chuck wrote:

Owen Duffy wrote:

On Fri, 14 Oct 2005 00:58:35 GMT, Owen Duffy wrote:



It is the ratio of signal to noise and distortion, and it is measured
by setting up a test where the receiver produces output from a SSG
(typically for a 1KHz audio output) and notching out the 1KHz output
to measure the noise and distortion wrt the filtered 1KHz output.



That should be: It is the ratio of signal and noise and distortion to
noise and
distortion, and it is measured by setting up a test where the receiver
produces output from a SSG (typically for a 1KHz audio output) and
notching out the 1KHz output to measure the unfiltered output wrt the
noise and distortion.

I should also have mentioned the EIA test requires the receiver be set
to rated output with 1mV RF input at 60% of rated modulation, then the
RF output reduced to find the input level for 12dB SINAD.

You could measure it with soething like a HP334A Distortion Analyser,
but it is pretty tedious if you are trying to find the RF input for a
particular SINAD. Hence you see boxes that have an AGC controlled
amplifier deliving a constant voltage to the filter block. I have a
Motorola one (R1013A) that works ok, there were also Sinadders. Even
more convenient are the ones integrated into a communications monitor.
I don't believe these boxes do true RMS measurements.

An alternative if you have a standalone SSG and want to do SINAD
measurement is to use a PC sound card and software that does an FFT
and calculates the SINAD (using true RMS measurement). Spectrum Lab
does it, its free, but it is such a flexible / general tool, it may be
a bit daunting to get it working. The SpectrumLab menu "Quick Settings
/ Rx Equipment Tests / SINAD test" is a quick path to setup... but it
is still a quite complex package. A whole lot better than the style of
a HP334A though! SL is at http://www.qsl.net/dl4yhf/spectra1.html .

Owen
--
Good info, Owen. I think the EIA test procedures really have FM or AM in
mind, rather than SSB or, what is exactly the same for SINAD purposes,
CW. The 60% figure just doesn't apply to SSB or CW. You would simply use
an unmodulated signal generator with the frequency offset to produce a 1
kHz tone in the receiver's audio output, preferrably centered in the
receiver's passband. Then a measure of rms af voltage at the receiver's
output with and without the 1 kHz filter would be made.

We don't hear much about SINAD testing procedures for SSB and CW. Even
the ARRL's test procedure manual glosses over the procedure for other
than FM.

Chuck
NT3G

SINAD measurements were cooked up because the audio quality of an FM
receiver depends both on the demodulator and on the noise
characteristics of it's front end. Just having a noise figure for an FM
receiver is pretty useless. Keep in mind that an FM receiver is usually
rated as "X dB SINAD for y microvolts input".

For SSB and CW, on the other hand, the noise is purely additive so all
you need to know is the receiver noise figure. Once you know that
(assuming that it's not a really strange radio) you know everything
about it's performance. Given the noise figure in dB you can easily
calculate the 12dB SINAD should you be so inclined, as well as any other
signal vs. noise figure you should want. You have a good reason to
believe that the noise is white so you can even take an SSB receiver and
calculate the noise figure of the thing after you tack on an audio
bandpass filter for CW. This is _not_ the kind of thing you could do
with FM.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

On Fri, 14 Oct 2005 10:50:03 -0700, Tim Wescott
wrote:


bandpass filter for CW. This is _not_ the kind of thing you could do
with FM.

Indeed Tim. The complications as I see it in predicting the SINAD for
FM when you know the receiver NF is that most narrow band
communications applications of FM are closer to PM because of the
pre-emphasis characteristic.

FM + 6dB/octave preemphasis over the entire modulation passband is PM.
(PM is where the modulation index (dev/fm) is independent of fm (the
modulating frequency)).

However, the receivers in my experience are over de-emphasised (at the
top end) presumably to get better SINAD. In fact, I think
specifications of the de-emphasis curve are commonly stated along the
lines of +1 to -3dB of -6dB demphasis relative to 1Khz over 300 to
3000Hz. This accomodates a over de-emphasis at the high end for little
loss in intelligibility and a dB or so improvement in sensitivity
figures.

Filter / demodulators and CTCSS IM also contribute to distortion
products significantly.

The result of demod distortion, PM with a slope across the passband,
and uncertain high pass filtering to accomodate CTCSS makes prediction
of S/N out from C/N in a bit of a guess in FM comms receivers, not
nearly as accurate as you suggest for linear receivers.

Owen
--

Originally, two way shops set FM rigs up for 20 dB quieting, on a volt
meter (crank up the signal until the AUDIO VOLTAGE, UNMODULATED, was
1/10th the voltage of a no signal audio output . Tho, for the most
part, this works well, there are constraints on sensitivity, because of
bandwidth concerns, and , as bandwidth is halved, the signal improvement
is 6 dB (quadrupled). Sinad is Signal/Noise /signal/(noise+distortion)
and in fact, in recent times , devices that will measure it are built
into many pieces of test equipment (IFR meters comes to mind), also
look for an outfit called "SINADDER" . The main thing is that it adds
a "Bandwidth" component to the sensitivity equasion. It is measured
with a 1 KHz tone, at (in FM), 3 KHz deviation- and the smaller the
signal that is detectable , with this constraint, the more sensitive
the reciever is considered to be! This also works at SSB/AM. Tho, it
is true that this measures Sensitivity, it includes a BANDWIDTH
component, that a (noise figure/ quieting) would NOT consider (at least
fully!) Hopefully, this is helpful-- Jim NN7K


For SSB and CW, on the other hand, the noise is purely additive so all
you need to know is the receiver noise figure. Once you know that
(assuming that it's not a really strange radio) you know everything
about it's performance. Given the noise figure in dB you can easily
calculate the 12dB SINAD should you be so inclined, as well as any other
signal vs. noise figure you should want. You have a good reason to
believe that the noise is white so you can even take an SSB receiver and
calculate the noise figure of the thing after you tack on an audio
bandpass filter for CW. This is _not_ the kind of thing you could do
with FM.