[ARC5] S+N/N ratio results.

[Richard Knoppow]

     I've just been reading an old book _Radio Engineering 
Handbook_ 4th Edition, Keith Henny editor, published by 
McGraw-Hill 1950.  In the section titled "Receiving Systems" 
there is a fairly lengthy discussion of the methods of 
measurement of receiver characteristics in use at the time. 
This includes the ENSI measurement of noise. ENSI means 
Equivalent Noise Sideband Input and is very similar to the 
test I described earlier. One difference, which I never 
noticed before, is that a sharp audio filter is used for the 
tone part of the test. This is 30% modulation at 400 hz. The 
filter is taken out for the carrier part of the test. I have 
not tried this because I just discovered it and don't have a 
suitable filter anyway. I suspect it results in lower signal 
levels than are read for the test without the filter.  I 
will give this a try on my AR-88 using the tone control. Not 
exact but will give some idea of how the test works. The 
ENSI test was a standard for some time but is NOT the test 
given in the handbooks for many communications receivers 
where carrier only noise is compared to _unfiltered_ 
recovered modulation and noise.  ENSI was designed mostly 
for home receivers.  There is also a brief section on noise 
figure and more in the section on FM receivers.  The noise 
source for N.F. is a starved diode. That is a diode with no 
space charge. The noise is related to the space charge so it 
can be calculated by means of plate current. The noise 
output with a dummy antenna is measured first and then the 
diode current is brought up until it doubles the output 
_power_ from the dummy load. The N.F. is the ratio in 
decibels of the two measurements. If you don't need 
laboratory grade measurements I think a source would not be 
difficult to make.
     The section on receiver tests is interesting because it 
explains what some of the specifications of old receivers 
really mean.


--
Richard Knoppow
Los Angeles
WB6KBL
dickburk at ix.netcom.com