I recall reading somewhere that the SINAD measurement is the residual signal after the 1kHz tone has been filtered out. Assuming you have a full-quieting input signal with 3kHz deviation and you've notched that out, there snould be nothing left to measure. As the signal gets noisier, the harmonics (if any) and the noise will be measurable. When that level gets to 12dB below the no-signal noise level, you've reached the point of 12dB SINAD. This is a very simplified explanation, and the 1kHz tone is used as the reference level.
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. The necessity of notching the 1kHz tone is what makes the 12dB SINAD measurement more difficult than the 20dB quieting method, but the SINAD method takes into consideration any distortion of the demodulated signal, which is certainly going to be audible. The 20dBQ method just goes for a reduction of wideband noise. 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. This will be the 12dB SINAD point, assuming that the wideband noise etc doesn't add up to more than the -12dB signal amplitude. Once you calibrate your equipment with a real SINAD setup, you should be able to continue using the audio SA. 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'm still an old-timer, and I prefer the 20dBQ method because it's easier to reproduce and only requires a simple AC Voltmeter. 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. Bob M. ====== --- Dave VanHorn <[EMAIL PROTECTED]> wrote: > > Ok, I wasn't thinking in this direction before, but > I do have an audio > band spectrum analyzer handy. > > Question is, how can I translate this to a Sinad > measurement? > > Looking at the receiver in question now, the second > harmonic of the > 1kHz tone is -40dB, and the noise is at about 5dB > below that. > > It's an interesting study in using the wrong > instrument for the job. > I get a very detailed look at the spectrum of the > audio output, but > what I need is a very non-detailed measurement of > out-of-band energy. > > Tried doing it on my scope too, which can subtract > Chan 1 from chan 2, > but there's almost 180 degrees phase shift, and the > amplitudes are very > different, and it would take some significant > messing about to fix that > up. > > I need a sinadder. __________________________________________________ Do You Yahoo!? Tired of spam? Yahoo! Mail has the best spam protection around http://mail.yahoo.com Yahoo! Groups Links <*> To visit your group on the web, go to: http://groups.yahoo.com/group/Repeater-Builder/ <*> To unsubscribe from this group, send an email to: [EMAIL PROTECTED] <*> Your use of Yahoo! Groups is subject to: http://docs.yahoo.com/info/terms/
