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Old 12th March 2008, 04:15 PM   #1
ECC33 is offline ECC33  United Kingdom
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Smile dBm or dBV Audio Voltmeter

Hello All,

I have been looking into getting an audio voltmeter with a dB scale for frequency response measurements.

Some have got a dBm scale; 0 dBm = 1 mW/600Ω, and some have got a dBV scale; 0 dB = 1 Volt.

What, in real-world terms, will be the difference and which is the one to get?

Many thanks for any help!
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Old 12th March 2008, 04:39 PM   #2
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0 dB = 0.775V at 600 Ohm,
i.e. (0.775)^2 / 600 Ohm for 1 db of power.



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Old 12th March 2008, 05:21 PM   #3
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Default Re: dBm or dBV Audio Voltmeter

Quote:
Originally posted by ECC33
Some have got a dBm scale; 0 dBm = 1 mW/600&, and some have got a dBV scale; 0 dB = 1 Volt.

What, in real-world terms, will be the difference and which is the one to get?
db(m) measures power ratios:

db(m)= 10log(Po / Pi) (Where P is in milliwatts)

db(v) measures voltage ratios:

db(v)= 20log(Vo / Vi)

Therefore, a change of +/- 3.0db(m) is a halving or doubling of the power, and a +/- 3.0db(v) change is by either sqrt(2) or sqrt(0.5).
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Old 12th March 2008, 05:29 PM   #4
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Default Re: dBm or dBV Audio Voltmeter

Quote:
Originally posted by ECC33
Hello All,

I have been looking into getting an audio voltmeter with a dB scale for frequency response measurements.

Some have got a dBm scale; 0 dBm = 1 mW/600&, and some have got a dBV scale; 0 dB = 1 Volt.

What, in real-world terms, will be the difference and which is the one to get?

Many thanks for any help!
Because the dB has no specific value and is only a reference, it must be referenced to something. The internationally accepted standard is now 1 milliwatt in a 600 ohm terminated line which is 0.775 volts for 0 dB(m). (The old standard, no longer used, was 500 ohms.) Almost all professional AC voltmeters use the 1mW/600 ohm standard.

Some less professional meters will reference 0 dB to end of scale at 1 volt as a convenience. This will let you measure differences in levels expressed in dBs (dBv), but you must always remember that the true reference for power levels is actually lower. (0.775v) This could lead to mistakes in some cases. So for best all around use, I'd stick with the 1mW/600 ohm 0dBm reference.

Since decibles are a logarithmic function, the dB divisions on a regular linear scale meter movement are compressed at the low end making readings slightly difficult. Because of this, some meter movements are made non-linear by magnetically biasing the movement toward the low end. Doing so makes the dB scale linear and easier to read. Unfortunately the voltage scale now becomes non-linear and a little harder to read. So it's a trade-off that you must choose between.

Still other meters simply put the dB scale at the top most part of the face so that the compression is more spread out and easier to deal with. I prefer Hewlett-Packard meters because they're easy to get on the used market and are the most accurate because HP individually calibrates each printed scale and movement together, as one, at the factory when made. And I assume that you want an analog meter and not a digital. Digitals are ok for absolute measurements, but are very nerve racking for watching changes.

Victor
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Old 12th March 2008, 05:57 PM   #5
AndrewT is offline AndrewT  Scotland
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I wouldn't use a meter to measure frequency response.
You may find that the meter is less accurate than the device being tested.
Compare input to output!!!!!
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Old 12th March 2008, 06:37 PM   #6
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Quote:
Originally posted by AndrewT
I wouldn't use a meter to measure frequency response.
You may find that the meter is less accurate than the device being tested.
Compare input to output!!!!!
Even great meters like the HP3478a and HP3456a limit their bandwidth to prevent RFI from screwing up your measurements. Analyzers like the Tektronix DA4084 and AA501, Boonton 1120 are good to a bit above 100kHz.

Most hand held DVM's are limited to a few kHz.
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Old 12th March 2008, 07:03 PM   #7
EC8010 is offline EC8010  United Kingdom
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Here we go again! dBm is history. Once upon a time there genuinely were 600 Ohm audio transmission lines (miles long) which were driven from 600 Ohm and terminated in 600 Ohm. Studio centres even used 600 Ohm internally. Meters had dBm written on their scales, and has been correctly pointed out by other posters, 1mW into 600 Ohms requires 0.775V. The thing is, such a meter read voltage, not power, and people didn't like all this 600 Ohm nonsense anyway, so a new standard was adopted; dBu. 0dBu is 0.775V and takes no account of impedance. Studios (the few that are analogue) drive from a low source impedance into a high load impedance, and even better, many high load impedances. This couldn't be done with the 600 Ohm nonsense - you needed a distribution amplifier with as many outputs as loads.

The trouble is, the instrument makers didn't see the joke, and persisted in labelling their scales dBm when they were really measuring dBu. Rant over.

My audio test set uses a nice analogue meter and can easily measure frequency response to 0.02dB from 10Hz to 300kHz. As pointed out earlier, DMMs are best for measuring mains; although the more expensive ones go up to tens of kHz.
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Old 12th March 2008, 08:01 PM   #8
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www.sowter.co.uk -has a chart for those lost with dB'ues. -> scroll down to < decibel > all is explained.

richj
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Old 13th March 2008, 07:52 PM   #9
ECC33 is offline ECC33  United Kingdom
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Many thanks to everyone who replied.
You have helped greatly.

Next question, any recommendations please?
DMMs have got their uses, but I was looking for an analogue meter.

Which meter have you got?
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Old 13th March 2008, 08:04 PM   #10
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Quote:
Originally posted by ECC33

Next question, any recommendations please?
DMMs have got their uses, but I was looking for an analogue meter.

Which meter have you got?
I use very old HP 400D vacuum tube voltmeter. It is warm and glows inside.
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