[Elecraft] Negative rail grounding

[Jim Brown]

On 4/18/2018 1:33 PM, Josh Fiden wrote:
> SS30 is an SMPS so a different beast than the prior discussed linear 
> supplies. It's possible that the DC return to chassis connection is 
> needed for EMC compliance. That's providing a low impedance return 
> path for noise generated by switch elements. Without the connection, 
> noise will take an unwanted path and come out as common mode noise. 

In fact, the opposite is true. When V- is NOT bonded to shack ground, 
the power cable forms a transmission line, confining the field for noise 
current to a small region around the pair. But when V- IS bonded to 
shack ground, return current divides between the shack ground path and 
the V- (black) conductor. The fields no longer cancel, the noise current 
takes whatever path it chooses, and a magnetic loop is formed between  
V+ and shack ground.

Common mode noise current can easily be blocked using ferrite chokes on 
the DC cable just was we do with coax.

> A frequent source of the noise is capacitive coupling from MOSFETs to 
> chassis, sometimes via a heat sink. 

Yes. Interior circuit design and layout is critical to low RF noise.

73, Jim K9YC