EMI and RFI

Keith Hilton · Post by **Keith Hilton** » 11 Mar 2000 12:37 pm

EMI-electromagnetic interference. RFI-radio frequency interference. Many people blame ground loops, and the equipment they are using for hum or buzz problems. Many times this is not the problem. There are many trnasmission towers, T.V. Radio, micro-wave, cell phone, and other signals floating around. These can enter electric power transmission lines and eventually enter the 120 volt AC line your equipment plugs in to.
This becomes a big problem when a electric power line is directly below a transmitter that is pumping out large amounts of energy. This energy gets into the electric power line ,and can cause excessive noise, that manifests itself as hum or buzz. Most product builders address this problem, but the problem has yet to be "TOTALLY" eliminated by any manufacturer. I know that Hartley Peavey is concernted about EMI and RFI, and so am I. Sometimes noise in equipment is not the manufacturers fault. It will probably never be eliminated totally, since there is more and more EMI and RFI transmitted energy.
Usually manufacturers address these issues with bypass capacitors, chokes, ferrite beads,good grounding, and many other things.
I would like to hear some solutions that have worked for you. Like absorptive low-pass filters comprised of ceramic-disc capactor and ferrite bead. Also methods of attenuating energy in noise spikes with transformers, and inductors used with other components. Would also like to hear some success stories with lossy ferrite beads, and wire thread-through techniques that have worked for you. I don't suppose this is out of place, since this is the electronics section.

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[This message was edited by Keith Hilton on 11 March 2000 at 12:38 PM.]

Jack Stoner · Post by **Jack Stoner** » 12 Mar 2000 7:08 am

Keith, I completely rewired Charlie Kellogg's "Big K" recording studio in Kansas City (north) years ago. His studio was in an old building about a block from an AM radio transmitter site.

I got the RF out (or attenuated to the point it was not a problem) of the recording equipment and his TEAC mixing board but never got it completely out of the headphone system.

Most of it consisted of .01 disc ceramic caps and shielding. In fact I didn't use a lot of the by pass capacitors, just made sure everything was shielded and grounded properly and it took care of most of his problems.

C Dixon · Post by **C Dixon** » 12 Mar 2000 9:19 am

Very interesting thread Keith and Jack.

Truly and engineer's nightmare, is the induced and developed, unwanted and extraneous sounds eminating from electronic gear.

I could site you horror stories all day concerning past and sometimes futile efforts to suppress and/or eliminate such sounds.

I do not envy any design engineer who has been given the task of getting rid of such unwanted noises. What hangs heaviest over the engineer's head is just about anything that he comes up with, can and often hurts the sound that we DO want. And herein lies the delimma.

At RCA's Princeton's labs, thousands of tried and failed circuits have been built. Only to be rejected for cause.

'Tis too bad indeed that the chosen power line frenquency had to be right in the audio pass band. Since none of this frequency is needed in amplifiers, (we just need the resultant power), it is a bear to deal with.

Given the levels that sound is played at nowadays, and the quietness of present day audio circuits, it is little wonder that we hear remnants of power line and induced noises "bleeding" thru.

Since I have been retired from my former company, I have no idea of the present strides that are being taken to greatly suppress or eliminate these unwanted noises.

But my heart is with you and whomever is trying to do this.

God bless you in your noble attempts,

carl[This message was edited by C Dixon on 12 March 2000 at 09:20 AM.]

Sam Marshall · Post by **Sam Marshall** » 12 Mar 2000 11:01 am

Much of the RFI concerns these days result from the radiation due to digital signals. High speed edge transistions on clocks, strobe, address, and data lines constitute RF energy eminating from a piece of equipment. Any gear with DSP or microprocessor control can be an offender and are subject to FCC regulations just like your computer. This falls more under the technical realm of EMC (Electromagnetic Compliance).

Regards,
Sam

Keith Hilton · Post by **Keith Hilton** » 12 Mar 2000 12:58 pm

Jack, I agree, .1uf caps to ground,placed all over the place works wonders.
Has anyone had any luck using "pi" filters? That is, putting a inductor between two capacitors to ground? Or, putting a common-mode transformer between two capacitors to ground? Please give values of the capacitors, inductor and transformer? If you were using these "pi" filters on a dual polarity power supply where would you put them in the chain? I know where I put mine, but I would like to hear some other ideas? Thanks!

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Mark Amundson · Post by **Mark Amundson** » 12 Mar 2000 6:40 pm

The RF problem generally can be just limited, not eliminated. Good shield cabling, circuits with .01uf to .1uf supply bypass capacitor (ceramics), and good quality metal chassis with few cracks or open seams are the best preventive measures.

Fortunately RF signals attenuate rapidly as the distance from the source is added (1/d^2). Your friend the RF engineer and fellow pedal steeler.

Mark,

Rich Paton · Post by **Rich Paton** » 17 Mar 2000 8:24 pm

Keith, in a related thread we discussed this in a similar vein recenty, and it's a
HUGE technical subject.

But...here's what I think is a good place to start any basic "RFI Control" research, at the Corcom RFI Faq.

http://www.cor.com/faq/rfifaq.htm

Keith Hilton · Post by **Keith Hilton** » 17 Mar 2000 9:26 pm

I went to the site you listed. Rich, all I can say is GREAT! Everyone should read and be aware of this stuff. As a manufacturer, I attempt to go overboard on filtering and noise reduction. I happen to think the problem is going to get worse. I think there is a lot of illegal stuff coming out of CHINA. If a person buys one of my products and gets a hum, he automatically thinks it is my fault. The same is true of Peavey. It may not be. He may of created a ground loop, or who knows what. A even more dangerous problem lies in the number of "INDIVIDUAL" power supplies used in all the equipment combinations. I've seen guys with 10 AC adapters, and cords run everywhere. What happens if something in the middle is turned off? These are called power sequencing problems. Unless designed protection is built into the device, sooner or later large surges ,or draws, will damage the transistor junctions of the device. I know one company building steel guitar electronic stuff that does not put "ANY" type of protection in their stuff. It is like playing RUSSIAN ROULETTE. I don't feel luck, so I load my stuff down with protection, and hope for the best. I just learned of a electronic part I can install, that will tell me if the electronics has been in water. When I have to repair something ,I want to know "WHY" it failed.
Some music gear is rode hard and put away wet!

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Rich Paton · Post by **Rich Paton** » 18 Mar 2000 6:33 am

Keith, here's another good one! There's a reference to excellent free info pamphlets from "Industrial Communications Engineers" towards the end of the webpage as well as a huge searchable database.

http://www.arrl.org/tis/info/rfigen.html

Keith Hilton · Post by **Keith Hilton** » 18 Mar 2000 10:05 am

Rich, you must be as paranoid about hum and noise as I am. This last web-site you listed is as good or better than the first.

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Rich Paton · Post by **Rich Paton** » 18 Mar 2000 11:39 am

Keith, I've been around equipment using Corcom brand RFI filters since 1971. That is what they do, so I figured it was a good shot.
With the ARRL, I knew they would have good RFI supression/control info, due in part to their personal amateur radio operating needs, and also there's the the fact that many a crackerjack Electrical Engineer lurks behind the guise of a mild-mannered Ham Radio enthusiast! The combined practical electronics knowledge at ARRL is beyond estimation.