ENGINEERING NOTES
CCL Engineering Note 290
Comparison of FCC Limits with CISPR Limits
September 1993
The Federal Communications Commission (FCC) has amended Part
15 to harmonize the United States standards for radio frequency
emissions from digital devices with the international emissions
standards for these devices.
The new rules permit parties seeking to authorize a digital device
to choose to demonstrate that the device complies with either
the Part 15 standards or the international standards found in
Publication 22 of the International Special Committee on Radio
Interference (CISPR).
In harmonizing the standards, the FCC agreed to accept measurements
demonstrating compliance with the standards in the 1985 version
of CISPR 22, as amended by the Draft International Standards already
adopted by CISPR. The FCC retained its existing standards for
emissions above 1000 MHz, but permitted emissions above 1000 MHz
to be measured at the test distances specified in CISPR 22. The
FCC also requires that testing be performed following the American
National Standards Institute (ANSI) C63.4-1992 measurement procedure.
The following tables and graphs illustrate the differences between
the FCC Limits and the CISPR Limits:
Limits on AC Powerline Conducted Emissions
| Class A Digital Devices |
|
|
|
|
|
| |
FCC Limits |
|
CISPR Limits |
|
|
| Frequency (MHz) |
Voltage Quasi-Peak |
(dBµV)* |
Voltage Quasi-peak |
(dBµV) Average |
|
| 0.15-0.45 |
No Limits |
No Limits |
79 |
66 |
|
| 0.45-0.5 |
60 |
None |
79 |
66 |
|
| 0.5-1.705 |
60 |
None |
73 |
60 |
|
| 1.705-30 |
69.5 |
None |
73 |
60 |
|
| Class B Digital Devices |
|
|
|
|
|
| |
FCC Limits |
|
CISPR Limits |
|
|
| Frequency (MHz) |
Voltage Quasi-Peak |
(dBµV)* |
Voltage Quasi-peak |
(dBµV) Average |
|
| 0.15-0.45 |
No Limits |
No Limits |
66-56.9** |
56-46.9** |
|
| 0.45-0.5 |
48 |
None |
56.9-56** |
46.9-46** |
|
| 0.5-5 |
48 |
None |
56 |
46 |
|
| 5-30 |
48 |
None |
60 |
50 |
|
*The comparison of the FCC and CISPR conducted limits
must take into account the differences in measurement
procedures. While the FCC does not have a limit on
the average value of conducted emissions, the measurement
procedures permit the FCC quasi-peak limits to be
raised by 13 dB if the difference between quasi-peak
and average measurements is 6 dB or greater. Under
this condition, the limit for Class B digital devices
becomes 61 dBµV (quasi-peak) and 55 dBµV (average,
representing the minimum 6 dB difference). Similarly,
for Class A devices the limits become 73 dBµV (quasi-peak)
and 67 dBµV (average) for the band 0.45-1.705 MHz
and 82.5 dBµV (quasi-peak) and 76.5 dBµV (average)
for the band 1.705-30 MHz.
**The limit decreases linearly with the logarithm
of the frequency.Limits on Radiated Emissions
| Class A Digital Devices |
|
|
| |
FCC Limits* |
CISPR Limits* |
| Frequency (MHz) |
Field Strength (dBµV/m) @ 10 meters |
Field Strength (dBµV/m) @ 10 meters |
| 30-88 |
39 |
40 |
| 88-216 |
43.5 |
40 |
| 216-230 |
46.4 |
40 |
| 230-960 |
46.4 |
47 |
| 960-1000 |
49.5 |
47 |
| > 1000 |
49.5 |
No Limit |
| Class B Digital Devices |
|
|
|
| |
FCC Limits* |
|
CISPR Limits* |
| Frequency (MHz) |
Field Strength (dBµV/m) @ 3 meters |
Field Strength (dBµV/m) @ 10 meters |
|
Field Strength (dBµV/m) @ 10 meters |
| 30-88 |
40 |
29.5 |
|
30 |
| 88-216 |
43.5 |
33 |
|
30 |
| 216-230 |
46 |
35.6 |
|
30 |
| 230-960 |
46 |
35.6 |
|
37 |
| 960-1000 |
54 |
43.5 |
|
37 |
| > 1000 |
54 |
43.5 |
|
No Limit |
*The FCC Class B limits were converted to 10 meters
using an inverse linear distance extrapolation factor
(20 dB/decade), as specified in 47 CFR Section 15.31(f)(1).
CISPR limits and FCC limits ≤ 1000 MHz are
based on quasi-peak measurements. FCC limits above
1000 MHz are based on the use of an average detector.
For emissions above 1000 MHz, 47 CFR Section 15.35
also limits the emissions, measured with a peak detector,
to 20 dB above the stated averagelimit, e.g. peak
emissions above 1000 MHz for Class A devices, measured
at a distance of 10 meters, shall not exceed 3000
æV/m (69.5 dBµV/m). Measurements above 1000 MHz are
required under 47 CFR Section 15.33 when the digital
device contains an oscillator operating at 108 MHz
or higher.
**CISPR Publication 22 states that if the field strength
measurement at 10 meters can not be made because of
high ambient noise levels or for other reasons measurements
may be made at a closer distance, for example 3 meters.
An inverse proportionality factor of 20 dB per decade
should be used to normalize the measured data to the
specified distance for determining compliance. Care
should be taken in measurement of large test units
at 3 meters at frequencies near 30 MHz due to near
field effects.
Reference: Federal Communications Commission,
Notice of Proposed Rule Making, ET Docket No. 92-152,
released July 30, 1992 and Report and Order (FCC 93-421)
adopted August 20, 1993. |
|
