The first part of this paper reviewed the first one-third of the report including the Title of the Paper, the Background to its development, the Members of the Subcommittee that developed the report, Definitions, Table of Contents, Scope and Section 4. This second part of the paper will look at Section 5 (Susceptibility of Communications Receivers to Commercial EDP/OE Emanations) of the Report.
In the middle of the 1970s, the United States Federal Communications Commission (FCC) began to look seriously at electromagnetic emissions from electronic data processing (EDP) equipment and office equipment (OE). This growing awareness on the part of the United States telecommunications regulation body was a result of the increasing number of computers being used by society and the increased potential for growth by licensed broadcast services due to the proliferation of electronic-computer sources. The Computer and Business Equipment Manufacturers Association (CBEMA) formed a technical subcommittee to assist in preparing
an industry response to the concerns of the FCC.
Title of Paper
The title of the published paper was Limits and Methods of Measurement of Electromagnetic Emanations from Electronic Data Processing and Office Equipment. The report was prepared by Subcommittee 5 on Electromagnetic Interference, which was organizationally part of the Environment and Safety Committee of CBEMA.
Table of Contents
The 183 page report included a Title Page, Foreword, Table of Contents, Scope, Definitions, Introduction, seven major sections, Conclusion, and one Appendix.
The seven major sections were:
- Section 4 – Electronic Data Processing and Office Equipment as a Source of Electromagnetic Emanations
- Section 5 – Susceptibility of Communication Receivers to Commercial EDP/OE Emanations
- Section 6 – Interference Potential of EDP/OE
- Section 7 – The Commercial EDP/OE Interference Models
- Section 8 – Emanation Limits for EDP/OE Products
- Section 9 – Comparison of Recommended Limits with Others
- Section 10 – Emanation Measurement
Section 5: Susceptibility of Commercial Receivers to Commercial EDP/OE Emanations
This section of the report investigated principal communication services in the Unites States of America. It divided services into two sections: low frequency-high frequency (LF-HF) and very high frequency-ultra high frequency (VHF-UHF).
Though not all services were studied, those that had the greatest potential for interference due to the sensitivity or number of their receivers were studied in great detail. Receiver populations and antenna density adjacent to electronic data processing/office equipment centers was carefully studied and reported on in Section 6 of the report. Receiver frequency selectivity and “electrical noise” tolerance were also investigated.
Furthermore, Section 5 looked at the derivation of signal-to-noise (S/N) ratios and the interference criteria for narrowband (NB) and broadband (BB) receiver responses.
Table 1 shows the Federal Communication Commission (FCC) frequency allocations in the LF-HF range in 1977.
Examples of VHF-UHF (30 MHz to 300 MHz) services include: government, public safety, industrial, transportation, amateur, television (TV), operational fixed, radio astronomy, aeronautical navigation, broadcast frequency modulation (FM) radio, airdrome control, aeronautical mobile, space research, maritime mobile, land fixed mobile, TV and FM links, and industrial, scientific and medical (ISM).
Receiving devices in the VHF-UHF range had common technical characteristics such as detector type, intermediate frequency (IF), bandwidth sensitivity, antenna type, and antenna polarization. The VHF-UHF range was divided into ten bands in Table 5.3 of the report and the bands were used to develop emanation limits with respect to receivers in those bands.
Typical received and recommended FCC field strengths in the AM radio band (535 to 1605 kHz) were listed in Table 5.4 in the report, as duplicated in Table 2.
A similar table was generated for the TV bands encompassing 54 – 88 MHz, 174 – 216 MHz, and 470 – 890 MHz. Similarly, other field strengths were analyzed for the remaining communication services in the study.
A communications receiver’s susceptibility to electromagnetic signals is determined by two major factors: its frequency selectivity as gauged by its bandwidth and its amplitude sensitivity as gauged by its signal-to-noise ratio (S/N).
