[amsat-bb] Re: SINAD Vs. Noise Figure Testing

Glen Zook gzook at yahoo.com
Tue Dec 9 14:53:17 PST 2008

The original SINAD was a 10 dB figure which was developed at least a couple of decades after the 20 dB quieting figure.  The "proper" way to measure the 20 dB quieting point was to use an AC voltmeter utilizing a cupric oxide rectifier (which was what most of the "olde tyme" VOM units used - i.e. Simpson 260).  With the squelch of the FM receiver "wide open" the voltmeter was placed across the speaker and the volume control adjusted for a convenient reading (1 volt was very common).  Then an on-frequency unmodulated signal was applied from a signal generator having an accurately calibrated attenuator and the signal level was adjusted to the point where 1/10th the voltage was recorded on the VOM.  Since a reduction in voltage by a factor of 10 represents a 20 dB reduction this point was referred to as the "20 dB quieting" point.

Most experienced two-way radio technicians could actually make a 20 dB quieting measurement "by ear" and did not require the use of an AC voltmeter.  When verified by someone watching a voltmeter while the technician did the measurement by ear virtually always resulted in a measurement that was well within 1 dB which was "close enough for government work" for field measurements.

Using a well calibrated dB meter (i.e. one made by Hewlett Packard) across the speaker will usually not result in the same reading as that done with a "plain old VOM" using the simple cupric oxide rectifier.  The 20 dB quieting measurement was done by field technicians who often did not have such fancy test equipment as a calibrated dB meter.

An easy to make and a very meaningful sensitivity measurement is the LDS (least discernible signal) which involves turning on the BFO, tuning the receiver for maximum signal strength, and then reducing the signal level (using a well calibrated attenuator on the signal generator) to the point at which the signal can just be detected by ear.  The reading in microvolts from the signal generator is the LDS.  Of course reducing the bandwidth does affect the LDS.  By doing an LDS measurement you are measuring the weakest signal that the receiver is capable of receiving through the noise generated within the receiving system.

Glen, K9STH

Website:  http://k9sth.com

--- On Tue, 12/9/08, Michael Heim <kd0ar at sbcglobal.net> wrote:

From: Michael Heim <kd0ar at sbcglobal.net>

Stan makes a good point here, especially for FM sensitivity.  I worked in the 2-way business for a long time, and typically a sinad measurement would be something like the value of microvolts at the antenna input for 20 dB of quieting of the receiver.  It is basically a measurement of FM receiver sensitivity.  It will also show mis-aligned IF strips and detector, but for the sake of arguement in this case, lets say the receiver is functioning normally.  The goal should be the minimum amount of signal input at the antenna connector quiets the receiver 20 dB.  
I am not certain that a sinad reading would be of much use on an SSB receiver.  Typically a similar measurement for an SSB receiver would be MDS, or "Minimum detectable Signal", which would be the amount of signal that is injected into the antenna connector that produces a faint but detectable signal in the receiver.


More information about the AMSAT-BB mailing list