Re: [amsat-bb] SNR measurements

[Stan <stan@xxxxxxxxxxx>]

Hello Lee,

I would consider using a SINAD technique,

SINAD is a SIgnal, Noise, And Distortion measurement technique.

Modulate your TX signal with a very clean 1 KHz tone in SSB mode for 
AO-40 or FM with UO-14/AO-27.
Take the received audio and put into the SINAD meter. directly read out 
the S+N+D to N+D ratio.
D is distortion and will probably be negligible, N is the noise, so 
essentially you get a S/N ration if S >> N.

Point your antenna to maximize the SINAD reading.
Tune the receiver to maximize SINAD, continue to correct for doppler.

As you lower your RF output power, the SINAD measurement also goes down.
As you raise your output power the SINAD reading will increase to the 
overall system AGC thresholds.
AGC action in your receiver and/or the satellite, will limit the SINAD 
maximum.
Then you meet LEILA.

I use a model SL105 SINADDER made by HELPER INSTRUMENTS in Florida
407-777-1440. I believe they are still around. Cost was about 100$ used, 
a few years ago.

It contains a true RMS audio voltmeter and a precision low distortion 
1000.0 Hz source.
Dynamic range is about  0 thru 32 dB for SINAD, 10mV thru 100V for the 
TRMS audio voltmeter.

Knowing the resolution bandwith (noise bandwidth) of the IF, you can 
reasonably calculate the noise figure or system sensitivity
from RF thru audio. Most noise figure meters only operate at RF or RF 
thru IF, not from RF thru IF to demodulated audio,
like the SINAD technique.

You can also optimize your system by radiating a test SINAD (1000.0Hz) 
signal ( you pick the frequency and  modulation)
into your system. tweaking every possible link in the receive chain for 
best SINAD (S/N).

Of course, many other folks make SINAD meters, HP/Agilent and Motorola, 
to suggest a couple.

Another trick...  with all of the gain in a RX block down converter, you 
can attenuate this gain until the SINAD reading is degraded.
Excess gain only reduces the overall RX system dynamic range. You want 
best sensitivity and greatest dynamic range.

any other questions, send them along..

Stan, WA1ECF      LM AMSAT       FN41sr  Cape Cod




zzzzz



Lee wrote:

>I've been trying to make signal to noise measurements on AO-40 over the
>weekend and seem to be getting numbers better than I would expect.  As a
>result I'm inclined to believe that my technique is in error since sun &
>ground noise measurements result in fairly reasonable values for system NF
>and gain.  What would be the best approach or software to use for making SNR
>measurements?  I would particularly like to find a technique for measuring
>the SNR of the MB's BPSK signal since that eliminates a lot of uplink
>related variables.
>
>Using Spectran, I tried measuring from the null in the center of the middle
>beacon to the peak during the idle period between blocks and get a value of
>30 dB when the transponder is on and 33 dB during the mode-K window from MA
>124-126.  While I suspect the absolute numbers, the change is consistent
>with the drop in noise floor that I see when the receivers are disconnected
>from the S2 tx.
>
>Following the mode-K break I looked at CW carriers at two power levels. 14w
>EIRP resulted in Spectran showing a carrier peak 11 dB above the noise
>floor.  Increasing the power to 140w EIRP resulted in an increase of the
>peak to 16 dB. During the measurements the U-Rx AGC varied from 6 to 13 dB
>with an average value of 10 dB.
>
>Comments and suggestions would be greatly appreciated.
>
>73,
>Lee-KU4OS
>
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>


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