13. Microwave Measurements

13.1 The vector network analyzer (VNA)

The most important instrument on any RF lab bench. Measures all four S-parameters of a two-port (or N-port) device versus frequency, magnitude and phase. A modern VNA spans DC to 67 GHz in benchtop form and to 220 GHz in extended-frequency systems.

   ┌──────────────────────────────────────────────┐
   │                                              │
   │   synthesized source         receiver A      │
   │      (1 MHz–67 GHz)             (mag/phase)  │
   │            │                       ↑         │
   │            │                       │         │
   │       ┌────┴───┐               ┌───┴────┐    │
   │       │ direc- │               │ direc- │    │
   │       │ tional │               │ tional │    │
   │       │coupler │               │coupler │    │
   │       └────┬───┘               └───┬────┘    │
   │            │                       │         │
   │      port 1│                       │port 2   │
   └────────────┼───────────────────────┼─────────┘
                │      DUT              │
                └────────[device]───────┘

Inside, the VNA has a synthesized source that sweeps across the band, directional couplers to separate forward and reflected waves at each port, and four heterodyne receivers measuring the four scattered waves. Calibration with known standards (short, open, load, thru, or TRL standards) removes the fixture's contribution. The output is $S(f)$ across the band.

A modern VNA's noise floor is around −120 dBm; dynamic range is 130 dB; phase accuracy is sub-degree. It is the workhorse of every RF design lab, antenna test lab, EW lab, and IC test floor.

13.2 Spectrum analyzer

A receiver that displays signal power as a function of frequency. Sweeps a tunable filter (or, in modern FFT analyzers, samples a wide IF and computes FFT) across the band of interest. Shows discrete signals as peaks.

What it does not do: measure phase. So spectrum analyzer is for amplitude-only diagnostics; VNA is for full-vector network analysis.

Used for: signal-presence diagnostics, harmonic measurements, spurious emissions, modulation analysis (with vector signal analyzer extensions), interference hunting.

13.3 Power meter

A bolometer or thermistor sensor measures absolute RF power. Slow but very accurate (better than 0.1 dB absolute accuracy). The reference instrument for any absolute power calibration.

Modern thermistor and diode power sensors run from microwatts to watts. Higher-power applications use calorimetric loads where water flow rate and temperature rise yield absolute power.

13.4 Noise figure meter

Specialized instrument that measures the noise figure of an amplifier. Uses a noise source (a special diode that produces calibrated broadband noise) switched on and off; measures the ratio of receiver output power in each state. The math (the Y-factor method) yields NF.

Modern NF meters are now usually integrated into VNAs that have a "noise figure" personality.

13.5 Time-domain reflectometry (TDR)

A short pulse is launched down the line; reflections from impedance mismatches return at times proportional to distance. By Fourier-transforming broadband VNA $S_{11}(f)$ data, one obtains the equivalent time-domain reflection response. Distance to a discontinuity is $d = v_g t / 2$ (round trip).

TDR uses:

• Cable fault location: damaged or kinked cable shows a reflection at the fault. Telco and CATV operators use this daily.
• PCB signal-integrity debugging: find impedance mismatches, vias, stubs.
• Connector quality: see the launch transition and its discontinuity.
• Tamper detection: a length change in a sealed cable (someone added a tap) changes its reflection signature. Crypto equipment in classified spaces uses TDR to verify cable integrity; deviations trigger alarms.

13.6 Slotted line (legacy)

Before VNAs, engineers used a slotted line: a section of waveguide or coax with a longitudinal slot through which a small probe sampled the field. Sliding the probe revealed standing-wave maxima/minima; the ratio gave VSWR, the position of minima gave reflection phase. Slow but conceptually clean. Still seen in older labs and teaching demos.