RadioBanter - What is SINAD?

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On Mon, 17 Oct 2005 16:39:25 GMT, chuck wrote:

Hello Owen,

Seems both average-responding and trms meters use rectifiers, so a
square wave input with perfect symmetry should result in BOTH meters
reading the same: an amount equal to the peak square wave voltage. Am I
confused on this?

Leaving aside the rectifier point which is arguable:

If we accept that the RMS responding instrument reads correctly on all
types of waveforms, the issue is with the average responding
instrument.

The average responding instrument is (usually) calibrated for the form
factor of a sine wave, and its scaling in RMS is only valid for
waveforms with the same form factor.

There are two cases to consider, the square wave, and the square wave
with the fundamental removed. The form factor of both are different to
the form factor of a sine wave, and more importantly to each other, so
the average responding meter does not provide an accurate ratio of the
(true) power of each wave.

Does that make sense?

Owen
--

"chuck" wrote in message
link.net...
Thanks for the info, Frank.

Actually, the RSS125 on the site doesn't contain the procedure, but
RSS181, also available at that site, does.

FWIW, the procedure is basically what has been discussed, except that
the signal generator output to be recorded as the receiver's sensitivity
is that level which produces a 12 dB SINAD at 50% of rated audio output!
Probably a more realistic test than allowing the AF stage to operate at
a low-distortion level of something like 1% of rated output.

73,

Chuck
NT3G

Thanks Chuck, forgot all about RSS 181, even though I have type approval
tested countless SSB transceivers to that specification. Interesting that
it has not been updated since 1971. Note that he sensitivity is defined as
that input that will produce 12 dB SINAD or that input which will produce at
least 50% of the rated audio output. The fact is that most of the testing
that I did was measured at full audio output, just below the threshold of
audio clipping. I do not recall any unit where the receiver gain was a
factor in sensitivity. Also note they still refer to "A3j", and not J3E,
etc. All these old specifications are based on tube designs. With TDA2002
type audio chips, you could typically get 5W out at 1% distortion. It was
so easy to drop in an extra IF stage if you could not meet the 50% min audio
output at threshold sensitivity.

73,

Frank

You're correct, of course, Steve. I was thinking the average-responding
meter was calibrated to display average levels, but it is not: it is
calibrated to give the rms value of a true sine wave with that average
value. So the only way to measure the average value of a non-sinusoidal
ac signal is to use an average-responding meter and correct the
displayed reading as you have noted. Not relevant to the SINAD
discussion but interesting.

Thanks for the clarification.

Chuck

Steve Nosko wrote:
Nope. See my previous post.

A square wave has an average equal to the RMS equal to the peak. It's just
like DC.

The "older types" RESPOND to average of a SINE (63% of peak) but display the
value for the RMS (71% of peak), so they have a 1.11 correction factor to
get from average to RMS.

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

"chuck" wrote in message
ink.net...

Hello Owen,

Seems both average-responding and trms meters use rectifiers, so a
square wave input with perfect symmetry should result in BOTH meters
reading the same: an amount equal to the peak square wave voltage. Am I
confused on this?

Chuck

Owen Duffy wrote:

On Mon, 17 Oct 2005 03:09:11 GMT, Owen Duffy wrote:

So, theoretically:
- an ideal average responding meter should read (1-2/pi)% which is
36.3% or 8.8dB on an perfect square wave;

I think this is close to the right answer, but for the wrong reason. I
think it needs to be evaluated iteratively, and I get an answer closer
to 34.3% or 9.3dB.

Owen
--

What is SINAD? Qiock test

On Mon, 17 Oct 2005 14:01:41 -0500, "Steve Nosko"
wrote:

"Owen Duffy" wrote in message
.. .
On Mon, 17 Oct 2005 00:05:18 GMT, Owen Duffy wrote:

- an ideal average responding meter should read (1-2/pi)% which is
36.3% or 8.8dB on an perfect square wave;

In another post ) I have
identified that that expression is wrong.

The correct ratios are closer to 34.3% or 9.3dB.

Wait a minute here. You're percents and dB is confusing.

Many Distortion analysers are calibrated in % where the % figure is a
voltage ratio.

I don't know about the 1-2/pi. It's been about a year or so since I went

It is wrong, see above.

through all this for that QST article using a serise resistor in the power
line to figure out power supply (and rig) power consumption - unfortunately
ignoring the pulsed nature of capacitor input power supply current,
BUT...

I don't remember the analytical expressions for these quantities. I'll use
the common numbers...

