Topband: Desktop Power Supply Brand/Noise Question

[Tom W8JI]

> On 7/21/2013 8:49 AM, Tom W8JI wrote:
>>
>> I can't understand why throwing some ferrite beads at a problem, or 
>> changing the supply, are the only two solutions.
>>
>> Many times, if not most times, a few .01 uF line voltage rated bypass 
>> capacitors are significantly better than a sting of cores, or a winding 
>> through cores.
>
> Yes, IF the problem is differential-mode coupling to the power line. But 
> often it is NOT --

Bypass capacitors will cure common mode also, because common mode cannot be 
generated without differential mode someplace in the system between two 
points.

In virtually every situation, the power supply is in a metal box. The trash 
is between the power line leads, and also between those leads and the case. 
This is the nature of switching supplies, because they tie a "chopper" with 
squarewaves across the power mains, and the system is not well balanced.

As a general rule the stuff exiting the dc side is much less problematic. If 
it is problematic, it needs cleaned up. Bypassing to the cabinet, 
groundplane, or case fixes or greatly assists in fixing it, too.

If a line is properly bypassed to the case, the system can't produce much 
common mode or differential mode on that particular line.
In the rarer case, where two or more lines are involved, they all must be 
suitably bypassed.

The same things that work for lightning protection work for RFI issues, and 
vice versa. Payback for doing things right, rather than just tossing beads 
at wires, is much wider than the initial target.

73 Tom