On Sat, 15 Oct 2005 00:40:34 GMT, "W3JDR" wrote:


Recently, it has become quite easy to do true RMS measurement at audio
frequencies using DSP techniques. In fact at audio you can even do an
accurate RMS measurement in DSP using a PIC microcontroller to sample the
signal and perform the calculations.

I mentioned in an earlier post that I had done some comparisons of
true RMS response based SINAD measurements and average responding
meters.

I have just rerun the test.

I have a receiver with 2400Hz wide IF , fed with SSG and connected to
a HP334A Distorion Analyser. I have adjusted the SSG for 12dB
indicated SINAD on the HP334A.

The HP334A's meter is boldly labelled RMS, but it is an average
responding meter scaled for RMS with a sine wave.

I measured the output from the HP334A using a no-name true RMS
voltmeter that covers the audio frequencies involved (trap there...
some dont make it past power frequencies), and measured SINAD of
11.3dB.

I connected the HP334A output to a PC running FSM and measured the
following figures for Vtotal and Vfiltered

total filtered
V Average 2708 679
V RMS 2753 763
V Peak 4287 2302

(The three detectors in FSM are all calibrated to read the same on a
sine wave.)

The FSM measurements indicate a SINAD of 11.1dB RMS responding and
12dB average responding.

Overall, the two / three methods are reasonably consistent indicating
around 12dB SINAD using an average response meter, and around 11.2 dB
using RMS responding meters.

That suggests to me that using an average responding instrument may
overestimate the SINAD by a little less than a dB. However, given the
statistical variance of the noise, I would not be fretting about it,
especially on an FM rx where it might only need a smaller change in
C/N for that SINAD change.

I connected the rx to a Motorola R1013A which indicated 12dB SINAD (it
is most unlikely to have an RMS responding ALC and meter).

Owen
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