{Collins} Re: 75A4 Sensitivity Redo



David,
Your measurement will work if you have a "True RMS" AC volt meter.  This 
involves actually taking the square root of the mean of the square of 
the AC voltage.  I don't know whether the HP400FL is capable of making 
this calculation.

Many AC volt meters rectify the AC waveform, measure the average DC 
value of the rectified AC waveform and multiply the average by 1.11 to 
get RMS voltage of a sine wave.  If this method is applied to white 
noise, which is not made up of sine waves, the RMS voltage reading will 
be 11.5% low.  You can correct this error by multiplying the reading of 
this type of AC voltmeter by 1.13 when measuring white noise.

Academically, it shouldn't matter what type of signal you are measuring 
as long as its spectrum fits within the measurement bandwidth and you 
measure for a period long enough to get a true average value of the 
signal.  Obviously, making this measurement on a CW signal would be 
faster, easier, and probably more meaningful for comparison than making 
the measurement for some complex modulated waveform.

Bob K4ERR

David C. Hallam wrote:
> I am hung up on this sensitivity measurement issue.  The 75A4 manual I have
> states "1.0 microvolt  for 6 db signal to noise ratio with 3 KC bandwidth"
> It does not state whether this is AM or CW sensitivity.
>
> Here is my method to measure:
> Mode: AM
> Pass band: Centered (0)
> Selectivity: 3.1 KC filter  I have only the 2.1 and 3.1 KC filters
> AVC: Off
> Noise Limiter: Off
> Rejection Tuning: Off
> Tuning set to center of band of interest, e.g., 3.7 MHz
> HP-400FL AC voltmeter connected to 4 ohm speaker terminals with speaker
> connected
> HP-606B signal generator set to same frequency as above and 30% modulation
> at 1000 Hz
>
> Connect 75A4 coax antenna terminal to 50 ohm dummy load and cap output
> terminal on signal generator.  Measure output voltage at speaker terminals.
> Remove dummy load and connect signal generator to antenna terminal.  Set
> output of signal generator so that the voltmeter reads 2X what the reading
> was with no signal.  Rock the signal generator frequency and pass band
> tuning to peak output on the voltmeter.  Adjust signal generator output
> level as necessary to obtain the 2X reading on the voltmeter.  The microvolt
> reading on the HP-606B meter is the sensitivity at 6 db S/N.
>
> Is this correct or am I doing something wrong?
>
> David
> KC2JD/4
>
>
> .
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.
SHARE THE FUN: JOIN THE COLLINS COLLECTORS ASSOCIATION TODAY !
      http://www.collinsradio.org/html/membership.html    -paypal accepted-
.
 NETS: Tues & Thur: 3.805 MHz-2000 Central + Fri: 3.895 MHz-2000 Pacific
  SUNDAYS: 14.263 MHz-2000 UTC  +  AM Net: 29.050 MHz-1200 Central
       1st Wednesday AM Net 3.880 MHz-2000 local (ET, CT, MT, PT)





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