Common mode
current, which is
what typically
bothers consumer
devices or causes
consumer devices to
bother us, comes
from a variety of
causes. Sometimes
several different
things combine to
make a problem. Very
rarely are harmonics
or other transmitter
maladies at fault.
The exception is VHF
or HF transmitter
harmonics to a
television or FM
receiver, and this
is very rare.
Common Mode RFI in Consumer Devices
To have common mode RFI there has to be a path THROUGH the gear, not just to the gear. The path is always over the outside of a shielded cable, although it can be on either or all conductors of an unshielded cable. With unshielded wires and cables, the problem is common mode on all conductors including any ground wires. Once in a while differential mode currents can cause a problem, but not nearly as often as common mode currents.
The problems is almost always never direct pickup, but most often from the power line acting like an antenna or ground and some other lead, such as speaker wires or antenna leads, providing the other "half" of the undesired antenna system. The problem with anything like this is very often multiple paths are involved, but power mains are usually one of them because of the sheer size of the "antenna" formed by the power mains. Whatever the path, almost always multiple wires or cables are involved in the path. The path can be from the power mains through the electronics to the speaker leads, it can be from speaker leads to the chassis of some other device connected to the amplifier and back through power lines, it can almost any path.
RF can flow along any path between external connections A, B, and C. Most often B, the power cord, is involved. The last thing we want to do is have individual pieces of equipment in an equipment group or cluster plugged into different electrical outlets.
The RFI path is
normally not a differential mode signal path. This means long conductors, like speaker
leads for external speakers, contribute to problems even when rectification is
not in the audio
output. Think of the
problem as the
speaker leads acting
like an antenna or a
ground, with the other leads (like power line or CATV lines) acting as the other
half or "pole" of a big dipole.
Switching a front panel switch to a different input port
might not change the interference, even when a cable to a particular port is
involved,
because cable shields
and other conductors remain
connected. Only
center conductors
switch, and with any properly connected
shielded cable the
center conductor always has
zero common-mode RF
current. Common mode RF current on shielded cables is always on the outside of
the shield. Switching off the center conductor often has very little effect.
Curing RFI in a Typical Installation
The installation below is in my family room. It has been there for about 7 years with no RFI or lightning damage.
I modified this box by adding two .01uF 250 VAC rated UL/CSA/VDE approved capacitors from each power lead to the safety ground. The 75-ohm cable fittings were already grounded to the safety ground, and the telephone jack bypasses were grounded there also.
Every power connector from the entertainment center, without exception, routes to this common outlet and cable box.
Hidden behind my stereo amplifier (and not shown in the picture) the external speaker leads are bypassed through .01 uF disc capacitors to a lug under a screw in the stereo amplifier's metal case. That unit's case is grounded to the electrical safety ground through the three wire cord.
This connects or bypasses every wire entering or leaving the immediate area of the entertainment center to the common "ground" point provided by the outlet strip.
Computer Center RFI
At every computer center, I also use a single outlet strip. The strips I use have internal MOV's, but I add .01uF 250 VAC UL/CSA/VDE approved capacitors as bypasses from each power line lead to the safety ground.
If a Telecom line has to come into the same area, I use a protection strip as used in my television installation above.
Every connection must always run through and be bypassed to one single point.
This not only helps for RFI coming in, it also protects against lightning damage and RFI going out.
Using the above methods I have virtually no TVI or RFI either to or from my consumer devices.
Background to the Above Method of RFI Protection
In the 70's, I engineered systems for a small cable operation with plants over northwestern Ohio and Southeastern Michigan. We had a few hundred systems of various sizes. We inherited a very large apartment complex from a cable operation because areas of the complex were built over the radial field of a 5 kW AM and 50 kW FM broadcast station. The original cable operator installed double shield cables and TVI filters throughout the system before giving up.
I had our crews replace all the cables with normal single-shield 75-ohm hardline, grounding our cables to the service feeds for each building. At those points we installed our distribution splitters, and followed the power feeds with our distribution cables to each apartment. At the apartments, we mounted our outlets next to the power feeds used by TV sets, and bonded out box grounds to the outlet box. Where this was not possible I made my own boxes, where the TV set power feed and CATV feeds were bonded.
The result of this was a nearly perfect RFI cure. Out of dozens of TV sets that were formerly unwatchable, only one or two televisions required additional work.
If you have a serious problem and need help with correcting RFI, need help figuring out how to modify a box, or are chicken and would like to purchase modified outlet boxes, contact me.
c2010 W8JI