[Milsurplus] Further note on WB vs NBFM

[gl4d21a at juno.com]

Part 2:

After I sent the previous information, I realized I overlooked a 
point which seems to cause a lot of confusion among those trying to 
figure out compatibility issues.  And it revolves around a 
breakthrough invention patented by Dan Noble, the noise balanced 
squelch.  The way this works depends on properly shaped, clipped and 
filtered transmit audio, and a modulation bandwidth which matches the 
receiver IF bandwidth.  But I am getting ahead of myself.

Carrier operated squelch is/was used on AM radios and early FM 
systems.  When the DC level from the detector passes a preset 
threshold, the audio circuit is completed, and something comes out of 
the speaker.  Unfortunately, that was too often noise, which caused 
the operator to "tighten" the squelch, thus requiring an ever 
stronger wanted signal to be necessary to "get through".  Noble's 
invention overcame a lot of that problem. 

I have seen only two examples of the noise balanced squelch applied 
to AM, the Morrow FTR mobile IF and successor receivers of the 1950s, 
and one model of Kaar HF monitor receivers of about the same 
vintage.  Motorola did not even use their own invention in their AM 
radios, or at least not any I saw.  All "modern" FM receivers use it, 
and it works from an amazingly simple basis.  If you have a receiver 
with, say 10 kHz wide IF passband (any bandwidth applies), when you 
look at the detector (AM, FM, PM, whatever) output, there will be a 
noise spectrum from low up to about 10 kHz.  Now, if you insert a 
carrier in the middle of that IF, the noise content above 5 kHz drops 
off faster than that below 5 kHz.  The noise from one edge of the 
bandpass no longer mixes with the other edge, but with the carrier 
about half way between.  So, by designing the gain in a noise 
detector channel broadly tuned up near 10 kHz to be about the same as 
that exhibited by the detector itself to noise, when input noise 
alone increases or decreases, the circuit remains balanced, but any 
carrier, even a weak one, will unbalance the amplitudes of the two 
channels and open the squelch.

What happens when you apply an over-deviated signal to a receiver 
containing such a squelch?  Well, two things, both undesirable.  
First, much of the energy of the modulated signal goes outside the IF 
passband on certain voice peaks, causing an apparent drop in the 
signal level at the detector and thus producing squelch closure, and 
some of that "audio" also gets into the noise channel, further 
contributing to closing the squelch.  Hence, "squelch clipping" 
occurs on any overdeviated signal referenced to the victim receiver 
IF passband.

I have not mentioned pre-emphasis and de-emphasis because it merits a 
more comprehensive treatment than I am willing to try to give it 
here.  See any of the good texts on FM for an analysis.  Just 
remember that "time constant", which the pre and de emphasis boys 
tend to use is the recriprocal of frequency roll off, which we use.

73,
George
W5VPQ


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