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Working principle of a (vector) network analyzer

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.

Describe the working principle of a network analyzer (VNA type).

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

 +----------+    +-----------+    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:
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.

✓ Answer checked by Claude Opus 5.5 (high effort)

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