[Milsurplus] 10 meters w/BC684,924 xmtrs

[gl4d21a at juno.com]

Back when Howard (That's Major Armstrong to you, sonny) invented FM, 
the idea was to make everything as linear as possible, and swing as 
wide as possible to take best advantage of the "FM Improvement" for 
high fidelity broadcasting.  Well, some of that was productive, the 
rest didn't really apply to communications circuits.  If you study 
the design of the original Motorola, General Electric, Link, and the 
other initial FM two way radios, you will find the same philosophy.  
Swing wide and let 'er rip.  However, by 1950 in the US mobile radio 
bands, narrower bandwidths and closer channel spacings were mandated, 
and this, of course, forced design changes.  Specifically, the audio 
stages in the transmitters were designed to go into saturation, a 
cheap version of limiting, which concentrated the audio into a 
narrower range of amplitudes, and thus also a narrower transmit 
bandwidth.  Soft peak clipping is also used to describe the speech 
processing.  This was supplemented with the addition of a low pass 
filter to clean off the clipping artifacts above 3 kHz.  Somewhere 
out in my vast storage area, I still have one of the original 
Motorola add-on audio amplifier/limiter kits which was designed for 
the "casket" radios with Loktal tubes, to make them meet the changed 
regulations.  So, there are several ways to accomplish this same 
effect in army radios, diode limiters plus low pass filter works 
pretty well.

Now, those misunderstood receivers.  First, a wideband receiver 
receives narrow band signals just fine.  What it does not do is 
reject adjacent channel interference.  Also, from actual development 
lab experience, I can state that properly limited +/-5 kHz bandwidth 
voice audio can sound as loud or louder than unlimited +/-15 kHz 
modulation.  You see, I was designing radios when the transition from 
15 to 5 happened, and we did a lot of exactly what is being discussed 
here.  

In some army radios, the receiver audio recovery may be marginal, and 
a transistor power amp built into the speaker is a good non-intrusive 
solution to that.  However, the 603 and 683 receivers I used to tune 
up on channels of interest for pre-scanner listeners all seemed to 
have plenty of audio recovery on narrow band commercial signals.  IF 
bandwidth, on the other hand, is a challenge.  Usually redesign of 
the IF cans is necessary, or insertion of a stage or two of narrow 
band IF just after the mixer is required to solve the adjacent 
channel problem.  The latest two-way IF and detector stips are so 
tiny that using one of them plugged into the grid pin of the first IF 
tube and out to the volume control will give you a fully reversible 
mod, and a HOT receiver.

Just some musings from the bench.  Hope they are useful to someone.

73,
George
W5VPQ


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