[Elecraft] K3: 6M SSB audio hash
Jim Brown
jim at audiosystemsgroup.com
Sun Jun 8 16:17:26 EDT 2008
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
More information about the Elecraft
mailing list