Yes. Common mode chokes can correct for serious design errors and the
penny-pinching cost reductions made by the bean-counters that degrade
the RF immunity of the product. Common mode chokes applied to noisy
cables are non-intrusive to the design -- for example, they don't affect
rise times that can seriously increase dissipation and cause equipment
failure. Capacitors and inductors applied to the differential circuit
CAN cause problems by affecting rise times.
As a SYSTEMS engineer who realizes the complexity of designing inside
the box, I have always concentrated on solutions that are outside the
box. And the differential circuit (like things that affect rise times(
is often inside the box.
If you haven't already done so, I STRONGLY urge you to study the app
notes on my website about "The Pin One Problem," which is, sadly, nearly
universal, and the primary cause of hum, buzz, and RFI in practical
systems. k9yc.com/publish/htm
In addition to adding ferrite common mode chokes to wiring that can act
as antennas, the most important solution is proper bonding. That's
addressed here. http://k9yc.com/GroundingAndAudio.pdf
73, Jim K9YC
and the On 2/8/2020 5:58 PM, David Eckhardt wrote:
One interview I ran while at HP trolling for a tech for our EMC group.
I asked the candidate what he though of ferrites. I don't like specific
questions on an interview. I'd rather ask general questions and see
where the interviewee takes it. His answer, "Ferrites are our
friends". I had to laugh a bit, but couldn't fault his answer.
If things are designed and executed properly with EMC/RFI in mind, there
should be no need for these clamp-on ferrite 'beads'. However, few
designs are bullet proof and most times compromised by budgets. Ferrites
are useful as a band aide to a marginal of compromised design. They
should be deployed intelligently, not haphazardly. I have to use them
generously on my wireless router to keep the RFI to acceptable levels
for radio astronomy. I have no control over the design inside the box.
In addition, there is no common mode choke on my PC power supply for
economic reasons, I'm sure. I put one there and its amazing how much
difference it makes. Ferrites are a good thing so long as they are
installed intelligently.
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