What is resolution bandwidth and how do I choose it?

Resolution bandwidth (RBW) is the bandwidth of the IF filter that determines the frequency resolution of the measurement. A narrower RBW can resolve two closely spaced signals that a wider RBW would merge into one lump. But narrower RBW means longer sweep time (proportional to span/RBW^2 for swept analyzers) and a lower noise floor (the displayed noise floor drops by 10 dB for every 10x reduction in RBW). For harmonics and spurious measurements, use a narrow RBW (100 Hz to 1 kHz) to maximize sensitivity. For modulated signals, the RBW must be wider than the signal bandwidth to capture the full power: a 20 MHz LTE signal requires at least 20 MHz RBW. For phase noise, use the narrowest RBW available and normalize the result to 1 Hz bandwidth.

What is DANL and why does it matter?

Displayed Average Noise Level (DANL) is the noise floor of the spectrum analyzer itself, typically specified at the narrowest RBW. It determines the weakest signal the analyzer can measure. A spectrum analyzer with 6 dB noise figure and 1 Hz RBW has DANL = -174 + 6 + 0 = -168 dBm. At 10 kHz RBW: DANL = -168 + 40 = -128 dBm. At 1 MHz RBW: -108 dBm. The DANL limits the measurement dynamic range: to measure a spurious signal at -80 dBm, the noise floor at the measurement RBW must be at least 10 dB below that (-90 dBm or better). Using a preamplifier lowers the system NF and improves DANL by the preamp gain minus its NF, but reduces the maximum input power before compression.

Swept-tuned vs. FFT: which analyzer should I use?

Swept-tuned (superheterodyne) analyzers tune the LO across the span and measure power at each frequency sequentially. They handle very wide spans (up to 50 GHz) and have excellent dynamic range but miss transient signals that occur between sweeps. FFT-based (real-time) analyzers digitize a wide bandwidth simultaneously and compute the spectrum via FFT. They capture every signal within the digitizer bandwidth (typically 40 to 500 MHz instantaneous) with zero probability of intercept loss, making them essential for pulsed signals, frequency-hopping, and interference hunting. Modern signal analyzers combine both: swept mode for wide spans and FFT mode for deep analysis within a narrower band.

Test Equipment

Spectrum Analyzer

Connect a cellular base station's antenna port to a spectrum analyzer. The display reveals everything the antenna sees: the desired carriers at −30 dBm, intermodulation products at −60 dBm from nearby transmitters, a harmonic at −70 dBm from the site's own PA, and the broadband noise floor at −105 dBm. The spectrum analyzer transforms the time-domain chaos at the antenna into a frequency-domain map where every signal, wanted or unwanted, has a distinct location and measurable power. It is the oscilloscope of the RF world, but instead of showing voltage versus time, it shows power versus frequency.

Category: Test Equipment

Display: Power (dBm) vs. Frequency

Key Specs: DANL, RBW, Phase Noise

Architecture and Key Specifications

Specification	Entry-Level	Mid-Range	High-End	Impact
Frequency Range	9 kHz to 3 GHz	9 kHz to 26 GHz	2 Hz to 50+ GHz	Measurement coverage
DANL (at 1 Hz RBW)	−145 dBm	−155 dBm	−168 dBm	Sensitivity to weak signals
Phase Noise (10 kHz)	−85 dBc/Hz	−105 dBc/Hz	−120 dBc/Hz	Close-in signal resolution
TOI (3rd-order intercept)	+5 dBm	+15 dBm	+25 dBm	Spurious-free dynamic range
RBW Range	1 Hz to 3 MHz	1 Hz to 10 MHz	0.1 Hz to 100 MHz	Resolution and sweep speed
Real-time BW	N/A	40 MHz	500+ MHz	Transient signal capture

Displayed noise floor:
DANL = −174 + NF_SA + 10·log(RBW) dBm
NF = 6 dB, RBW = 10 kHz: DANL = −174 + 6 + 40 = −128 dBm

Sweep time (swept-tuned):
T_sweep ≈ k × Span / RBW² (seconds)

Noise floor reduction:
10× narrower RBW = 10 dB lower DANL = 100× longer sweep

Common Questions

Frequently Asked Questions

How to choose RBW?

Narrower = better resolution and lower noise floor, but slower. Harmonics: 100 Hz to 1 kHz for max sensitivity. Modulated signals: RBW ≥ signal BW. Phase noise: narrowest available, normalize to 1 Hz. Every 10× narrower: 10 dB lower DANL, 100× slower sweep.

What is DANL?

The analyzer's own noise floor: −174 + NF + 10·log(RBW). 6 dB NF at 10 kHz RBW: −128 dBm. To measure a −80 dBm spur, need DANL < −90 dBm. Preamplifier improves DANL but reduces max input power.

Swept vs. FFT?

Swept: wide spans (50+ GHz), great dynamic range, misses transients. FFT (real-time): captures all signals within digitizer BW (40 to 500 MHz), essential for pulsed/hopping signals. Modern analyzers combine both modes.

Related Terms

← Smith Chart Spurious Emission →

Measurement Setup

Spectrum Analyzer Settings Guide

Enter your measurement type (harmonic, spurious, channel power, ACLR) and signal parameters. Get recommended RBW, VBW, span, reference level, and attenuation settings.

Get Settings