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  #1  
Old 10-23-2007, 09:52 PM
cheddy cheddy is offline
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Default Multimeter measuring AC voltage

I bought a 29 range digital multimeter from radio shack "22-813" (Link to Owners Manual)

1. What is a "AC voltage riding on a DC source bias" mean?

2. The specifications say that it can measure AC 600 V RMS Maximum at 50/60Hz. Does that mean that it can't measure any voltage above 50/60Hz or it just can't measure 600V at 50/60Hz?

I am trying to measure what should be 4.5 V AC at 58kHz and am not getting an accurate reading. Is it possible my multimeter is just a piece of crap?
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Old 10-23-2007, 10:33 PM
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It sounds like the meter is optomized for 50/60 Hz RMS measurements. The maximum voltage is 600 VAC. I would be surprised if it did an accurate measurement at 58 KHz, though. I'm not sure my Fluke 23 would do well. It depends on the measurment circuit in the meter.
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Old 10-23-2007, 10:56 PM
cheddy cheddy is offline
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Thanks for the response. I would like to hear more peoples opinions on the matter. Still does anyone know what an AC voltage on a DC source bias means?

Also, is it possible that an analog multimeter would give an accurate reading on such a high frequency that a digital multimeter just can't read?
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Old 10-23-2007, 11:55 PM
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the bias is the DC offset. If you had a one volt peak signal and a 0.5V bias, the peaks would be 1.5 volts and neg 0.5 volts.
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Old 10-24-2007, 12:21 AM
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The best way to make that measurement would be with an oscilloscope.

Multimeters, analogue or otherwise, are just not designed for measuring anything other than mains frequency AC @ 50 or 60hz.
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Old 10-24-2007, 01:02 AM
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The DMM or Analog meter is limited. You would have to look at a frequency response correction factor .... they did publish them with the old Simpson 260's ... up to about 1 MHz if my memory serves me correctly.
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Old 10-24-2007, 05:01 AM
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Good old Simpson 260's - and that's about what they cost back in the early 70's.

DMM/DVM's are pretty hard-pressed to accurately measure AC waveforms above 10KHz; even there, you really need to be looking at a chart.
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Old 10-24-2007, 05:52 AM
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The specifications for AC measurement say "Average responds, RMS calibrated, DC coupled". Apparently (you should check this) if you have it set to measure AC volts, the meter will respond to DC. Set it to measure 4 volts AC, and see what it reads if you connect the leads to a flashlight battery. Some voltages you want to measure may have an AC component riding on top of a DC component. If you want to measure just the AC component, you must block the DC component. That's the purpose of the .1 uF capacitor they tell you to put in series with the meter.

This meter probably won't measure high audio frequency AC voltages accurately. Check this by connecting a variable frequency oscillator to the meter and sweep from 50 Hz up to 100 kHz; see if the meter reads the same at all frequencies.
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Old 10-24-2007, 03:26 PM
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ac voltmeters are designed to measure the rms values of sine waves of 60 hertz. any other wave form or frerquency will provide a certain amount of error. if you measure a dc source (battery) with an ac voltmeter it will read 0 vac, as there is no ac component. if you read an ac voltage (say you duplex receptacle outlet) with a dc voltmeter it will read o vdc, as there is no dc component. an ac signal on a dc offset will read those repective values individually.
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Old 10-24-2007, 09:22 PM
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Quote:
Originally Posted by cheddy View Post
I am trying to measure what should be 4.5 V AC at 58kHz and am not getting an accurate reading. Is it possible my multimeter is just a piece of crap?
Well, that is possible but not likely. As I said in the other thread, you need to understand the tools and their limitations. What you really need is a 'scope and understanding about DC and AC components and the differences between RMS, average etc and what each meter indicates and its limitations.
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