Physical Address
304 North Cardinal St.
Dorchester Center, MA 02124
CISPR 20: Sound and Television Broadcast Receivers – Immunity Requirements
Immunity requirements for AM/FM radio and TV broadcast receivers against conducted and radiated electromagnetic disturbances from 0.15 MHz to 150 MHz
Introduction to CISPR 20
CISPR 20 specifies the immunity requirements for sound and television broadcast receivers and associated equipment in the frequency range 0.15 MHz to 150 MHz. This standard ensures that consumer audio/video equipment can operate as intended in the presence of common electromagnetic disturbances encountered in domestic environments. The standard covers AM and FM radio receivers, television receivers (analog and digital), audio amplifiers, and auxiliary input/output devices. Immunity testing per CISPR 20 evaluates both conducted disturbances (via power and signal cables) and radiated electromagnetic fields, with specific performance criteria for audio and video degradation.
CISPR 20 immunity limits are closely aligned with the emission limits of other CISPR standards, creating a balanced EMC compliance framework. A receiver compliant with CISPR 20 should be able to operate without perceptible degradation in the presence of emissions from neighboring equipment that comply with their respective emission standards.
Immunity Test Methods and Performance Criteria
CISPR 20 defines several immunity test configurations covering conducted disturbances on antenna terminals (0.15-30 MHz), conducted common-mode disturbances on audio/video/control ports (0.15-30 MHz), and radiated RF electromagnetic field immunity (30-150 MHz). For each test, specific performance criteria are defined: Class A (no perceptible degradation), Class B (temporary degradation with automatic recovery), and Class C (temporary degradation requiring manual reset). Audio performance is assessed through the SINAD (Signal-to-Noise and Distortion) ratio, while video performance is evaluated against the CCIR five-grade impairment scale.
| Test Type | Frequency Range | Severity Level | Performance Criterion |
|---|---|---|---|
| Conducted RF (antenna port) | 0.15 – 30 MHz | 126 dBµV (EMF) – AM band, 120 dBµV – FM band | SINAD ≥ 20 dB for AM, ≥ 30 dB for FM |
| Conducted CM (audio/video ports) | 0.15 – 30 MHz | 126 dBµV (EMF) | Class A — no perceptible degradation |
| Radiated RF field | 30 – 150 MHz | 3 V/m (unmodulated) | Class A — no perceptible degradation |
| Power port conducted | 0.15 – 30 MHz | 126 dBµV (EMF) | Class A — no perceptible degradation |
The antenna terminal immunity test is particularly important for FM broadcast receivers. A 20 dB SINAD degradation at the limit level is considered the pass/fail threshold — values below 20 dB indicate unacceptable audio quality degradation in real-world listening conditions.
Engineering Design for Broadcast Receiver Immunity
Designing broadcast receivers that meet CISPR 20 immunity requirements involves multiple engineering disciplines. Front-end selectivity is critical — properly designed input bandpass filters with high-Q inductors and low-temperature-coefficient capacitors reject out-of-band interference before it reaches the mixer stage. The intermediate frequency (IF) filtering chain must provide adequate adjacent-channel rejection, typically 60-80 dB for FM receivers and 40-60 dB for AM receivers. Modern receivers employ surface acoustic wave (SAW) filters and ceramic resonators for precise IF selectivity.
PCB layout and grounding strategy significantly affect immunity performance. A solid ground plane on the top layer, careful separation of analog and digital sections, and ferrite bead isolation on all cable entry points are essential practices. Differential signaling for audio paths (rather than single-ended) provides 20-30 dB common-mode rejection improvement. Shielded enclosures with proper gasketing around connector openings can provide 10-20 dB additional immunity margin for radiated field tests.
Implementing a Pi-type EMI filter (ferrite bead + 100 pF capacitor to ground + ferrite bead) on each audio input/output line can improve conducted immunity by 15-25 dB with negligible audio signal degradation and a component cost of less than $0.10 per channel.
Correlation Between Laboratory Testing and Real-World Reception
Laboratory immunity testing per CISPR 20 uses standardized disturbance levels that may not perfectly represent all real-world scenarios. The 3 V/m radiated field level corresponds to a relatively strong interference source at 3 m distance. In practice, receivers may encounter higher field strengths from nearby transmitters or intentional radiators. Engineers should design with 6-10 dB of immunity margin above the CISPR 20 requirements to ensure robust performance in challenging electromagnetic environments. Field testing with actual interference sources — such as nearby power line communication transmitters or switching power supplies — provides valuable validation beyond laboratory compliance testing.
Receivers installed within 100 m of high-power broadcast transmitters may experience field strengths exceeding 10 V/m at the antenna input. CISPR 20’s 3 V/m test level is not sufficient for these installations — additional front-end filtering and shielding are mandatory for co-located receiver sites.
Frequently Asked Questions
Q: Does CISPR 20 apply to digital television receivers?
A: Yes, CISPR 20 covers both analog and digital TV receivers. However, digital receivers may show a sharper degradation threshold (cliff effect) compared to the gradual degradation of analog systems.
A: Yes, CISPR 20 covers both analog and digital TV receivers. However, digital receivers may show a sharper degradation threshold (cliff effect) compared to the gradual degradation of analog systems.
Q: What is the difference between CISPR 20 and CISPR 35?
A: CISPR 20 is the older standard focused on broadcast receivers. CISPR 35 (published 2016) supersedes CISPR 20 and extends immunity requirements to multimedia equipment including computers and gaming consoles.
A: CISPR 20 is the older standard focused on broadcast receivers. CISPR 35 (published 2016) supersedes CISPR 20 and extends immunity requirements to multimedia equipment including computers and gaming consoles.
Q: How do I test the immunity of a receiver with an integrated antenna?
A: For receivers with integrated antennas, radiated field testing (30-150 MHz) is the primary evaluation method. The receiver is placed in the anechoic chamber at the specified field strength and performance is assessed functionally.
A: For receivers with integrated antennas, radiated field testing (30-150 MHz) is the primary evaluation method. The receiver is placed in the anechoic chamber at the specified field strength and performance is assessed functionally.
Q: Can software-based signal processing compensate for inadequate hardware immunity?
A> Partially — adaptive filtering and digital notch filters can mitigate some interference types, but they cannot compensate for front-end overload or mixer intermodulation. Hardware immunity design remains essential.
A> Partially — adaptive filtering and digital notch filters can mitigate some interference types, but they cannot compensate for front-end overload or mixer intermodulation. Hardware immunity design remains essential.
