On Thu,1/29/2015 1:23 PM, Tom W8JI wrote:
People just love adding series impedances, that what we do as hams. :)
Bypassing is usually much more effective than simply adding series
impedances. Sometimes bypassing alone is more than enough. 90% of the
time when I clean up an offshore SMPS for Ham use, it is just moving
ground leads and bypassing. :)
This snide remark ignores the fact that it is generally not practical
(nor is it good politics) to go inside your neighbor's equipment to add
bypass capacitors. Nor is it practical to open up a wall wart, nor even
most consumer equipment. It also ignores the fact that much of RFI at HF
is common mode, not differential mode, caused by bonding failures (the
Pin One Problem and its power systems equivalent). And it ignores the
fact that it is often the cable shield or the power system equipment
ground that is carrying the RF noise current, for reasons noted in my
earlier post.
The use of common mode chokes that have a high value of resistive
impedance at the desired operating frequency is an effective fix for
common mode RFI the results from poor equipment design. Your only
problem is that you didn't think of it first.
Think about it -- a single ferrite core applied to wiring from a noise
source has a low Q parallel resonance in the range of 150 MHz, causing
to appear as a resistance in the range of several hundred ohms. (Look
at the impedance curves on the data sheet for any Fair-Rite suppression
part. If you don't recognize that as a parallel resonant circuit, you
failed electrical circuits 101.) The impedance of that single core is
enough to get the equipment past Part 15 in the VHF range, so they stop
there. The resonance is in the wrong place for HF, so we wind turns to
move it down where we need it, and in the process, multiply both R and L
by the square of the turns, and C by the number of turns. Again, all of
this is fundamental circuit concepts.
Do all systems NEED 5-10K ohms resistive Z? Of course not, BUT most
systems are used over a wide range of frequencies and have a wide range
common mode impedances -- anything from high to low, capacitive to
inductive to resistive. A simple inductor resonates with a capacitive
common mode circuit, which increases the common mode current, making the
problem worse. But resistance ALWAYS reduces the current -- by how much
depends on the impedance of the circuit.
I have always recommended high values of common mode Z because it never
makes the problem worse, because it doesn't require the person with the
problem to be a trained engineer, because it's relatively simple and
inexpensive, and because we don't have to go "inside the box." The only
potential problem is if the conductor being choked has enough common
mode current to fry the choke -- for example, a high power antenna system.
If you happen to BE the product engineer, certainly you should be
solving the problem inside the box before the product goes out the door.
73, Jim K9YC
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