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

Edward Cole kl7uw at acsalaska.net
Wed Dec 10 00:14:26 PST 2008

In this modern day, soundcard programs display very weak signals much 
below what can be heard.  The method described below can be modified 
using a soundcard spectrum program whereby you can lower the signal 
on the display to equal the noise floor and thus achieve very close 
to S+N/N = 0.  Measuring the signal in dBm that produces this level 
will give one a good measure of sensitivity.

For example, my SDR-IQ is calibrated in dBm so I can see this 
directly.  Typically with 2.1 KHz wide SSB the SDR-IQ has a noise 
floor of -130 dBm at 28-MHz into a 50-ohm load.  On an antenna you 
see a much higher noise floor that shows the amount of noise the RF 
environment adds.

When I use a 144/28 MHz convertor before the SDR-IQ, the absolute 
reading is likely shifted by the gain and NF of the convertor.  Use 
of a calibrated signal generator will give a good benchmark for 
sensitivity.   One must insert a test signal using a directional 
coupler to know what actual sensitivity is when connected to an antenna.

73, Ed - KL7UW

At 01:53 PM 12/9/2008, Glen Zook wrote:
>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.
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