Laboratory crystal detector (microwave power measurement) – Typical standing wave ratio In standard laboratory setups using a crystal (or Schottky) detector for relative microwave power measurements, what is a representative VSWR on the detector port when properly matched?

Introduction:
Microwave crystal detectors are commonly used in labs for relative power measurements (for example, in slotted-line or coupler setups). Their input match affects measurement accuracy; thus the typical standing wave ratio (VSWR) at the detector port is specified and controlled to keep mismatch ripple and calibration errors small.

Given Data / Assumptions:

• Detector used in its square-law region with appropriate impedance matching.
• Standard waveguide/coax fixtures with typical adapters and short leads.
• Goal is relative power measurement, not precision absolute power metrology.

Concept / Approach:

VSWR represents the degree of mismatch: VSWR = (1 + |Γ|) / (1 − |Γ|). Practical lab detectors are designed to keep reflections modest; a commonly quoted value around 1.25:1 corresponds to |Γ| ≈ 0.111, which is sufficiently low to reduce standing-wave errors while remaining achievable without overly complex matching networks over a useful bandwidth.

Step-by-Step Solution:

Verification / Alternative check:

Datasheets for laboratory crystal detectors (waveguide or coax) commonly list VSWR values between 1.2 and 1.5 across band; 1.25 is a representative nominal figure balancing bandwidth and match complexity.

Why Other Options Are Wrong:

VSWR = 0.1 is impossible (minimum is 1). VSWR values of 5 or 10 are far too mismatched for lab measurement accuracy. 2.5 is higher than desirable and would produce large ripple in standing-wave systems and poor repeatability.

Common Pitfalls:

Confusing return loss (in dB) with VSWR; forgetting that good but not perfect matches are typical for broadband detector heads and that adapters and connectors also contribute to the overall VSWR seen by the system.

Final Answer:

1.25.