At 7/6/2009 17:39, you wrote: >Ian, > >I suspect that the problem is "talk off" caused by overdeviation of the mike >audio. When the CTCSS tone is mixed with the mike audio prior to the >deviation limiter stage, an excessive voice signal will be hard limited >along with the CTCSS tone, causing a distorted signal. This problem occurs >quite often in Amateur-grade equipment, since the makers often have very >"hot" tone deviation that is set with a fixed resistor rather than >adjustable as in most commercial-grade radios. Alinco, Icom, and Yaesu >handhelds seem to have this problem all the time. Every Alinco handheld >I've tested has a PL deviation above 1 kHz, and a few samples were up to 1.5 >kHz- about three times the necessary amount. Needless to say, a tone >deviation of 1.5 kHz doesn't leave much room for voice deviation, and all it >takes is a loud talker to cause the limiter to "squash" the modulating >signal and distort the CTCSS tone enough for the repeater to drop.
Since the CTCSS is injected into the modulator after the limiter, a higher CTCSS deviation level wouldn't cause increased clipping in the TX. In fact, I suspect the reason for the higher CTCSS level settings from the factory is to overcome the lack of any high pass filtering in the voice modulation path. Ideally, everything below 300 Hz would be filtered out of the voice audio, but most (cheap) radios don't have this filtering (gotta save that $0.25 per radio by omitting a couple more Rs & Cs). If this is the case, lowering the CTCSS level will probably make the problem worse. Perhaps lowering the voice (clipped) deviation level so that the total deviation is somewhat less than 5 kHz will reduce the voice to CTCSS ratio to the point that the CTCSS decoder can "find" the tone amongst all the sub-300 Hz voice audio. Another solution is to install a bottle of helium with a dispensing tube run up the mic cable with a solenoid valve activated by the PTT switch ;) Bob NO6B
