> I would infer from this that if you knew what was inside the boxes, it
> might be possible to build a fairly decent filter by putting one of type A
> and one of type B in series. Is that correct?
We have to watch the safety ground. If it passes through the filters without
being filtered the combination can be nearly the same as no filtering at
The series common mode impedance offered by ferrites wound with or slipped
over line cords, speaker cords, or shielded cables is is generally
inadequate. The ability to substantially modify the system impedance is
unpredictable at best, because we have no idea what the commion mode
impedances in the system are UNLESS we establish them with bypassing and
common very short connections.
We can establish the common mode impedances the fileters see at a low
impedance by proper single point grounding of all filters or (safe to
ground) cables entering the sensitive device, and treating that point as a
barrier or wall. Usually that cures most problems anyway.
One of the most effective cures for RFI with coaxial cable fed TV sets is to
ground the coaxial cable shield (for RF) directly to the electrical safety
ground of the same outlet feeding the TV set! That single simple almost
free change cured most cases of RFI I have seen.
As I've said, I go to MCM or someone and buy a cheap multioutlet lightning
protection strip with telco and CATV outlets as well as power outlets.
That's what I modify to use as a filter (by adding UL/CSA approved parts
like line bypass caps), since I "cluster" all of my electronics in a room in
one small area. Those strips "ground" the CATV cable shield directly to the
I wouldn't think it necessary to use two filters. It really isn't a critical
application. If we do things correctly with "so-so" parts it would cure
virtually all problems.
The most common thing I see are ground or RF loops through the gear, and
instead of providing an alternate path we just start adding clamp on cores.
A string of 73 material beads a foot long is only 1000 ohms or less across
most of HF. The common mode impedance already might be several hundred ohms
or more (and of the same sign) so what big change can we expect if we only
Bypassing around the devices is the key, not adding some small value of
> Alternatively, I see on page 1077 of the current Mouser catalogue that
> Schurter appears to make some 1 and 2 stage filters that do this all in
> package. Do any of those meet your specs for HF rejection?
Using a "standard" to judge while placing a filter into an unknown impedance
to remove an unknown level from an unknown sensitivity device doesn't really
make sense to me. I use progressive modification until the system is
The problem with filter measurements is when the system changes from the
test standard, which it always will, the results of the tests are often
useless. A very good filter in a controlled test can be a real dog in the
actual system compared to a more mediocre looking filter.
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