{Collins} 75A4 Sensitivity Measurement Redo



After reading all the suggestions and advice I received about how to measure
sensitivity on the 75A4, here's what I did.

Specification in the 75A4 manual: 1.0 uV for 6 db S/N at 3 KC with no
additional information given.

HP-606B signal generator
HP-400FL AC voltmeter
Output of signal generator set to CW
75A4 Mode SSB/CW with pass band set to my best judgment as to what was a
1000 Hz tone.
75A4 selectivity 3.1 MHz filter  (Had I used the 2.1 MHz filter, the
measured sensitivity should have been -2 dbm better)
4 ohm speaker connected to the receiver and the voltmeter connected to the
receiver output terminals
Receiver noise measurements taken with a 50 ohm terminator on the antenna
input and BNC grounding caps on the HP-606B output jacks.  Audio gain was
set to give a reading of 35 mV and left at that point.
S/N (S+N/N) measurements were taken at the center of each band by increasing
the signal generator output until the HP-400FL reading was 2X (70 mV) the no
input reading.

80, 40, 20, 15, 10L, and 10H all measured in the range of 0.26-0.28 uV at 6
db S+N/N
160 measured 0.6 uV
11 measured 1.6 uV  I did not do any further work to see if this could be
improved.

All measurements are at the mercy of the calibration of the signal generator
and the fact that the 400FL is an average reading AC voltmeter.  I have
since purchased a Ballantine 323 true rms voltmeter and might consider
taking additional measurements to see if there is any significant
difference.

David
KC2JD/4


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