The big change was the way we indicated the onset of limiting in the transmitter. The LED was changed to green to warn our users that this was a different system than in the past. The previous red LED on the B version came on just before limiting and users were setting the gain too low on the transmitter and having noise artifacts from both the compander and RF link. On the C version, the green LED is turned on by the same matched FET as is used in the limiter itself so the LED comes on when you are in limiting and not before. Too many users were being "scared" by the previous limit LED even though the limiter is pretty smooth and IMHO, does little damage to the signal.
In scientific tests performed on anybody we could drag into engineering, we found that the gain on the C version is set about 10 dB higher by the average user than on the B version. What is really important though, is the questions and complaints from end users about low level noise, artifacts, etc. have dropped off by a factor of ten. In reality, if the gains are set the same on the B and C versions they will perform the same.
The biggest unseen change to the C transmitters is the addition of a circular isolator or circulator to the output stage of the transmitter. This is a magnetically polarized non linear ferrite device that has three ports, any of which can be used as input or output. What's black magic about this device, is power applied to port A goes to port B, but power applied to port B goes only to port C, and power applied to port C goes only to port A. What this does for a transmitter is this: the output stage is connected to port A of the circulator and the power is delivered to port B which is connected to the antenna. The transmitter acts just like a regular transmitter so far. However, if the antenna tied to port B picks up power from another transmitter such as a two way radio or more commonly another wireless , this power doesn't get back into the output stage that is connected to port A but gets transferred to port C. Port C is tied to a 50 Ohm resistor and the incoming RF is simply dissipated as heat. The circulator reduces intermodulation between transmitters by 30 to 40 dB. Intermodulation between transmitters is probably as common as intermodulation in receiver front ends and some interference attributed to the receiver may be really due to transmitter intermod.
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