[amsat-bb] SINAD Vs. Noise Figure Testing

[Stan W1LE]

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