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