[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




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