--- In Repeater-Builder@yahoogroups.com, "Bob M." <[EMAIL PROTECTED]>
wrote:
>
> I recall reading somewhere that the SINAD measurement
> is the residual signal after the 1kHz tone has been
> filtered out.
Right
> When that level gets to 12dB
> below the no-signal noise level, you've reached the
> point of 12dB SINAD.
Well, if everything were simple, that would be it.
What I measure, is a noise floor that slopes 20dB between the lowest
frequency I can measure (near 0) and 2kHz, with the 1kHz tone in the
middle. So do I average that noise floor, or take the peak reading,
or something else?
> A distortion analyzer is basically the same thing. A
> narrow filter notches out the fundamental, and the
> voltmeter reads what's left. HP331, 332, 333, and 334
> units are fairly cheap and they can do triple duty as
> an AC voltmeter, distortion analyzer, and SINAD meter.
That's why I was thinking of the SA in this application, because it
gives a good measure of noise and distortion, but it gives me too
much detail, and takes about 10 sec to do a sweep.
> With that audio spectrum analyzer, adjust it for full
> scale on the fundamental, and look at the noise and
> any harmonics. Increase the RF signal level until this
> drops to 25% (1/4) of the level of the 1kHz tone.
:) I have to mod my receiver then, I can't adjust the squelch to hold
in that low. These daniels receivers don't come with the ability to
run open squelch, unless you hold down a front panel button, which I
may change over to a toggle switch.
> A poor-man's SINAD would have an adjustable amplifier,
> a relatively sharp filter at 1 kHz, and a voltmeter
> following it. This might be easier to come up with
> than a real SINAD meter.
I don't have any problem doing the filtering, but not a lot of data
out there on how sharp the filter needs to be, or what frequency
response the system should have outside the filter.
One approach would be to do it with a boxcar integrator, which can
act as almost an arbitrarily narrow filter. You average up a copy of
the tone, and then subtract that from the output.
Another way would be to do it in DSP, and not mess with the analog at
all.
> I'm still an old-timer, and I prefer the 20dBQ method
> because it's easier to reproduce and only requires a
> simple AC Voltmeter.
True, but it can end up with the bandwidth too narrow.
I made that mistake here, and the local 2M club machine, not tuned by
me, has the same problem. When the weak guys deviate a bit more than
normal, they fall out of the squelch. They aren't over-deviating,
it's just that the receiver does better on unmodulated carrier.
>Plus, I've learned that on
> MaxTracs, this level of quieting occurs when all the
> crackles on a dead carrier disappear. I don't even
> need a meter. If nothing else, it gives me a way of
> comparing one radio to another just by feeding in a
> weak signal.
Yup, makes a good quick quality check.,
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