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Re: SINAD Vs. Noise Figure Testing



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.  

The way it works is this:  an FM modulated carrier is injected into the receiver with a modulating frequency of 1000 Hz (at typically around 30% modulation).  A sinadder is simply an audio level meter that has a deep notch at 1000 Hz.  Therefore, any audio that it sees that is not on 1000 hz is read as noise (or distortion).  As you increase signal strength, the noise drops off, and when the signal voltage reaches 1/10th of the unsquelched noise with no signal present, the amount of signal injected into the antenna connector is read. That would be the signal necessary to quiet the receiver 20 dB.

A typical reading for sinad from a typical amateur (barefoot) rig today is such that .25 - .35 microvolts will quiet the receiver 20 dB.  It will be even lower with a preamp in line.

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.

Basically, SINAD and noise figure measurements produce the same result.  They both are a measurement of receiver sensitivity, however sinad is reliable only for an FM receiver.  A sinad meter is very easily constructed, and can be purchased much more inexpensively than a noise figure meter, so therefore, if your receiver is capable of FM reception, you can tune up your system by using a sinad measurement.  The only difference that I can really think of between the two measurement systems is that noise figure is an absolute measurement that is applicable to all modes of operation, whereas sinad is only applicable to FM.  If, however you place a generator at the preamp input, and monitor in the FM mode as you make adjustments to your preamp, you will also be tuning for best signal to noise, which will correspond to lowest noise figure.  You just will not know what THAT value is, unless it can be calculated, which I do not know the formula if it is possible
 to do so.

If I needed to tune a preamplifier for absolute best noise figure in a labrotory environment, then, yes, I would tune with the help of a noise figure meter, but for most amateur purposes, I see no reason why one could not tune up using sinad as the criterion.

Michael Heim
Chief Engineer, Forever Broadcasting
New Castle PA
WKST  WJST  WWGY
814-671-0666
Chapter Chair, SBE-122
ARS KD0AR


--- On Tue, 12/9/08, Stan W1LE <stanw1le@verizon.net> wrote:

> From: Stan W1LE <stanw1le@verizon.net>
> Subject: [amsat-bb]  SINAD Vs. Noise Figure Testing
> To: AMSAT-BB@amsat.org
> Date: Tuesday, December 9, 2008, 3:03 PM
> Hello The Net:
> 
> Upon the suggestion of Domenico, I8CVS, I conducted some
> tests to 
> compare SINAD to Noise figure testing.
> 
> Noise figure testing was performed using a HP8970B with 346
> noise source 
> on a ARR preamplifier,
> one of their switched units, model SP144VDG.
> Performance was optimized at  144 MHz.  for a noise figure
> of 0.9 dB and 
> gain of 25 dB.
> 
> The preamp was added before the IC-910H, which was set to
> 144.000 MHz in 
> a SSB (USB) mode.
> 
> SINAD , SIgnal + Noise + Audio Distortion testing allows a
> complete 
> receiver to be tested
> from the RF input port, thru the IF's and demodulation
> to audio.
> 
> For SSB mode testing I injected a low level RF carrier, (~
> -120 dBm),
> no modulation, into the preamplifer with RX.
> I tuned the RX frequency to 1000 Hz less than was actually
> injected.
> 
> example: for a RF of 144.000 MHz, I tuned the RX,  in a SSB
> (USB) mode, 
> to 143.999 MHz.
> SINAD instruments use a 1000 Hz tone for further
> processing.
> 
> Connecting the audio output to a SINAD meter, in my case a
> Helper 
> Instruments, Inc. model Sinadder Linear 5,
> I was able to get a SINAD reading from the meter.
> 
> While monitoring the SINAD meter, I tweaked the
> preamplifier tuning and
> could not improve the original SINAD measurement.
> 
> Conclusion: If the noise figure is optimized, the SINAD
> measurement will 
> also be optimized.
> 
> An additional test was performed, adding a fixed 10 dB BNC
> attenuator after
> the preamplifier and before the RX.   The SINAD measurement
> did not change.
> This indicated that at least 10 dB of excess gain was
> present and can be 
> eliminated.
> It is always best to minimize system gains to maximize the
> ability to 
> handle strong signals
> and to reduce higher order intermodulation distortion.
> Minimize system interstage gains until the system SINAD
> sensitivity or 
> the noise figure is slightly degraded.
> 
> In this example, if I was able to reduce 10 dB of excess
> gain, this 
> would result in lowering
> the 3rd order intermodulation distortion by 30 dB .   !!!
> 
> I hope this helps folks to consider SINAD testing of their
> system, to 
> verify performance.
> 
> Stan, W1LE   FN41sr    Cape Cod
> _______________________________________________
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