roy@alanine.phri.nyu.edu (Roy Smith) (07/29/90)
I'm trying to compare several Marine VHF/FM tranceivers. In the
spec sheet, there are figures for Sensitivity, subheading "12 dB SINAD".
I've never seen the acronym SINAD before; what does it mean? The ratings
run from 0.20uV to 0.35uV. Are these good numbers? They also list "20 dB
quieting" (0.25uV to 0.40uV), but I'm not sure exactly what that means
either.
--
Roy Smith, Public Health Research Institute
455 First Avenue, New York, NY 10016
roy@alanine.phri.nyu.edu -OR- {att,cmcl2,rutgers,hombre}!phri!roy
"Arcane? Did you say arcane? It wouldn't be Unix if it wasn't arcane!"brian@ucsd.Edu (Brian Kantor) (07/29/90)
The two measurements are often complementary. 20db quieting level is how much signal is required to cause the no-signal unsquelched output of a receiver to decrease by 20db as the limiters saturate out the noise; it's a simple quick measurement of receiver sensitivity. .2uV to .4uV is typical of modern medium-price highband (150-170MHz) receivers. SINAD (Signal, Interference, Noise, And Distortion) measurements are made by applying a modulated signal to the receiver and increasing the rf carrier level until there is a 12db signal-to-(noise+distortion) ratio, which is supposed to simulate the level at which a typical human can clearly understand the received signal. 20db quieting is a fast and simple measurement which does not take into account things like hum, distortion of received audio, and miscellaneous crud in the receiver. SINAD is about 20 years newer, but some people don't quote it, even though it's well over 20 years since it was standardized. An interesting comparison to make is to look at the respective figures for two receivers. If the ratio between the quieting and SINAD for one receiver is different from the ratio for another, there is quite likely going to be a notable difference between their performance in weak signal conditions. I'd choose the one with the more sensitive SINAD figure if all other things were equal. Of course, you have to trust the manufacturer to have made the measurements accurately. Most service shops only check the quieting quieting when they do a repair job, since setting up for a SINAD measurement is a pain unless you have just the right instrument. Normally a repair that restores the quieting sensitivity will also restore the SINAD, so most don't check. Typical service monitors which include SINAD capability might well run the service shop over $15,000 per instrument. Probably only the bench tech has one if they have one at all; the field techs usually get the cheaper instruments. - Brian
larry@kitty.UUCP (Larry Lippman) (07/29/90)
In article <1990Jul29.015732.21869@phri.nyu.edu>, roy@alanine.phri.nyu.edu (Roy Smith) writes: > I'm trying to compare several Marine VHF/FM tranceivers. In the > spec sheet, there are figures for Sensitivity, subheading "12 dB SINAD". > I've never seen the acronym SINAD before; what does it mean? The ratings > run from 0.20uV to 0.35uV. Are these good numbers? They also list "20 dB > quieting" (0.25uV to 0.40uV), but I'm not sure exactly what that means > either. Your data sheet must have a typographical error. In the particular case of marine radios, the acronym SINBAD is used, which is a subjective sensitivity rating originated by Sinbad the Sailor. I'm sorry; the Devil grabbed hold of my keyboard and made me write the above. :-) Seriously, though, SINAD is a widely accepted approach for the specification of receiver sensitivity. SINAD is an EIA method which expresses in decibels the ratio of [SIGNAL + NOISE + DISTORTION] to [NOISE + DISTORTION]. A 12dB ratio is used as an arbitrary point for reference sensitivity. The 12 dB merely indicates the measurement ratio, and is not a specification for the receiver per se. 12 dB SINAD sensitivities of 0.20 uV to 0.35 uV are good numbers; offhand, on a top-of-the-line commercial VHF-FM radio radio like say, the Motorola Micor, I don't ever recall seeing anything better than 0.175 uV - and that was with the preamp option. 20 dB quieting sensitivity is used as a means to express the noise figure of the radio in uV as opposed to dB. The 20 dB is a fixed ratio used to define a measurement condition. The 20 dB quieting sensitivity will always be a higher value than the 12 dB SINAD value. The figures you give above are good numbers, and they don't get any better for this type of radio. A specification which you haven't mentioned is EIA SINAD selectivity, which would be my next concern. I would not settle for anything less than -95 dB @ +/- 30 kHz. In addition, I would look for spurious and image rejection of at least 95 dB. Larry Lippman @ Recognition Research Corp. "Have you hugged your cat today?" VOICE: 716/688-1231 {boulder, rutgers, watmath}!ub!kitty!larry FAX: 716/741-9635 {utzoo, uunet}!/ \aerion!larry