Core idea
The receiver bandwidth is the slice of spectrum admitted into the detector. A narrow CW signal is mostly unchanged once the filter is wide enough to contain it. Broadband noise and impulses change more because a wider filter captures more energy and responds differently in time.
RBW / resolution bandwidthFrequency resolving width of the IF filter. Narrower RBW separates closer carriers and lowers the displayed noise floor.
6 dB bandwidthThe filter width measured where response has fallen 6 dB from the peak. MIL-STD-461G explicitly uses this convention for emission receiver bandwidths.
Impulse bandwidthThe effective receiver bandwidth for short pulses. It depends on the full filter shape, not just the -6 dB points.
DetectorThe circuit or algorithm after the filter: Peak, Average, RMS or Quasi-Peak. Same signal, different detector, different displayed value.
1. Tool: two narrow signals through a receiver filter
Move the bandwidth. At small bandwidth the two carriers are resolved. At large bandwidth they merge into one broad measured response.
Can the receiver resolve both carriers?Yes
Rule of thumb: resolution improves with narrower bandwidth, but sweep time increases and the noise floor falls.
2. Tool: broadband noise captured by bandwidth
For broadband noise, measured noise power increases approximately 10 log₁₀(BW ratio). This is why using a larger bandwidth can raise the displayed level.
Noise increase vs 10 kHz10.8 dB
This is conceptual: real readings also depend on detector, filter shape and signal statistics.
3. Tool: 3 dB vs 6 dB bandwidth
The same filter can be quoted at different down-points. This does not change the filter; it changes where you measure its width.
3 dB width85 kHz
6 dB width120 kHz
6 dB / 3 dB≈ 1.41
MIL-STD-461G, DEF STAN wording and CISPR-style EMI receiver filters are based around 6 dB bandwidth / impulse response behaviour, not generic analyser 3 dB RBW.
4. Tool: CISPR-style impulse response
Select a CISPR band. The page shows a conceptual time response of a receiver IF filter. Wider bandwidth means a shorter response in time.
Selected 6 dB BW120 kHz
Conceptual impulse BW≈ 120 kHz
Relative pulse durationMedium
Important: this diagram is educational, not a CISPR tolerance mask. CISPR receivers are verified using detailed requirements for 6 dB bandwidth, impulse response, selectivity and detector behaviour.
5. Tool: detector functions
The filter decides what reaches the detector. The detector decides how the receiver displays it.
Displayed value for burst train68.0 dBµV
Quasi-Peak is intentionally weighted by repetition rate. In commercial EMC it can penalise frequent pulses more than rare ones.
6. Instrument mode: analyser vs EMI receiver
Spectrum analyser mode
- RBW often specified at 3 dB.
- Excellent for diagnostics and troubleshooting.
- May not provide CISPR/MIL compliant IF filters unless EMI options are enabled.
EMI receiver mode
- Uses defined EMC receiver bandwidths.
- Supports Peak, Average, RMS and/or Quasi-Peak detector functions.
- Best choice for formal compliance measurements.
For formal work, select the receiver mode/filter family required by the procedure and record the bandwidth/detector in the report.
7. Standards comparison
| Standard / family | Bandwidth convention | Typical emission bandwidths | Detector notes | Practical note |
|---|---|---|---|---|
| MIL-STD-461G | Bandwidths specified at the 6 dB down points of the receiver selectivity curve. | 10 Hz, 100 Hz, 1 kHz, 10 kHz, 100 kHz and 1 MHz, selected by frequency range. | Peak detector for frequency-domain measurements. | Do not treat listed bandwidths as generic 3 dB analyser RBW unless your procedure/equipment evidence supports equivalence. |
| DEF STAN 59-411 Part 3 | Uses wording such as receiver 6 dB (impulse) bandwidths. | Defined by test method/frequency range tables in the standard. | Peak detector generally, with average detector where required by specific tests. | Confirm bandwidths used in the test report. |
| CISPR 16 / EN 550xx family | EMI receiver specification includes 6 dB bandwidth, impulse response and filter selectivity requirements. | 200 Hz, 9 kHz, 120 kHz, 1 MHz bands are commonly encountered. | Peak, Quasi-Peak, Average and RMS depending on standard and frequency range. | CISPR compliance is about the complete receiver behaviour, not just the label “120 kHz”. |
| DO-160 emissions | Uses specified receiver bandwidths/detectors in the procedure; check section and category. | Commonly 10 kHz, 120 kHz and 1 MHz style bandwidths depending on test/frequency. | Peak and/or Average depending on section. | Always use the section-specific table rather than memory. |
8. References and further reading
- MIL-STD-461G, 4.3.10.3.1: measurement receiver bandwidths are specified at the 6 dB down points for the overall receiver selectivity curve.
- DEF STAN 59-411 Part 3, 4.16: discusses receiver 6 dB (impulse) bandwidths and detector function, and requires bandwidth use to be confirmed in the test report.
- CISPR 16-1-1: defines radio disturbance measuring apparatus including IF bandwidth, impulse response, selectivity, detector functions and overload behaviour.
- Tektronix EMI diagnostics guidance: explains the practical difference between -3 dB and -6 dB filter definitions.
- Schwarzbeck CISPR receiver note: provides a useful explanation of CISPR receiver impulse behaviour.
Report wording suggestion: “Measurements were performed using the bandwidths and detector functions required by the applicable test method. Receiver bandwidths were selected using EMI receiver mode / compliant filters where applicable.”