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.

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

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.

73, Steve, ,K.9;D'C'I



"W3JDR" wrote in message
news:ljM3f.39197$q81.11651@trnddc06...
It is the ratio of (Signal + Noise+Distortion) to (Noise+Distortion) as
measured at the receiver audio output. It is measured using an RMS-reading
AC voltmeter , typically with a 1Khz modulation tone on the signal applied
to the receiver under test.

First you measure the audio signal out of the receiver using the AC RMS
meter. Then you apply a notch filter at the modulation frequency and
measure
the residual noise+distortion, again using the RMS AC voltmeter. SINAD is
the ratio of the two measurements.

Joe
W3JDR



"crb" wrote in message
...
Is it only valid for AM and FM measurements?

I know its receiver sensitivity.

Is it Signal divided by Noise with distortion??

12 dB SINAD means what? The signal is 12 dB greater than noise and
distortion or
is it more complicated than that?

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

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.

Hi Steve,

You don't need a "real" RMS meter. The expressed requirement for a
pure 1kHz tone provides the necessary sine wave shape such that it
simply becomes a matter of scale calibration. If you had said a
square wave 1KHz tone (nothing pure about that), then you would have
to dig deep for a "real" RMS meter. That too, could be scaled, but I
wouldn't count on it because it would be a rare amplifier chain that
could faithfully keep it square - and the notch would inject it into
the measurement as distortion and noise.

73's
Richard Clark, KB7QHC

Richard,
What you said is largely accurate, however at low S/N ratios, or where the
distortion becomes comparable to the signal level, the reading of the
composite signal (signal+noise+distortion) with anything other than an RMS
meter could produce erroneous results.

Joe
W3JDR

"Richard Clark" wrote in message
...
On Fri, 14 Oct 2005 13:08:22 -0500, "Steve Nosko"
wrote:

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.

Hi Steve,

You don't need a "real" RMS meter. The expressed requirement for a
pure 1kHz tone provides the necessary sine wave shape such that it
simply becomes a matter of scale calibration. If you had said a
square wave 1KHz tone (nothing pure about that), then you would have
to dig deep for a "real" RMS meter. That too, could be scaled, but I
wouldn't count on it because it would be a rare amplifier chain that
could faithfully keep it square - and the notch would inject it into
the measurement as distortion and noise.

73's
Richard Clark, KB7QHC

On Fri, 14 Oct 2005 20:28:17 GMT, "W3JDR" wrote:

Richard,
What you said is largely accurate, however at low S/N ratios, or where the
distortion becomes comparable to the signal level, the reading of the
composite signal (signal+noise+distortion) with anything other than an RMS
meter could produce erroneous results.

Hi Joe,

In the practical world of SINAD (having tuned a number of GE and
Motorolas), one is not very interested in how poor your set is, but
rather meeting a service standard (that 12 dB which is as arbitrary as
any).

I doubt if many of the current generation of commercial surplus
equipment comes with a stock tester employing what would have been an
expensive converter chip to insure RMS measurements. I come by that
assessment by noting those I used employed standard meter movements.
The first RMS meters I calibrated in the mid 70s came from Fluke (just
up the highway), and the components of that circuit were scrubbed of
all identification numbers or cast in epoxy. Such was the cachet of
being hi-priced, and having others try to break into the market with
knock-offs.

My Radio Shack multimeter makes that claim (ca 1995) and if memory
serves, that Micronta's "True RMS" was barely capable of poor voice
grade bandwidth. This was 20 years after Fluke, costing about as much
(economic inflation), and not performing as well (technical
deflation).

73's
Richard Clark, KB7QHC

Richard,

I understand the historical difficulties of making accurate RMS
measurements, however I didn't know the original post only solicited ways to
make the measurement with "current generation of commercial surplus
equipment ". My intention was to point out some measurement nuances that
might not be obvious at first glance.

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.


Joe
W3JDR


"Richard Clark" wrote in message
...
On Fri, 14 Oct 2005 20:28:17 GMT, "W3JDR" wrote:

Richard,
What you said is largely accurate, however at low S/N ratios, or where the
distortion becomes comparable to the signal level, the reading of the
composite signal (signal+noise+distortion) with anything other than an RMS
meter could produce erroneous results.

