Spectrum Analyzer

Test & Measurement System

Component Descriptions

Signal Chain Walkthrough

A swept-tuned spectrum analyzer measures signal power as a function of frequency by sweeping a local oscillator across the band of interest. At each frequency point, the IF filter selects a narrow slice of the spectrum, and the detector measures the power in that slice.

Preselector

A YIG-tuned bandpass filter tracking the LO prevents image and spurious responses. Essential for measuring signals in the presence of strong out-of-band interference.

Resolution Bandwidth (RBW) Filter

The IF bandpass filter that determines the frequency resolution. Narrower RBW reveals closely-spaced signals but increases sweep time. Typical RBW range: 1 Hz to 10 MHz.

Logarithmic Detector

Converts the IF signal amplitude to a logarithmic (dB) scale, providing wide dynamic range (80-120 dB) on a single display.

Video Bandwidth (VBW) Filter

A low-pass filter after the detector that smooths the displayed trace by averaging noise. Setting VBW < RBW reduces displayed noise level by 10·log(VBW/RBW) dB.

Typical Specifications

Component Specifications

Component	Parameter	Typical Value
Frequency	Range	DC to 110+ GHz
DANL	Noise Floor	-150 to -170 dBm/Hz
RBW	Range	1 Hz - 10 MHz
Dynamic Range	Display	80 - 120 dB
Phase Noise	@10 kHz offset	-100 to -120 dBc/Hz

Design Note: The spectrum analyzer sweep time is proportional to span/RBW². Reducing RBW by 10× increases sweep time by 100×. Modern signal analyzers use FFT-based digital IF processing to dramatically reduce measurement time compared to traditional swept architectures. For pulsed or transient signals, real-time spectrum analyzers with 100% POI are required.

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