For the meter that responds to average (63% peak - I think this is 2/pi) ,
but shows RMS which is .707 of peak (1/root2), the ratio for average input
to reading = 0.707/.63 . For this I get 2/(2* root2)

Average of a square wave is equal to the peak.

So a 1 volt (pk) square wave should measure 1.11 Volts on one of these (sine
average responding, RMS displaying) meters and 1V on an rms meter.

I think I did that right?

The problem is that while the RMS meter provides a true power
indication on the square wave, and the filtered square wave (ie
without fundamental), the average responding meter does not give an
accurate power ratio because the form factors (RMS/AVG) of the two
waveforms is different.

Owen
--

On Mon, 17 Oct 2005 05:09:06 GMT, Owen Duffy wrote:

On Mon, 17 Oct 2005 03:09:11 GMT, Owen Duffy wrote:

So, theoretically:
- an ideal average responding meter should read (1-2/pi)% which is
36.3% or 8.8dB on an perfect square wave;

I think this is close to the right answer, but for the wrong reason. I
think it needs to be evaluated iteratively, and I get an answer closer
to 34.3% or 9.3dB.

An analytical approach to solution of the problem.

A unit height square wave has a fundamental component of amplitude
4/pi.

To find the area under the filtered curve, I think we are looking for
the integral from 0 to pi/2 of absolute(1-4/pi*sin(theta))). I will
divide the integral at theta=asin(pi/4) to deal with the absolute
function since the problem function is positive from 0 to asin(pi/4)
and negative from asin(pi/4) to pi/2.

Here is some Perl to evaluate the ratio:

#find the zero crossing point
$theta=asin($pi/4);
#find the area under the curve
$area=$theta-4/$pi*(-cos($theta)+cos(0)); #first part
$area+=4/$pi*(-cos($pi/2)+cos($theta))-($pi/2-$theta); #second part
#divide by area under unit square wave
$ratio=$area/($pi/2);
print "Average response ratio is $ratio \n";

And the answer is 0.3430678471... or 9.3dB. It is about 9.4dB if you
only consider the harmonics up to 50KHz.

One of you mathematical whizzes might know a better way to put this!

Owen
--

Owen,

[[[ while I suspect the difference is so small that it makes little
difference in the receiver sensitivity number arrived at, I proceed
anyway. ]]

From your last two posts, it appears you/we have vastly different
interpretations of what the question was. You talk about removing the
fundamental from the square wave and this puzzles me. I do not believe the
intent was to measure SINAD using a square wave modulation. Therefore
removing its fundamental and measuring its RMS does not fit the situation.

My understanding of the issue was measuring SINAD with the older
"Average-measuring-, RMS-reading" type (call it the "AVG-Type") of meter vs,
a "true RMS" reading meter.

While I did talk about measuring a square wave with the "AVG_Type" meter,
that was a digression only ment to show (what I think is) the difference for
the previously proposed reference measurement. Namely try simply measuring
a square wave for comparing the two meter readings, not SINAD measurements.
I did this to show a calculation of how the two meters would read knowing
the average and RMS values of _some_ waveform. I was thinking that I could
do this for the two SINAD waveforms, but as you see here, I gave up...

Back to SINAD.
Therefore, the issue I was addressing was the following:
(for the normal SINAD technique, 1 kHz sine wave tone):
A- Assume the RMS meter gives the "correct" reading.
B- What does the "AVG_Type" show on the display/scale?

So, B has two parts.

1- What does the "AVG-Type" read for the un-notched signal, and
2- What does the "AVG-Type" show for the notched signal.

1- As a first approximation, lets say the un notched reading is dominated by
the sine wave. With this assumption, they both read the same. To refine
this estimate, I am unable to assess, easily, the effect of the noise on
either measurement except that the True RMS mwter will give an indication of
the total tone and noise power (actually Erms^2). As a first approximation,
we could say that the "AVG-Type" reads the average of the sine plus the
average of the noise voltages...and my estimation powers peter-out right
there.
2- What does the "AVG_Type" read on the noise (notched signal)... yep, peter
once again.

Therefo

GOTO [[ my statement in brackets above ]]

Then there's the pronunciation. Some say "sin' add" and some say "sign'
add". We said SIN add.

Nice exercise, time for me to move on to other things.

