On Sun, 8 Jun 2008 14:28:45 -0400, Joe Subich, W4TV wrote: >Then I should not have said "PTT Ground" but that is how Icom, >Kenwood and Yaesu all label their mic connectors. In all three >brands, the "PTT ground" is DC return and is, in fact, chassis >ground - usually a short jumper from the "PTT Ground" pin to >the chassis or the ground trace on a circuit board which contains >mic connector which is tied to chassis with multiple grounding >and mounting screws.
Joe, Thanks for hanging in on this issue, because it is VERY important. My point is that what you have described is ALSO WRONG. That short jumper can cause common impedance coupling, as the voltage drop across it is added to circuit common. That "short jumper" has inductance, and at some frequency, and/or at some power level, the voltage drop across it becomes strong enough to cause RFI. On my website are photographs of very high quality (and very expensive, German-made) condenser mics with serious RFI problems. In one of them, the cable shield goes to the shell of the mic with a jumper less than one inch long. In downtown Chicago, where TV transmitters are on tall buildings, that mic begins to detect TV broadcast stations at roughly 180 MHz. An older mic from the same manufacturer begins detecting at TV channel 2 (54-60 MHz) and FM broadcast! Mics with shorter jumpers begin having trouble only on higher UHF channels. They ALL have trouble with cell phones. Documentation of this, along with the extensive testing I've performed, are on my website. http://audiosystemsgroup.com/publish The voltage drop across that inductance is proportional to frequency, inversely proportional to distance between the victim equipment and the transmitting antenna, and proportional to the square root of the transmitter power! I will keep repeating this until it becomes clear. The ONLY good place to terminate a cable shield is to the shielding enclosure DIRECTLY. Any other connection sets up the possibility of common impedance coupling -- what pro audio folks call "the pin 1 problem." >The whole point is that all of the "big three" manufacturers use >the shield in their microphones improperly by connecting it to an >ungrounded mic return instead of the chassis ("PTT Ground"). In >the schematics I have checked, every one provides a DC return for >the mic/preamp using an RF choke but connects the mic shield to >the mic return instead of the chassis. I also see this, and I strongly agree with you that it is VERY WRONG, and is often the cause of RFI into the ham rig (what we call RF feedback). This improper connection of cable shields is the primary cause of RFI of all types, including RF in the shack. Add to this the US members of that group, which should more properly be called the big five. The K2 and the K3 have improperly terminated shields at some connectors. So does my Ten Tec Omni V. One piece of good news -- the front panel mic connector on the K3 IS mounted directly to the shielding enclosure. So are all the RF connectors. n Sun, 8 Jun 2008 09:39:20 -0700, Brian Lloyd wrote: >I guess that people forget that, in shielding equipment, they are >building a Faraday cage around it. That means that you need to >terminate your shield at the OUTSIDE of the equipment, not inside. One >wants to continue the Faraday cage all the way out to the input >device. This means that the shield of any wire needs to be attached to >the chassis externally. That isn't hard to understand. I know that I >solved the problem in my designs by using shielded twisted-pair for >phono cartridge input and tying the shield to the chassis. YES! 73, Jim Brown K9YC _______________________________________________ Elecraft mailing list Post to: [email protected] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
