Immunity of Television Receivers and Video Cassette Recorders (VCRs) to Direct Radiation from Radio Transmissions, .5 to 30 MHz
|Publication Date:||1 May 1987|
The 0.5 to 30MHz frequency range has been designated initially because the majority of interference complaints fall within this range.
The two field strength levels specified recognize that specific authorized services are responsible for the highest field strengths and most potential interference.
Fields in excess of 1 V/m can exist close to AM broadcast stations or amateur radio stations employing maximum power and a directional antenna.
CB transmitters are low power (4w). However, they are consumer devices and have achieved high market penetration. Hence other consumer devices (e.g., TV receivers) must operate in close proximity to CB transmitters.
The 1 V/m guideline for the designated bands and 0.3 V/m for other frequencies permits the concentration of efforts by manufacturers where there are real problems without diluting these efforts or compromising situations needlessly to improve immunity at frequencies where no high level interfering sources are likely to be found.
The 1 V/m value corresponds to the American National Standards proposal in ANSI C63.12 as the minimum susceptibility guideline for electronic devices.
The Canadian Department of Communications has published the electromagnetic field strength probability profiles for three Canadian cities. One percent of the locations exceeded 2.6 V/m in Ottawa. Less than 1% of the locations in Toronto and Montreal exceeded 1 V/m.
The guideline presented here covers entertainment products. Personal safety and protection of property are not involved. A 1 V/m guideline is considered appropriate for this class of device. It represents an achievable challenge to manufacturers.
The guideline and the method of measurement are supported by the results of measurements made by an EIA consultant on a group of 24 TV receivers and VCR's from 11 manufacturers at RCA Corporation, Indianapolis, September 1983, in a 2m TEM cell. The test methods were refined on a similar group of receivers measured in a similar TEM cell at NAP Consumer Electronics Corp., Knoxville, Tennessee, in late 1984.
The frequency band necessary to modulate the video signal on VCR tape contains the same frequencies (AM broadcast and amateur radio) from which a VCR is to be protected. Video signal levels induced at the video heads are very low; therefore, interference from external signals becomes a serious matter. Because there must be provision for insertion of a cassette, shielding of the head area is a difficult design consideration. The orientation of the VCR affects immunity, as pickup is dependent on the direction of the magnetic component of the external RF field.
Large screen (greater than 25 inches diagonal) TV receivers may be physically and electrically too large for a 2m TEM cell or an open stripline. An open field measurement site may not be permitted. Therefore, further study is necessary.
The measurement of immunity of TV's, VCR's, and other electronic devices, such as radios and tape recorders, to common mode conducted interference on leads (including the AC power cord) is currently being studied and may be the subject of a separate EIA standard. Measurement of both direct radiated and conducted interference may be necessary to define the immunity of a product.