On Fri, 14 Oct 2005 13:08:22 -0500, "Steve Nosko"
wrote:


Owen, (& crb)
Your words are contrary to the way we measured it (Motorola). You say
..."It is the ratio of signal to noise and distortion,..", but we measured
not just the signal, but everything for the "top" of the ratio (which is
used more like a reference as it is the more stable as signal level varies).
I believe your last line saying wrt to the filtered tone supports this. It
implies that the tone is (bandpass) filtered for one of the measurements and
we don't do that.

It was a shabby description in my first post Steve, I wrote filtered
instead of unfiltered, had the "wrt", terms back to front at the and,
as you quoted signal to (N+D) when it is the total to (N+D). Wasn't
really worth 2/10, was it! I re-read it when I came back to add some
detail re a software approach and redrafted it.

My recollection was then when the scheme was introduced, one used a
Distortion Analyser, and I can't remember the early HP instrument, the
HP334A I mentioned in my follow up was a newer one. They are tedious,
whereas the R1013 or Sinadder or much more convenient, and the
integrated ones (like in the R2000) are much better.

Once you sort the issues of getting audio samples to a PC sound card
without hum and clipping, that approach can work well, and Spectrum
Lab works well... just it is a multipurpose tool for a simple job.
There are probably other software tools that are more targetted and
simpler to use.


Joe's is a bit better of an explanation (the RMS meter and quantities
ratioed).

Yes, but I don't believe most of the instruments acutally incorporate
a true RMS meter. I set about measuring the difference about six
months ago when I was doing calibration / validation measurements for
FSM (http://www.vk1od.net/fsm/) using custom software against the
R1013, and R2000, and a couple of HP334As. The error in using a
rectifier-average responding meter (as is typically done) is small
relative to the variance of such readings because of the variance of
the noise component.


A meter, a pure 1kHz tone modulated signal generator and a 1kHz notch is all
that is needed. What happens if you don't have a "real" RMS meter? I don't
know.

Yes, I didn't mention that the 1KHz tone needs to be relatively low
distortion. For measuring 12dB SINAD, the demand is not onerous, but
it is important. The frequency of the tone is important as the notches
in semi automatic instruments are typically +/10Hz or so, so one needs
to verify that the SSG modulation oscillator is close enough. More
importantly when testing an SSB receiver (where you use a CW carrier),
that the carrier is kept "on frequency" for a 1KHz beat note.

Owen
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