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

"Owen Duffy" wrote in message
...
On Mon, 17 Oct 2005 05:09:06 GMT, Owen Duffy wrote:

On Mon, 17 Oct 2005 03:09:11 GMT, Owen Duffy wrote:

So, theoretically:
- an ideal average responding meter should read (1-2/pi)% which is
36.3% or 8.8dB on an perfect square wave;

I think this is close to the right answer, but for the wrong reason. I
think it needs to be evaluated iteratively, and I get an answer closer
to 34.3% or 9.3dB.

An analytical approach to solution of the problem.

A unit height square wave has a fundamental component of amplitude
4/pi.

To find the area under the filtered curve, I think we are looking for
the integral from 0 to pi/2 of absolute(1-4/pi*sin(theta))). I will
divide the integral at theta=asin(pi/4) to deal with the absolute
function since the problem function is positive from 0 to asin(pi/4)
and negative from asin(pi/4) to pi/2.

Here is some Perl to evaluate the ratio:

#find the zero crossing point
$theta=asin($pi/4);
#find the area under the curve
$area=$theta-4/$pi*(-cos($theta)+cos(0)); #first part
$area+=4/$pi*(-cos($pi/2)+cos($theta))-($pi/2-$theta); #second part
#divide by area under unit square wave
$ratio=$area/($pi/2);
print "Average response ratio is $ratio \n";

And the answer is 0.3430678471... or 9.3dB. It is about 9.4dB if you
only consider the harmonics up to 50KHz.

One of you mathematical whizzes might know a better way to put this!

Owen
--

On Tue, 18 Oct 2005 10:37:15 -0500, "Steve Nosko"
wrote:

Owen,

From your last two posts, it appears you/we have vastly different
interpretations of what the question was. You talk about removing the
fundamental from the square wave and this puzzles me. I do not believe the
intent was to measure SINAD using a square wave modulation. Therefore
removing its fundamental and measuring its RMS does not fit the situation.

Some discussion arose about the extent of errors when using an aveage
responding meter to measure SINAD (compared to a true RMS meter).

My experimental evidence is that when measuring SINAD on a 2.4KHz wide
receiver at SINAD=12dB, the error is less than 1dB. Of course, it will
be less for higher SINAD ratios, and worse for lower ones.

I offered that a simple test of whether a SINAD meter was average
responding or true RMS responding, was to measure the SINAD of a good
square wave. The average responding meter will indicate about 9.3dB
whereas an RMS responding meter will indicate around 7.3dB.

I think we both understood that.

Back to SINAD.
Therefore, the issue I was addressing was the following:
(for the normal SINAD technique, 1 kHz sine wave tone):
A- Assume the RMS meter gives the "correct" reading.
B- What does the "AVG_Type" show on the display/scale?

So, B has two parts.

1- What does the "AVG-Type" read for the un-notched signal, and
2- What does the "AVG-Type" show for the notched signal.

1- As a first approximation, lets say the un notched reading is dominated by
the sine wave. With this assumption, they both read the same. To refine
this estimate, I am unable to assess, easily, the effect of the noise on
either measurement except that the True RMS mwter will give an indication of
the total tone and noise power (actually Erms^2). As a first approximation,
we could say that the "AVG-Type" reads the average of the sine plus the
average of the noise voltages...and my estimation powers peter-out right
there.
2- What does the "AVG_Type" read on the noise (notched signal)... yep, peter
once again.

As I stated above, and you stated, the comparison will depend on the
extent to which the sine wave dominates the total signal, and so will
depend on the SINAD being measured. I suggest that it will also depend
on the noise bandwidth.

Repeating, my experimental evidence is that when measuring SINAD on a
2.4KHz wide receiver at SINAD=12dB, the error is less than 1dB.

At the end of the day, it doesn't matter that much for normal
applications if the SINAD meter is average responding, they appear to
overestimate the SINAD, but by a very small amount.

Therefo

GOTO [[ my statement in brackets above ]]

You are trying to send Wes in a loop.

Then there's the pronunciation. Some say "sin' add" and some say "sign'
add". We said SIN add.

Dangerous territory, this could become an international incident when
you bring pronunciation into scope. There, and you all (mostly)
thought I didn't know how to spell... all those esses where they
should be zeds, no thats zees isn't it.

Owen
--

"Owen Duffy" wrote in message
...
On Tue, 18 Oct 2005 10:37:15 -0500, "Steve Nosko"
wrote:

[...]

I offered that a simple test of whether a SINAD meter was average
responding or true RMS responding, was to measure the SINAD of a good
square wave. The average responding meter will indicate about 9.3dB
whereas an RMS responding meter will indicate around 7.3dB.

I think we both understood that.

Nope, I missed that concept. Thanks for the clarify.