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CISPR 23: Limits for Industrial, Scientific and Medical Equipment Radio Interference
Radio interference limits for ISM RF equipment covering designated frequency bands and spurious emission requirements from 0 Hz to 400 GHz
Introduction to CISPR 23
CISPR 23 defines the limits of radio interference produced by Industrial, Scientific and Medical (ISM) RF equipment operating in the frequency range 0 Hz to 400 GHz. ISM equipment includes RF induction heating units, dielectric heating equipment, medical diathermy devices, RF welders, industrial microwave generators, and plasma generators. These devices intentionally generate RF energy for non-communication purposes but can cause severe interference to radio services. CISPR 23 specifies both the permitted ISM frequency bands (where high emissions are allowed for functional purposes) and the spurious emission limits outside these bands that must be met to protect radio communication services.
ISM equipment using frequencies outside the designated ISM bands (6.78 MHz, 13.56 MHz, 27.12 MHz, 40.68 MHz, 433.92 MHz, 915 MHz, 2.45 GHz, 5.8 GHz, and 24.125 GHz) must meet significantly more stringent emission limits — typically 20-40 dB lower than for in-band operation.
ISM Frequency Bands and Emission Limits
CISPR 23 divides ISM RF emissions into two categories: fundamental emissions within designated ISM bands, and spurious emissions outside these bands. Within the designated ISM bands, equipment may emit high RF power levels necessary for their industrial function — no specific limits apply to these fundamental emissions. However, harmonics and parasitic oscillations that fall outside the ISM bands are subject to spurious emission limits. The standard specifies both conducted limits (on the power supply line) and radiated limits, with measurement procedures following CISPR 16 methodology.
| ISM Frequency Band | Typical Applications | Spurious Limit (0.15-30 MHz) | Spurious Limit (30-1000 MHz) |
|---|---|---|---|
| 6.78 MHz ± 15 kHz | RF induction heating | 54 dBµV (QP) | 44 dBµV/m at 10 m |
| 13.56 MHz ± 7 kHz | RF welders, plasma generators | 54 dBµV (QP) | 44 dBµV/m at 10 m |
| 27.12 MHz ± 163 kHz | Industrial heating, medical diathermy | 54 dBµV (QP) | 44 dBµV/m at 10 m |
| 40.68 MHz ± 20 kHz | RF drying, medical therapy | — | 44 dBµV/m at 10 m |
| 2.45 GHz ± 50 MHz | Microwave ovens, plasma processing | — | 54 dBµV/m at 10 m (1-3 GHz) |
RF welding machines operating at 27.12 MHz are among the most problematic ISM interference sources. The fundamental emission at 27.12 MHz is permitted without limit, but the second harmonic at 54.24 MHz falls in the FM broadcast band (87.5-108 MHz) only if we consider higher harmonics — the third harmonic at 81.36 MHz and fourth at 108.48 MHz directly affect VHF TV and FM radio bands and must be stringently controlled.
Engineering Design for ISM Equipment EMC
Controlling spurious emissions from ISM equipment presents unique challenges because the RF power levels involved are high — typically 1 kW to 100 kW. The primary mitigation techniques include: (1) proper shielding of the RF generation and application sections using copper or aluminum enclosures with seam bonding at intervals ≤ λ/20; (2) balanced RF output configurations to minimize common-mode currents on supply and control cables; (3) RF output filtering using low-pass or band-reject filters to attenuate harmonics; and (4) ferrite common-mode chokes on all penetrations of the RF enclosure including power, coolant, and control lines.
Power supply filtering is particularly critical for conducted emission compliance. ISM equipment often uses three-phase power, requiring three-phase LISNs and filters rated for the full operating current (often 50-500 A). Custom-designed harmonic filters using tuned LC traps at the dominant harmonic frequencies (2nd, 3rd, and 4th harmonics) provide 30-50 dB attenuation at specific frequencies. Installation considerations — including equipment room shielding, cable tray bonding, and separation of ISM supply cables from sensitive signal cables — are as important as equipment design for overall EMC compliance.
A well-designed RF shielded room with 60 dB attenuation at 100 MHz, combined with proper mains filtering (200 A three-phase feedthrough filter), can reduce ISM equipment radiated emissions by 40-60 dB and is the most reliable approach for high-power ISM installations.
Measurement Challenges for High-Power ISM Equipment
Measurement of ISM equipment emissions per CISPR 23 presents practical difficulties due to the high RF power levels involved. Direct connection of measuring instruments to the RF output is dangerous and may damage sensitive receivers. The standard allows for alternative measurement methods including: substitution methods using calibrated antennas, near-field scanning with probe correction factors, and reverberation chamber testing. For large industrial installations, on-site measurements following CISPR 16-2-3 procedures are often necessary, with careful attention to ambient noise levels and the use of time-domain averaging to separate the ISM emissions from background signals.
Never connect measurement equipment directly to high-power ISM RF outputs without appropriate attenuators (minimum 40-60 dB rated for the full power level). Attempting to measure a 10 kW RF welder output directly will destroy the measuring receiver input stage instantly.
Frequently Asked Questions
Q: Are microwave ovens in homes covered by CISPR 23?
A: Yes, domestic microwave ovens are ISM equipment. However, most countries require household microwave ovens to comply with specific product standards (e.g., IEC 60335-2-25 for safety and CISPR 11/14-1 for EMC in residential environments).
A: Yes, domestic microwave ovens are ISM equipment. However, most countries require household microwave ovens to comply with specific product standards (e.g., IEC 60335-2-25 for safety and CISPR 11/14-1 for EMC in residential environments).
Q: What is the difference between CISPR 23 and CISPR 11?
A: CISPR 11 has largely superseded CISPR 23 for most practical purposes. CISPR 11 provides more up-to-date limits and measurement methods for ISM equipment, while CISPR 23 remains as a reference for specific frequency allocations.
A: CISPR 11 has largely superseded CISPR 23 for most practical purposes. CISPR 11 provides more up-to-date limits and measurement methods for ISM equipment, while CISPR 23 remains as a reference for specific frequency allocations.
Q: Do ISM equipment manufacturers need to test every unit?
A> Type testing (representative sample) is generally accepted. However, CISPR 23 advises that production units should maintain at least 2 dB margin below the limits due to component tolerances and manufacturing variation.
A> Type testing (representative sample) is generally accepted. However, CISPR 23 advises that production units should maintain at least 2 dB margin below the limits due to component tolerances and manufacturing variation.
Q: How do I calculate the required shielding effectiveness for an ISM equipment room?
A> The required shielding effectiveness (SE) is: SE = P_ISM – Limit + Margin, where P_ISM is the expected field at 10 m without shielding, Limit is the applicable radiated limit, and Margin is typically 10 dB. For a 10 kW ISM source, SE of 50-70 dB is typically required.
A> The required shielding effectiveness (SE) is: SE = P_ISM – Limit + Margin, where P_ISM is the expected field at 10 m without shielding, Limit is the applicable radiated limit, and Margin is typically 10 dB. For a 10 kW ISM source, SE of 50-70 dB is typically required.
