

Wendy

Joined Mar 24, 2008
21,914
There was a time I might have bought that meter, But I have been unemployed for 3years (The stroke happened on my way home from a job interview. I have several nice DVMs, but they aren't that nice. For me this is very much a theoretical exercise.I will get back in the swing if and when I can walk again. My left arm will likely take longer. So I think of what I would like to do, and exercise my mind.which was also hit by the stroke I love concept circuits, drawing schematics and writing. Thanks for the heads up on the BK meter.
 

Reloadron

Joined Jan 15, 2015
4,749
For what it may or may not be worth I ran a 100 KHz signal into my scope. I set the amplitude up for 8 Volts peak to peak. Doing the math this would be just about 2.82 volts RMS. Using a Fluke 87 I read about 2.9 volts and then using one of my old Simpson 260 VOMs I read about the same. The old analog Simpson has uncertainty specified by range and using the 10 Volt range it was just about right on. So what it comes down to is either the old Simpson 260 or the new Fluke 87 have no problem with measuring an AC voltage at 100 KHz. While not exactly a high end calibration frequency response test both meters certainly did well enough for general use.

Ron
 

MrAl

Joined Jun 17, 2014
6,833
Most DVMs are rated 40-400 Hz, Any suggestions beside an oscope how to measure this frequency?
Hello,

Partly depend on the amplitude you need to measure. At 0.1vrms it is harder to read than with 10vrms at 100kHz.

A Schottkey diode and cap works, but you really need to calibrate it with an oscope first for the low level stuff.
If you can get someone to calibrate it for you at several voltages levels, youve got the cheapest AC meter on the planet :)

A preamp would work nice too as you thought of already as long as it can handle the frequency and output amplitude with appropriate slew rate.

Many years ago we did this for a project that went up to around 300kHz so that all of the readers could measure their signals without having to buy an oscope.

BTW i like the abbreviation "oscope" as i hate having to type out the full word :)
I think some people use >o'scope< but im fine with any way of abbreviating it.

The thing to watch out for using a regular meter is not how well it measures a voltage at a single frequency, but how the reading changes with frequency.
 

DickCappels

Joined Aug 21, 2008
6,014
I think this will work well. You would use a compensated attenuator for voltages above 200 mv. It takes a lot of fun out of designing a voltmeter but it makes a very accurate True RMS measurement and has plenty of bandwidth.

upload_2017-12-18_19-31-46.png
upload_2017-12-18_19-29-56.png
http:// www.linear.com/product/LTC1968
www.linear.com/product/LTC1968 There are similar parts on the market, but I don't know if they can handle 400 kHz satisfactorily.
http://www.linear.com/product/LTC1968
As an alternative, I have a box containing 10,000 custom 0.5V 8.0 ma incandescent bulbs that can do the job nicely -the photodetector is available separately.
 

MrAl

Joined Jun 17, 2014
6,833
I think this will work well. You would use a compensated attenuator for voltages above 200 mv. It takes a lot of fun out of designing a voltmeter but it makes a very accurate True RMS measurement and has plenty of bandwidth.

View attachment 141940
View attachment 141939
www.linear.com/product/LTC1968 There are similar parts on the market, but I don't know if they can handle 400 kHz satisfactorily.
As an alternative, I have a box containing 10,000 custom 0.5V 8.0 ma incandescent bulbs that can do the job nicely -the photodetector is available separately.
Hi,

Oh that's cheating :)

Seriously did you find one in a dip pack? That would make a nice addition to some circuits i have.
I little bit pricey perhaps at 8 dollars a pop, but perhaps worth it for some stuff.

That would make a nice attachment for any meter. Build a tiny box to hold it, some banana plugs to connect to your DVM.
 

DickCappels

Joined Aug 21, 2008
6,014
Analog Devices makes something very similar in an 8 pin DIP, the AD737 (Also what looks like an 8 pin SO). They were about $4 each in small quantity. The performance does not approach that of the Linear Technology offerings but it was just fine for audio device R&D.
 

MrAl

Joined Jun 17, 2014
6,833
Analog Devices makes something very similar in an 8 pin DIP, the AD737 (Also what looks like an 8 pin SO). They were about $4 each in small quantity. The performance does not approach that of the Linear Technology offerings but it was just fine for audio device R&D.
Hi,

One of the things i liked about the previous chip (LT device) you found was that it goes up to 100kHz and beyond. I think that is a big plus for measuring AC signals which might be 100kHz or a little above.
20kHz wouldnt be too bad i guess.
 

Thread Starter

Wendy

Joined Mar 24, 2008
21,914
Actually accuracy may not be important. My original thought was comparisons for ratio measurements Love the IC suggestions,
 
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