[Fwd: LF: Rcvr Sensitivy tests]
Sun, 03 Jan 1999 16:18:07 -0500
Andy Talbot wrote:
> >> Basically, we used a HP generator tuned to 136 kHz, connected it to the
> >> receiver under test, and reduced the generator's output until we could
> >> no longer hear the signal. Not a very scientific test, but good enought
> >> to help us select the receivers we will take with us for the LF
> >> receiving test at Nags Head on 15-18 January
> >> The minimum output power we could reliably measure was -127 dBm into 50
> >> ohms (calibrated against another HP spectrum analyzer)
> >> Rycom 3121 (transistorized) -106
> >> Icom IC-735 -116
> >> Rycom 2174A (tube type) -116
> >> Icom IC-720 -126
> >> Cubic (US Navy rcvr) > -127
> >> NRD 525 > -127 dbm
> >> Icom R-71 A >-127
> Some of those are rather poor !!!
> To add another Rx to the list ..
> Racal RA1792 -135 dBm ( but with a different pair of ears listening
> for 'not audible' )
> Calibration was with both HP Vector voltmeter and HP scope to double check
> (+/- 0.2 dB). Signal source was a portable DDS based module operated off
> batteries. Any attempt to use a mains supply for this, or an Ailtech signal
> generator, caused signal leakage of around -100dBm into the Rx via ground
> loops etc.
> This leakage is a potential route for QRM on these bands.
> The same leakage problem is not evident when frequency rises into the MHz
> region and falls well below audibility. Has anyone noticed similar effects
> on other receivers or have I got a grounding problem on the RA1792.
> Since it was difficult to ascertain when the signal was "no longer
> audible" the S/N of this -135 dBm signal was measured using Dopplergram
> in on it's widest setting. (Gram is equally suitable when used in spectrum
> analyser mode)
> 27dB S/N in a 0.7 Hz bandwidth which equates to 5 dB S/N in 100 Hz - and
> seems about correct for listening to a CW tone. Rx noise figure is 14 dB
> based on these calculations, which is roughly what it should be according to
> spec and taking into account the error in calculating S/N from a spectrum
> Andy G4JNT / G8IMR