Reciever interference has both objective (technical) criteria and subjective (human differentiation) criteria. If multiple lines of energy are captured by the bandwidth of a receiver (in addition to the intended signal), the resultant “interference” sound like buzzing and popping on an AM radio band. If a single line of energy is captured in conjunction with the intended signal, a herringbone pattern may be displayed on a TV screen. The report clearly deferentiated between the two types of interference; the first being broadband (multiple lines of energy) and the second being narrowband (single line of energy). The physiological response of the human eye and ear varies from human to human. No single EMI receiver could duplicate all the various communication receivers in existence, so the report used the following criteria: a pulse repetition rate (PRR) of greater than 1000 pulses per second would be narrowband for the study, and a PRR of greater than 100,000 pulses per second would be narrowband for radiated emissions. PRRs of less than those values would be considered broadband.
Signal-to-noise ratios of receivers were collected from available literature for most receivers in the report. The thresholds of detectable interference (TDI) were generated by critical listeners and observers. The TDI and corresponding field strength level were used to calculate either the peak or quasi-peak S/N ratio for use in the radiated model, or the TDI was used directly in the conducted model.
In the empirical part of the report, it was decided that actual EDP/OE equipment would not be used because its signal amplitudes could not be adjusted to search for receiver noise thresholds. Therefore, radio frequency (RF) sine wave generators and square wave generators were used to simulate the EDP/OE emissions under controlled laboratory conditions.
Audio TDIs of AM radio receivers were determined in a low-audio ambient environment (40 – 45 dBA); approximately 45 receivers were evaluated. The receivers were tuned to 1 MHz and the carrier was modulated with a 1 kHz audio tone (1 kHz was selected because it is in the most sensitive region of the human ear). The BB audio output of the radios was found to be somewhat independent of the pulse width so a pulse width was selected to produce a nearly uniform spectral amplitude throughout the passband of the receiver at the chosen carrier frequency. Both the NB and BB distributions of the TDI for the receivers studied were identified as “normally distributed”. When the carrier was modulated with actual music or voice, the threshold of detectable interference was 6 dB greater than the RF source level. An audio masking factor of 9 db was used for NB signals and 8 dB for BB signals.
Figure 1: Figure 5-2 from the study
For the case of a radio with an ungrounded neutral for the radio, a worst-case factor of 6 dB was assumed.
Published S/N ratios for TV also were researched for the report. Accepting a Grade 3 quality picture (as defined by TASO – the Television Allocation Study Organization) as the standard, a 45 dB S/N tolerance was used for a narrowband source. Due to lack of information in the literature of the day, an empirical study for BB interference was done using 34 TV receivers. The observed TDI was highly dependent on the PRR. It was noted, among other conclusions of the BB study, that color TV sets were more sensitive than Black and White TV sets. In general, S/N ratios for other communication receivers were found in the literature and used in the report.
The method used by the 1977 study for developing BB peak and quasi-peak EMI limits was based on consideration of the empirically determined TDI levels (which are PRR dependent) evaluated at the selected BB pulse repetition rates for each of the services.
Table 5-13 in the study summarized the BB signal-to-noise ratios that were used in the development of the BB limits in Section 8 of the report. A portion of Table 5-13 is reproduced in Table 3.
Table 3 (click image for larger version)
Section 5 provided a detailed summary of the primary communication services in the USA (along with their allocated frequencies) and typical received field strengths under defined conditions. The response of a receiver to EDP/OE emanations is a function of pulse repetition rates (PRRs), S/N ratios, and receiver bandwidths. The PRRs determined whether the interference was narrowband or broadband. The result of the studies and empirical data generated in this section provide a measure for TDI relative to broadband and narrowband sources. A “transition PRR” is acknowledged where the observable effects change from BB to NB. The analysis was performed for conducted emissions from 450 kHz to 30 MHz and for radiated emissions from 30 MHz to 1000 MHz.
|Daniel D. Hoolihan
is the Founder and Principal of Hoolihan EMC Consulting. He is a Past-President of the EMC Society of the IEEE and is presently serving on the Board of Directors. He is presently an assessor for the NIST NVLAP EMC and Telecom Lab Accreditation program. Also, he is the Vice-Chair of the ANSI ASC C63® committee on EMC.