What are common N-dB bandwidth settings?

The most common settings are: 3 dB for half-power bandwidth (filter characterization, amplifier bandwidth), 6 dB for voltage half-amplitude (sometimes used for antenna patterns), 20 dB for filter shape factor denominator, 60 dB for filter shape factor numerator and selectivity assessment, and OBW (occupied bandwidth) mode for regulatory compliance (typically 99% power). The shape factor (SF = BW_60dB / BW_3dB) characterizes filter steepness: ideal rectangular filter SF = 1.0, Butterworth 4-pole SF ≈ 3.6, Chebyshev 4-pole SF ≈ 2.5. Lower shape factor means steeper skirts.

What spectrum analyzer settings affect bandwidth accuracy?

Three settings critically affect bandwidth marker accuracy. RBW (Resolution Bandwidth) must be much narrower than the signal bandwidth being measured, typically 1/10 or less. If RBW is too wide, it broadens the displayed signal and overestimates bandwidth. Span must be wide enough to capture both N-dB crossover points. Video bandwidth (VBW) should be ≥3x RBW for amplitude accuracy on modulated signals. Trace averaging should use RMS detection for modulated signals. A common error is measuring filter bandwidth with RBW wider than the filter bandwidth, which displays the analyzer's own filter shape rather than the device under test.

RF Test & Measurement

Bandwidth Marker

Q: How does the N-dB bandwidth marker work?

The spectrum analyzer places a marker at the signal peak, then searches left and right along the trace to find the two frequencies where the signal amplitude drops to N dB below the peak. The bandwidth is the frequency difference between these two points. For a 3-dB bandwidth marker, the result matches the classical half-power bandwidth. For a 60-dB marker, the result shows the signal's spectral extent including sidelobes and skirts. The analyzer interpolates between trace points for sub-bin accuracy. Modern analyzers display bandwidth, center frequency, Q-factor (f_center/BW), and the N-dB reference level simultaneously.

/BAND-width MAR-ker/

A spectrum analyzer measurement function that automatically displays the bandwidth between two amplitude-referenced points on a spectral trace. The user specifies an N-dB down level (typically 3, 6, or 60 dB), and the analyzer finds the two frequencies where the signal drops to N dB below the peak, then displays the frequency difference as the measured bandwidth. Used for filter characterization, signal bandwidth verification, shape factor assessment, and occupied bandwidth measurement for regulatory compliance.

Common N values: 3, 6, 20, 60 dB

OBW: 99% power method

RBW rule: < 1/10 of signal BW

Understanding Bandwidth Markers

The bandwidth marker is one of the most frequently used measurement functions on a spectrum analyzer. Rather than placing individual markers at the −3 dB points manually and calculating the difference, the bandwidth marker automates this process. The analyzer locates the signal peak, then searches symmetrically (or asymmetrically) for the specified N-dB crossing points, and displays the resulting bandwidth directly on screen.

For filter characterization, the bandwidth marker provides critical data at multiple N-dB levels. The 3 dB bandwidth gives the passband width. The 60 dB bandwidth reveals the filter's stopband extent. The ratio of these two (shape factor SF = BW_60dB/BW_3dB) quantifies the filter's selectivity. A shape factor close to 1.0 indicates near-ideal rectangular response, while values above 3 indicate gradual skirt rolloff.

Bandwidth Measurement Settings

N-dB Bandwidth:
BW_NdB = f_upper(peak − N dB) − f_lower(peak − N dB)

Shape Factor:
SF = BW_60dB / BW_3dB
Ideal rectangular: SF = 1.0
Butterworth 4-pole: SF ≅ 3.6
Chebyshev 4-pole: SF ≅ 2.5

Accuracy Requirements:
RBW < BW_signal / 10
Span > 2 × BW_NdB
VBW ≥ 3 × RBW (for modulated signals)

Common N-dB Settings

N-dB Level	Application	Physical Meaning
−3 dB	Filter passband, amplifier BW	Half-power points
−6 dB	Antenna patterns, voltage half	Half-voltage amplitude
−20 dB	Shape factor denominator	1% power
−60 dB	Selectivity, shape factor	0.0001% power
OBW (99%)	Regulatory compliance	99% of total power

Common Questions

Frequently Asked Questions

How does the N-dB bandwidth marker work?

Marker at peak, searches left/right for N-dB crossover. BW = frequency difference. 3 dB = half-power. 60 dB = spectral extent with sidelobes. Modern analyzers display BW, center, Q-factor, and reference level simultaneously.

What are common N-dB settings?

3 dB (half-power), 6 dB (voltage half), 20 dB (shape factor denominator), 60 dB (selectivity). Shape factor: SF = BW₆₀/BW₃. Butterworth 4-pole: 3.6. Chebyshev 4-pole: 2.5. Ideal rectangular: 1.0.

What analyzer settings affect accuracy?

RBW < 1/10 signal BW (wider overestimates). Span covers both crossover points. VBW ≥ 3x RBW for modulated signals. RMS detection for modulated signals. Common error: RBW wider than DUT bandwidth shows analyzer filter shape instead.

Related Terms

← Bandwidth (Isolator) Bandwidth (Network) →

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