Hello Mark.
04 Aug 97 16:22, Mark Magill wrote to Jeff Edmonson:
 MM>
 JE>> Should be simple enough to build something, with a 555 IC,
 JE>> and R/C component to obtain the frequency.
 MM>
 JE>> However you decide to come up with the actual tone, remember
 JE>> to inject it at the CARRIER level, and not in the audio
 JE>> section.  Remember to keep the ouput low enough to not be
 JE>> heard by human ears, yet detectable by the repeater
 JE>> reciever.  They're -supposed- to be sub-audible ;-)
 MM>
 MM>         That's why I wondered why our association would pick an
 MM> AUDIBLE sub-audible tone frequency. Some places I have heard of use
The only reason it's audible is because it's injected into the mic circuit. 
Inject it into a final stage, like the center of the dev pot and set it to 
dev the output at 500hz.  Nobody will hear it.
 MM> the lower ones 40.x hz or the like. I do have one commercial kit that
 MM> generates a single tone, but it uses the no-longer-common 567 chip
 MM> that Radio Shack -USED- to sell.
Uh, subaudible tones start at or near 67hz.  You can still buy 567 from RS.  
Or any small electronic supply outfit.
 MM>  Also, it generates more of a square-ish wave than a sine. It sounds
 MM> 'funny' to those who hear me on the air with it, and friends who hear
 MM> me talking on a non-PL repeater say "hey, turn your tone off!" to
 MM> remind me.. :-) But at least it works.
Doesn't matter the wave pattern.  It's because you injected the signal into 
the mic circuit.
MM>
 MM>         Good point about carrier-level injection, since as someone
 MM> else pointed out that injection at the audio-processor level would
 MM> result in clipping during voice peaks - and our local repeater will
 MM> drop you like a hot stone the second that PL disappears! The fellow
 MM> who installed the kit in my radio for me did it at mic input cicuit
 MM> level, so I have to be careful not to speak overly loud into it or I
 MM> can cause a drop out. But it DOES work, within those limits, and  he
 MM> did the best he knew how to for me.
Find the deviation pot, move the input there and adjust for 500hz deviation.
... "A phaser is the universal communicator."  - Worf
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