Hi Joe,

In the practical world of SINAD (having tuned a number of GE and
Motorolas), one is not very interested in how poor your set is, but
rather meeting a service standard (that 12 dB which is as arbitrary as
any).

I doubt if many of the current generation of commercial surplus
equipment comes with a stock tester employing what would have been an
expensive converter chip to insure RMS measurements. I come by that
assessment by noting those I used employed standard meter movements.
The first RMS meters I calibrated in the mid 70s came from Fluke (just
up the highway), and the components of that circuit were scrubbed of
all identification numbers or cast in epoxy. Such was the cachet of
being hi-priced, and having others try to break into the market with
knock-offs.

My Radio Shack multimeter makes that claim (ca 1995) and if memory
serves, that Micronta's "True RMS" was barely capable of poor voice
grade bandwidth. This was 20 years after Fluke, costing about as much
(economic inflation), and not performing as well (technical
deflation).

73's
Richard Clark, KB7QHC

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

I understand the historical difficulties of making accurate RMS
measurements, however I didn't know the original post only solicited ways to
make the measurement with "current generation of commercial surplus
equipment ".

Hi Joe,

That was interjected by me, knowing the market of the past several
years being flooded after trunk systems began replacing older service.

My intention was to point out some measurement nuances that
might not be obvious at first glance.

Useful information, that.

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.

Yes, the miracle of Moore's law. 20 years ago I was with HP, here, to
help them introduce their 100KHz real-time dual channel audio spectrum
analyzer. That was a tremendous effort with a million lines of Pascal
code and 5 years in the making when most HP instrumentation hit the
market in 18 months from inception.

I got to know the range of FFTs under some of the most brilliant minds
on the topic. One, Nick Pendergrass, went on to teach at an eastern
university.

Today, it is an underclass topic, probably occupying no more than 6
weeks of instruction coupled to other interests. Still and all, I see
considerable errors of omission in the discussion. Such errors often
make the difference in delivering a serviceable performance compared
to that which is 100 times better (actually a million times, but few
could get their imagination around a number that big so I understate
it).

73's
Richard Clark, KB7QHC

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
--

On Mon, 17 Oct 2005 00:05:18 GMT, Owen Duffy wrote:

This is seriously bad, replying to one's own post... but.

It occurs to me a quick test to reveal whether a SINAD meter is RMS
responding or average responding is to test it with a 1KHz square
wave. I am not suggesting this as a cal procedure, just a test that is
more sensitive to the meter response than noise testing.

IIRC, the Taylor series coefficients for a square wave a all even
harmonics are 0, the others are 4/pi/n.

So, theoretically:
- an ideal average responding meter should read (1-2/pi)% which is
36.3% or 8.8dB on an perfect square wave;
- an ideal RMS responding meter should read
(1-(2^-0.5*4/PI())^2)^0.5*100% which is 43.5% or 7.23dB.

Does the maths make sense?

I observe that my R1013A indicates 9dB on a good square wave, and the
HP334A around 35% (9.1dB)... so another indication that they are
average responding. I expect the readings a little low because neither
instrument has infinite bandwidth.

Owen
--

"Richard Clark" wrote in message
...
On Fri, 14 Oct 2005 13:08:22 -0500, "Steve Nosko"
wrote:

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.

Hi Steve,

You don't need a "real" RMS meter. The expressed requirement for a
pure 1kHz tone provides the necessary sine wave shape such that it
simply becomes a matter of scale calibration. If you had said a
square wave 1KHz tone (nothing pure about that), then you would have
to dig deep for a "real" RMS meter. That too, could be scaled, but I
wouldn't count on it because it would be a rare amplifier chain that
could faithfully keep it square - and the notch would inject it into
the measurement as distortion and noise.

73's


Hi Richard,

I don'r know about that. For the un-notched signal, yes, where the
dominant component is the sine wave. However, not knowing how a non-RMS
meter may respond to the notched-out (predomanantly noise) signal, I'd thing
there is a possible cause for error compared to an RMS meter.
73, Steve, K,9.D;C'I

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
--