On 10/10/2010 9:56 AM, G. Beat wrote: > It is not unusual for newer microphones to separate the microphone > shield/ground for Audio from the DC ground used for Push to Talk (PTT) > switching.
Yes, mics and mic cables are wired in a variety of ways. Some have four conductors plus a shield. Some have two tiny coaxial cables plus an overall shield. Some have only two conductors plus a shield. Some have three conductors plus a shield. When wiring a mic to a ham rig, we must figure out which of these we have. To do that, we must get our words straight. The return for a mic capsule is not MIC GROUND, it is mic RETURN or COMMON. The return for PTT is not PTT GROUND, it is PTT RETURN or PTT COMMON. These points will nearly always look the same with a DC Ohmeter, but at RF they may have significant inductance between them, and that inductance creates a Pin 1 Problem. Cable shields can act as receive antennas, so they can carry RF current (like from our transmitter). The ONLY proper connection for an overall cable shield is the CHASSIS. If you connect it anywhere else (for example, to a circuit board, where it wanders around and eventually finds the chassis), you have a PIN 1 PROBLEM, which is an open door to RFI (RF in the shack). If you have only one cable shield, it MUST GO TO THE CHASSIS to prevent RFI. Even though that shield may be used for mic RETURN and PTT return, it still must go to the CHASSIS. Inside the radio, MIC return and PTT return are connected to the chassis via circuit traces. Those circuit traces are plenty good enough to carry mic audio and PTT return (DC), but they MUST NOT CARRY THE CABLE SHIELD. That's a Pin 1 problem. For more on The Pin 1 Problem, see http://audiosystemsgroup.com/RFI-Ham.pdf 73, Jim Brown K9YC ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
