Mark; After re-reading my reply I may need to clarify a point or 3 :-) The repeater controller normally strips off CTCSS/PL from the rx audio from an incoming mobile by use of a high pass filter set to roll off all below 300hz. The controller decodes it first if it has that feature but it is stripped out... The controller, if so equipped ( in this case it is) will regenerate a new CTCSS/PL signal and encodes it out on a separate lead destined for an ENCODE input of the repeater transmitter, NOT the MIC input as that is filtered as well to strip low freq audio below 300 hz. The only exception is if the radio transmitter has a PAGING ( or now days 9600 baud FSK ) audio input, this input is normally flat pretty much from DC to daylight.... (at least through the whole audio pass band) In that case it IS POSSIBLE to shove both encode and voice into that port but is probably going to sound like garbage as it will have harmonic mix's that sound nasty..
SO NORMALLY there is a separate ENCODE port into the modulator, that is NOT the mic jack, or the MIC circuit ... that is defined for encoding tones below 300hz as a separate path entirely through the transmitter.. In synthesized radios it is normally a port coming pretty much from the microprocessor which generates the tone/dpl, after it is processed , filtered and cleaned up.. If you can SPOT this port, you can break the circuit and insert your own encode here ( again we are talking about a conversion mobile transmitter not a Mastr II / Micor etc.. that have dedicated CTCSS inputs... It is normally directly into the circuit of the VCO reference oscillator or very close to it as the CTCSS/PL can be viewed as a frequency aberration to the PLL loop filter if it is injected to late into the circuit like where the voice modulation is input.. Low freq DPL/CTCSS looks like an "unlock condition" to the loop filter /PLL if it is present in the normal voice process... and the PLL will try to "CORRECT" itself which would have the result of either killing the "aberration" or severely messing with it to "FIX" the problem. This loop filter/ PLL correction issue is a key factor in hooking up FSK stuff like 9600 baud packet ( Or D-Star as we are soon going to be seeing) into a radio .. as the modulation input is NOT linear across the audio pass band and the low freq rolloff KILLS 9600 baud components that approach DC in frequency under normal circumstances...UNLESS it is specifically engineered to do that ( some are ) or the loop filter component values can be changed to make it less concerned about low frequency aberrations ( more specifically slow the integration time so they are ignored). Radios with Packet inputs ( for 9600 FSK) have already dealt with the engineering side but you still need to match audio impedances to keep the processing clean. Doug KD8B
