Why does EMI bandwidth matter for compliance testing?

EMI bandwidth directly affects measured emission levels. A wider RBW captures more noise power, increasing the measured amplitude. If the wrong bandwidth is used, measured values will not match the limits published in EMC standards like CISPR 32, FCC Part 15, or MIL-STD-461. For example, broadband noise measured with 120 kHz RBW (Band C/D) will read approximately 22 dB higher than with 9 kHz RBW (Band B). Standards specify exact bandwidth for each frequency range to ensure all test labs worldwide produce comparable, legally defensible results.

What is the difference between CISPR and military EMI bandwidths?

CISPR bandwidths (200 Hz, 9 kHz, 120 kHz, 1 MHz) are used for commercial EMC standards. Military standards like MIL-STD-461 use different bandwidths: 10 Hz below 1 kHz, 100 Hz from 1-10 kHz, 1 kHz from 10 kHz-150 kHz, 10 kHz from 150 kHz-30 MHz, and 100 kHz above 30 MHz. The narrower military bandwidths provide finer frequency resolution for identifying individual emitters, which is critical for military RF spectrum management. MIL-STD also mandates peak detection rather than quasi-peak, resulting in more conservative measurements.

How does EMI bandwidth relate to detector type?

EMI bandwidth and detector type work together. CISPR receivers use quasi-peak (QP) detectors with specific charge/discharge time constants matched to each bandwidth. The QP detector weights impulsive signals lower than continuous signals, reflecting human perception of interference on AM radio. Peak detectors capture worst-case levels and are faster to measure. Average detectors are used for assessing interference to digital communications. The combination of bandwidth and detector determines the measured value: changing either one changes the result. Modern EMI receivers simultaneously measure with all detector types.

EMC Testing

EMI Bandwidth

/ee-em-eye BAND-width/

The resolution bandwidth (RBW) defined by CISPR 16-1-1 for electromagnetic interference measurements. Four standard bandwidths are specified, each matched to a frequency range: Band A (200 Hz), Band B (9 kHz), Band C/D (120 kHz), and Band E (1 MHz). These standardized RBW settings ensure repeatable, comparable EMC test results across all accredited laboratories worldwide. Using incorrect bandwidth invalidates the measurement.

Standard: CISPR 16-1-1

Range: 200 Hz to 1 MHz RBW

Detector: Quasi-peak, Peak, Average

Understanding EMI Bandwidth

In EMC testing, the resolution bandwidth of the measuring receiver determines how much spectral energy is captured. Wider RBW captures more broadband noise power, while narrower RBW provides finer spectral resolution. CISPR standardized the RBW for each frequency range to balance measurement speed, spectral resolution, and sensitivity. The bandwidths were historically chosen to match the characteristics of radio broadcast services that EMI regulations were designed to protect.

Band B (9 kHz, covering 150 kHz to 30 MHz) matches the IF bandwidth of shortwave and AM broadcast receivers. Band C/D (120 kHz, covering 30 MHz to 1 GHz) matches VHF/UHF receiver bandwidths. Band E (1 MHz, above 1 GHz) reflects wideband digital communication systems. These choices ensure that the measured EMI level correlates with actual interference potential to the protected services.

CISPR EMI Bandwidth Standards

CISPR 16-1-1 Bandwidths:
Band A: 9 kHz – 150 kHz → RBW = 200 Hz
Band B: 150 kHz – 30 MHz → RBW = 9 kHz
Band C/D: 30 MHz – 1 GHz → RBW = 120 kHz
Band E: 1 GHz – 18 GHz → RBW = 1 MHz

Broadband Noise Correction:
ΔdB = 10 × log₁₀(RBW₂/RBW₁)
Example: 120 kHz vs 9 kHz = 10 log(120/9) = +11.2 dB

Measurement Time per Frequency:
t ≅ 1/RBW × N_averages
Band B (9 kHz): ~111 µs per step

CISPR vs. Military EMI Bandwidths

Frequency Range	CISPR RBW	MIL-STD-461 RBW	Ratio
10 kHz – 150 kHz	200 Hz	1 kHz	5:1
150 kHz – 30 MHz	9 kHz	10 kHz	1.1:1
30 MHz – 1 GHz	120 kHz	100 kHz	0.83:1
1 GHz – 18 GHz	1 MHz	1 MHz	1:1

Common Questions

Frequently Asked Questions

Why does EMI bandwidth matter for compliance?

Wider RBW captures more noise power, increasing measured amplitude. Wrong bandwidth invalidates measurements. 120 kHz reads ~22 dB higher than 9 kHz for broadband noise. Standards specify exact bandwidth per frequency range for worldwide comparability and legal defensibility.

CISPR vs. military bandwidths?

CISPR: 200 Hz, 9 kHz, 120 kHz, 1 MHz (commercial). MIL-STD-461: 10 Hz to 1 MHz (finer resolution for individual emitter identification). MIL-STD uses peak detection (more conservative). CISPR uses quasi-peak (weights impulsive signals lower).

How does bandwidth relate to detector type?

Bandwidth and detector work together. QP detector has charge/discharge time constants matched to each RBW. Peak captures worst-case. Average assesses digital comms interference. Changing either bandwidth or detector changes the result. Modern receivers measure all detectors simultaneously.

Related Terms

← Bandwidth (Antenna) Bandwidth (ET) →

EMC Testing

Waveguide for EMC Test Systems

RF Essentials provides precision terminations and custom waveguide assemblies for EMC test chambers, receiver calibration, and compliance measurement systems.

Request a Quote