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Re: [RFI] Multi-Stage High Efficiency Air Conditioner Units

Subject: Re: [RFI] Multi-Stage High Efficiency Air Conditioner Units
From: Jim Brown <jim@audiosystemsgroup.com>
Reply-to: jim@audiosystemsgroup.com
Date: Sun, 9 Feb 2020 01:22:33 -0800
List-post: <mailto:rfi@contesting.com>
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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