[sdiy] Looking for a handheld, analog, true RMS dBu or dBm meter

Neil Johnson neil.johnson71 at gmail.com
Sun Mar 23 00:09:54 CET 2014


cheater00 wrote:
> as in the subject, I'm looking for a recent or not so recent unit
> that's a hand-held meter, has a needle display, true RMS measurement
> and dBu or dBm scale. I would appreciate any pointers.

Tricky one.

Real true-rms is either done with a thermocouple (expensive, thermal
lag) or a moving iron meter movement (crazy scale).  Both are rather
ancient approaches, so look to HP.

The next step in the RMS game are the RMS converters.  Available from
ADI, LTC and That Corp (and maybe others) they convert an AC voltage
into a DC voltage that is the RMS equivalent.  Much cheaper that
fiddly thermocouples, but they only work over a range of waveforms -
crest factor is the important detail here.  If all you are measuring
is sinewaves then there is nothing to worry about.

Finally, many wideband AC voltmeters from the likes of HP, Leader,
Ballantine, etc, use diode recitifers and a scale adjusted to
correctly show the RMS value of a sine wave.  For waveforms other than
sinewaves you have to apply a correction factor.  But they are
cheaper.

A lot depends on what you intend to use the meter for.  Also what
range of signal levels - handheld DMMs don't tend to go down too far
(the classic Fluke 8060A is specified down to -50dBu).

Neil
--
http://www.njohnson.co.uk



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