--- title: "Working principle of a (vector) network analyzer" subject: "RF and Microwave Engineering" chapter: 8 key: "semester_7/rf_and_microwave_engineering:network-analyzer-working" source: https://www.bctengineer.com/q/semester_7/rf_and_microwave_engineering/network-analyzer-working site: BCT Engineering --- # Working principle of a (vector) network analyzer RF and Microwave Engineering · Chapter 8: RF and Microwave Measurements - Asked in: 2078 Chaitra, 5 marks; 2073 Magh, 8 marks (asked 2 times) - Video: [Network Analyzer (Basics, Parameters, Block Diagram, Working & S Parameters) Explained](https://www.youtube.com/watch?v=tG3oIOIFdhU) — Engineering Funda - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_7/rf_and_microwave_engineering/network-analyzer-working - Whole chapter: https://www.bctengineer.com/md/semester_7/rf_and_microwave_engineering/chapter-8 ## Question Describe the working principle of a network analyzer (VNA type). ## Short answer A VNA measures magnitude and phase of S-parameters using a swept source, directional couplers to sample incident, reflected and transmitted waves, and mixers with a common LO to preserve phase. Ratios give S11 = b1/a1, S21 = b2/a1, etc. SOLT or TRL calibration removes systematic errors. ## Answer A **Vector Network Analyzer (VNA)** measures the magnitude and phase of the S-parameters of a device under test (DUT) over a range of frequencies. A scalar analyzer measures magnitude only. #### Block diagram ```text +----------+ +-----------+ port 1 +-----+ port 2 | Swept RF |--->| Test set: |============>| DUT |=========+ | source | | couplers, |<============| |<========+ +----------+ | switch | +-----+ +-----------+ | a1 b1 b2 (a2) v +--------------------------+ | Receivers: mixers (LO), | | IF filters, ADC | +--------------------------+ v +--------------------------+ | Processor: ratios, error | | correction, display | +--------------------------+ ``` #### Working principle 1. **Source:** a synthesised swept source generates the test signal over the chosen band. 2. **Test set:** a switch sends the signal to port 1 (forward) or port 2 (reverse). Directional couplers (or bridges) sample the **incident** wave a1 and the **reflected** wave b1 at port 1, and the **transmitted** wave b2 at port 2. 3. **Receivers:** the sampled waves are down-converted by mixers with a common LO to a low IF, keeping their phase relations; the ADC digitises them. 4. **Ratio measurement:** the processor computes complex ratios: ```text S11 = b1/a1 (a2 = 0) S21 = b2/a1 (a2 = 0) S22 = b2/a2 (a1 = 0) S12 = b1/a2 (a1 = 0) ``` Since the ratios are taken, source power variations cancel. 5. **Calibration (error correction):** systematic errors (directivity, source match, load match, tracking, isolation) are removed using known standards — **SOLT** (short, open, load, thru) or TRL. This is the 12-term error model for two ports. 6. **Display:** results are shown as log magnitude, phase, group delay, SWR, Smith chart or time-domain (via inverse FFT). #### Uses Measuring S-parameters of amplifiers, filters, antennas (return loss, SWR), cables; impedance matching; fault location in lines using time-domain mode. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What does a VNA measure that a scalar analyzer does not? A. Phase as well as magnitude of S-parameters B. Only magnitude C. Only DC resistance D. Only noise figure Answer: A. A VNA measures complex S-parameters (magnitude and phase) over frequency. (Key point: VNA vs scalar) 2. How is S21 obtained? A. S21 = b2/a1 with a2 = 0 B. S21 = b1/a1 with a2 = 0 C. S21 = b1/a2 with a1 = 0 D. S21 = a1/b2 Answer: A. S21 is the transmitted wave b2 over the incident wave a1 with port 2 matched (a2 = 0). (Key point: S-parameter ratios) 3. Why are ratios of waves taken? A. Source power variations cancel B. It removes the need for a source C. It makes the DUT linear D. It converts to DC Answer: A. Both waves scale with the source, so their ratio is independent of source power variation. (Key point: Advantage of ratios) 4. What is the purpose of SOLT/TRL calibration? A. To remove systematic errors (directivity, source match, load match, tracking, isolation) using known standards B. To change the DUT's S-parameters C. To raise the source power D. To display the Smith chart Answer: A. Known standards allow the 12-term error model to be solved and removed from the measurement. 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