Sensitivity Measurement

For measuring sensitivity of SSB and CW signals, the procedure is basic and undisputed.  In either case, you measure the noise with the signal absent and the signal plus noise with the signal present.  But with AM sensitivity measurements, there are two ways that are presented by various sources.

Sensitivity is defined as signal plus noise over noise, commonly expressed in decibels.  The signal plus noise portion of the measurement is often taken with 30% modulation and 4KC bandwidth for the R-390A.  The question that arises in some discussions is how to measure the noise-only portion.  Is this measurement to be taken with no RF at all (as it is when making CW or SSB sensitivity measurements), or should it be taken with the carrier present.

If the measurement is taken with no signal present, the noise will NOT be the same level of noise as taken when measuring signal plus noise.  Because always with an AM signal, there is a carrier along with the modulation.  The noise level changes with the presence of the carrier.  So to measure signal plus noise over noise, the noise-only measurement must be taken with the unmodulated carrier present.  Otherwise, the definition of "noise" changes between the two measurements taken.

In order to avoid confusion, I use the following terms to designate which type of sensitivity measurement is used:

N1 is the receiver noise in the presence of an AM carrier

N2 is the receiver noise with no signal present

S+N1/N2 : Sensitivity, where the noise-only measurement is taken with no RF present

S+N1/N1 : Sensitivity, where the noise-only measurement is taken with the unmodulated carrier present

The S+N1/N1 measurement is the one presented most in the vast majority of the literature and in the Field Depot manuals for the R-390 and R-390A receivers.  I "cut my teeth" on the S+N1/N2 method, and many of those measurement results are recorded on this site.  Future measurements will use the S+N1/N1 method.  I will be making updates to this site to specify the method used as new measurements are added.

A more detailed discussion of sensitivity measurements (by Dallas Lankford) can be found here:

James Moorer's Noise and Sensitivity Page

 

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© 2005 by Walter Wilson, KK4DF.  All information is offered for non-commercial use without warranty, expressed or implied.

 

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