From: utzoo!decvax!ucbvax!mhtsa!eagle!karn
Newsgroups: net.followup
Title: RFI & Terminals
Article-I.D.: eagle.573
Posted: Sun Oct 24 20:40:21 1982
Received: Mon Oct 25 02:14:51 1982

I have learned a good deal about this topic, because I have been both
the generator and the victim of computer RFI.  I am an active amateur radio
operator interested in computers, and as such have torn my hair out many
times when trying to use my home computer in persuit of my hobby,
especially when receiving very weak signals from satellites.

Anyway, the rules are like this:  All new computer equipment manufactured
after a certain date (I don't know exactly when offhand) must be
certified by the manufacturer as complying with either "Class A" or
"Class B" emission limits.  A device intended for a commercial
environment (a VAX, for example) will have to meet Class A.
Class B requirements, which are MORE strict, will have to be met by
computing devices intended for residential environments (games, personal
computers, etc).  The FCC allows uncertified equipment to be
manufactured up to the deadline, but requires a warning notice to be placed on
the equipment that says, in effect, that the equipment may cause
interference if used in a residential area and that the owner would be
required to take whatever steps are necessary to correct any interference.
For a sample of this notice, look in those little plastic pockets on the
back door of any VAX made in the last two years or so.

Many newer terminals are coming out that are certified as meeting either
the Class A or B RFI limits.  My home terminal, the Heath H-19A, meets
the B limits. It can be operated ten feet away from a TV with rabbit
ears and not generate any RFI.  Unfortunately, retrofitting earlier
terminals to make them RFI tight is much more difficult than designing
it into the terminal to begin with.

It has ALWAYS been the case that the owner of any device (whether intended
to be a radio transmitter or not) that interferes with another service by
radiating energy within the frequencies assigned to the other service has
the legal responsibility to correct the problem.  You'd be amazed at the
variety of non-communications devices that can radiate objectionable
amounts of RF: computers, terminals, power lines, oil burners, microwave
ovens, electric motors, etc.

This is not to be confused with interference caused by the inability of
a device to reject authorized transmissions to which it isn't supposed to
respond, e.g., a record player that picks up a local amateur radio
transmitter.  President Reagan recently signed a bill that gives the FCC
to set rules requiring the manufacturers of home entertainment electronics
meet certain minimal standards for the rejection of out-of-band RF
energy.

Hope this helps.

Phil Karn, KA9Q
BTL Murray Hill