[amsat-bb] Re: SINAD Vs. Noise Figure Testing
kl7uw at acsalaska.net
Tue Dec 9 23:49:01 PST 2008
As far as I know SINAD is only used for FM sensitivity measurements,
whereas the older standard that is used for AM is 20-dB quieting. Of
course SSB is another form of AM. But as an (relatively) inexpensive
tool for tuning RF stages for maximum sensitivity (which should
coincide with minimum NF) the SINADer should work fine. Most SSB VHF
receivers also include FM mode. Merely, tune for 12-dB SINAD and it
should result in the best sensitivity in for SSB, as well.
If you are wanting to know the absolute noise figure then you will
need to measure it using a NF meter or other standard method. But
since our goal is to achieve the best sensitivity, whatever NF that
results in, it seems to me that SINAD can be used for adjustments.
I have performed so many FM alignments over the years, I can tell
when I hear 12 dB SINAD without use of the meter with pretty good
accuracy. I perform probably 500+ of these a year. BTW state of the
art FM receivers run about 0.125uV these days (most ham VHF radios
are this sensitive). AT UHF they are often 0.25uV. The old standard
of 25-years ago was 0.35uV for VHF and 0.50uV for UHF. MOSFet
front-ends on commercial radios are pretty good. A good GasFet
preamp will achieve a NF of 0.5 dB which is about -145dBm at SSB
bandwidths. I don't convert dBm to uV much as my equipment has both
scales on them, but I know that 0.25uV = -119 dBm FM.
73, Ed - KL7UW
At 12:48 PM 12/9/2008, Michael Heim wrote:
>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.
>Chief Engineer, Forever Broadcasting
>New Castle PA
>WKST WJST WWGY
>Chapter Chair, SBE-122
>--- On Tue, 12/9/08, Stan W1LE <stanw1le at verizon.net> wrote:
> > From: Stan W1LE <stanw1le at verizon.net>
> > Subject: [amsat-bb] SINAD Vs. Noise Figure Testing
> > To: AMSAT-BB at 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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