rms meter reading on function generator

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THENOOF
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rms meter reading on function generator

Post by THENOOF » Mon Jan 28, 2008 9:39 am

I have 5vp on my oscope my meter is not showing 3.5v rms is there something I am not doing correct?
Brad

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haklesup
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Post by haklesup » Mon Jan 28, 2008 11:33 am

What are you getting?

Bigglez
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Re: rms meter reading on function generator

Post by Bigglez » Mon Jan 28, 2008 12:05 pm

Greetings Brad,
THENOOF wrote:I have 5vp on my oscope my meter is not showing 3.5v rms is there something I am not doing correct?
Your meter may be calibrated to read ACrms for sinewaves
only. Is the waveform you're measuring a sinewave?
Also, the meter will have a limited bandwidth, trying to
measure signals outside of that range will give erronoeous
readings. Is the meter analogue or digital?

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jwax
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Post by jwax » Mon Jan 28, 2008 4:22 pm

Welcome Brad, but if you present your query with all the details, you'll stand a fair chance of getting an informative answer.
What's 5vp? What waveshape? What kind of meter?

Dean Huster
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Post by Dean Huster » Mon Jan 28, 2008 4:32 pm

Don't forget that an oscilloscope measures PEAK-TO-PEAK voltage and not peak voltage. 5v p-p translates to 2.5v pk or 3.55v rms, so that scope reading you had must have been p-p and not peak.

As mentioned, that math only works with sine waves.

Dean
Dean Huster, Electronics Curmudgeon
Contributing Editor emeritus, "Q & A", of the former "Poptronics" magazine (formerly "Popular Electronics" and "Electronics Now" magazines).

R.I.P.

THENOOF
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Post by THENOOF » Mon Jan 28, 2008 8:27 pm

I am getting like 3 milli volts rms. The wave form is a sin wave. I can't remember the freq it was probably over 10 kHz it was a 10 V peak to peak voltage the meter is a fluke 179 is the freq to high?
Thanks brad

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Post by Bigglez » Tue Jan 29, 2008 12:25 am

Greetings brad,
THENOOF wrote:I am getting like 3 milli volts rms. The wave form is a sin wave. I can't remember the freq it was probably over 10 kHz it was a 10 V peak to peak voltage the meter is a fluke 179 is the freq to high?
The meter is spec'd to cover 100KHz, so that's probably
not the issue. Time to do a bit of detective work.

How confident are you that the scope and your DMM
are operating correctly?

How confident are you that you are driving the scope
and the meter correctly?

What happens if you use the meter to measure a
known (but not necessarily accurate) voltage?
If you put the meter in an electrical outlet does it
read 120V +/- 5%?

If you use the meter to measure the frequency of
the AC mains, what do you get?

When you run these checks on the scope, what
readings do you get?

Finally, where is the signal (that you can't measure)
coming from?

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haklesup
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Post by haklesup » Tue Jan 29, 2008 1:14 pm

That's about what I would recommend, Do a reality check on a known source like a wall receptacle or a battery. Short the leads in Ohms mode to verify they are not open. The meter may simply be broken in some way or your test leads may be open or damaged.

Don't be offended if I suggest operator error since I don't have any idea of your skillset (well, you are apparently using the oscilloscope properly) . Easy mistakes on a DMM include plugging the probe into the A instead of the V jack and setting the meter to read DC instead of AC (I contatntly do the opposite of that since my Tek meter defaults to AC, though not very likely with Fluke since AC and DC are on the main selector, not a secondary button). Another common cause is a blown fuse in the meter but that usually just kills the current measurements.

3mV is essentially open and reading ambient noise. I'd trust the scope since they don't just make stuff up for the most part. The fluke also reads "true RMS" so waveshape shouldn't have messed it up so much. Try poking the range button, maybe its stuck in an invalid range.

Finally about the point you are probing. Is the board coated? are you probing the same points as you connected the scope probe? Maybe its just a contact problem at the probe tip and the PCB contact point. Try scraping the metal before probing. This happens to me a lot because we coat our boards.

THENOOF
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Post by THENOOF » Tue Jan 29, 2008 7:46 pm

I am sure the meter is good. this is in a class room setting all five students were getting the same results. the AC was coming from a function generator that was not hooked up to a load we were just learning how to read a sine wave from a generator I had them hook up the fluke and see what the rms value was they calculated it and were going to verify it with the meter.

Brad

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haklesup
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Post by haklesup » Tue Jan 29, 2008 8:44 pm

Exactly where were the probes applied to the signal generator. Only thing I can think of is an open ground or not common ground point.

I would use a BNC to banana adapter and just meter off the two terminals but in the absence of an adapter, meter from center conductor to shell of a BNC (jack or plug).

Bigglez
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Post by Bigglez » Tue Jan 29, 2008 11:22 pm

Greetings Brad,
THENOOF wrote: I am sure the meter is good.
Then prove it!
THENOOF wrote: this is in a class room setting all five students were getting the same results.
So set a good example by not making assumptions.
"Trust but Verify" Ronald Reagan, 1989

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Dean Huster
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Post by Dean Huster » Wed Jan 30, 2008 5:08 am

I had a student once who drug me around the classroom with his circuit showing me how every single one of the Tektronix 2213 scopes was bad with exactly the same failure. He had a little trouble with common denominators in math, too.

Dean
Dean Huster, Electronics Curmudgeon
Contributing Editor emeritus, "Q & A", of the former "Poptronics" magazine (formerly "Popular Electronics" and "Electronics Now" magazines).

R.I.P.

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