Spectrum Analyzer
Understanding Spectrum Analyzers
The spectrum analyzer is one of the two most important RF test instruments (alongside the VNA). While the VNA characterizes devices, the spectrum analyzer characterizes signals. It answers the question: what frequencies are present and at what power levels?
Spectrum Analyzer Types
- Swept-tuned (traditional): A superheterodyne receiver sweeps its LO across the frequency range, displaying power vs. frequency. Simple but cannot capture transient signals.
- FFT-based (real-time): Digitizes the signal and computes the spectrum using FFT. Captures transient events. Limited by ADC bandwidth.
- Real-time spectrum analyzer (RTSA): Continuously computes FFT with no gaps. Captures 100% of signal events. Used for interference hunting and signal intelligence.
Key Settings
- Resolution bandwidth (RBW): The IF filter bandwidth. Narrower RBW reveals closer-spaced signals but slows the sweep.
- Video bandwidth (VBW): Post-detection filter that smooths the trace. Does not affect spectral resolution.
- Span: The frequency range displayed.
- Reference level: The power level at the top of the display.
Frequently Asked Questions
What does a spectrum analyzer measure?
A spectrum analyzer displays signal power vs. frequency. It shows carrier power, harmonics, spurious emissions, noise floor, modulation bandwidth, and interference. It is essential for transmitter testing, EMC compliance, and signal monitoring.
What is resolution bandwidth?
Resolution bandwidth (RBW) is the IF filter bandwidth that determines how closely spaced signals can be resolved. Narrower RBW sees closer signals but takes longer to sweep. Choose RBW narrow enough to resolve the signals of interest but wide enough for reasonable sweep time.
What is the difference between a spectrum analyzer and a VNA?
A spectrum analyzer measures signals (what frequencies are present and at what power). A VNA measures devices (how a component modifies a known test signal). The SA has one input and no signal source. The VNA has a source and multiple ports for stimulus-response measurement.