Sensitivity measurement of Collins 51J-4 receiver

I got  today my new (second hand) Motorola R1013A SINAD meter. I made a new cable for it and measured the AM and CW sensitivity of my recently restored and realigned 51J-4. The receiver is ready apart from  the reconditioned PTO which is not yet installed.

The AM sensitivity measurement is carried out by injecting RF carrier to the receiver input, AM modulated by 1 kHz at 30% depth. The receiver is set to 6 kHz mechanical filter, XTAL filter off, AGC on. Then the carrier level is decreased until the S/N ratio (or SINAD, to be precise) in the receiver output drops to 10 dB.

I went through all of the bands between 3.5 MHz and 18 MHz. The AM sensitivity is in all bands between 2.0uV and 1.2uV. The table of sensitivities is given at the end of this page. These are fairly good figures since Collins only promised sensitivity “better than 5 uV” for a new unit. I believe I can get better AM sensitivity than 1uV by a more careful alignment and controlling the external noise better. The odd bands seem to have more noise, indicating need of more careful alignment.

The CW sensitivity is measured by injecting unmodulated carrier to the receiver input, again at 10dB SINAD level. The receiver is set to 1 kHz mechanical filter, BFO on, AGC off, and using the liberty of playing with the XTAL rejection notch.  The sensitivities are between 0.25uV and 0.35uV.  Morse code is readable better than 3dB over the receiver noise at the input level of 0.1uV. This is the lowest level my RF generator HP8656B can give.

I am rather satisfied with these results.

Collins 51J-4                             17.3.2017

 f (kHz)  AM sensitivity (uV)  CW sensitivity (uV)
 3950     1.20                 0.20
 4950     1.45                 0.25
 5950     1.40                 0.20  (*)
 6950     2.00                 0.35
 7950     1.20                 0.20
 8950     1.70                 0.25
 9950     1.25                 0.20  (*)
10950     1.50                 0.25
11950     1.50                 0.25
12950     1.70                 0.25
13950     1.45                 0.25
14950     1.90                 0.25
15950     1.40                 0.20
16950     1.45                 0.20
17950     1.30                 0.20

(*) Some external QRM/QRN.