What is the difference between insertion loss and return loss and how are they measured?
IL and RL Measurement Techniques
Insertion loss and return loss are the most frequently measured parameters in RF engineering, used to evaluate cables, connectors, filters, amplifiers, antennas, and every other RF component.
| Parameter | SOLT Cal | TRL Cal | eCal |
|---|---|---|---|
| Accuracy | Good | Excellent | Good-very good |
| Standards Needed | 4 (S,O,L,T) | 3 (T,R,L) | 1 (module) |
| Bandwidth | Broadband | Band-limited | Broadband |
| Setup Time | 5-10 min | 10-20 min | 1-2 min |
| Best For | Coaxial, general | On-wafer, waveguide | Production, speed |
- Performance verification: confirm specifications against the application requirements before finalizing the design
- Environmental factors: temperature range, humidity, and vibration affect long-term reliability and parameter drift
- Cost vs. performance: evaluate whether the application demands premium components or standard commercial grades
Frequently Asked Questions
What return loss is considered acceptable?
Depends on the application: for antennas: RL > 10 dB (VSWR < 2:1) is the common specification. RL > 14 dB (VSWR < 1.5:1) is considered good. RL > 20 dB (VSWR < 1.2:1) is excellent. For connectors and adapters: RL > 20 dB is typical for SMA at < 18 GHz. RL > 26 dB is expected for precision connectors. For amplifier ports: input RL > 10-15 dB is typical (some low-noise amplifiers sacrifice input match for noise performance). For filters: passband RL > 15-20 dB is typical (determines passband ripple: RL = 20 dB corresponds to ±0.04 dB ripple). For cable assemblies: RL > 20 dB at each connector and > 26 dB along the cable itself.
Can insertion loss be negative?
Yes. Negative insertion loss means the device has gain (output power > input power). An amplifier with 20 dB gain has IL = -20 dB. The VNA displays S21 = +20 dB. A negative insertion loss in a passive device indicates a measurement error (the calibration reference is wrong, the cables have changed, or the DUT is oscillating). Some passive devices appear to have slightly negative IL at certain frequencies due to mismatch effects: if the DUT provides a better impedance match than the calibration THRU, the mismatch loss decreases, making the DUT appear to have slight gain. This is a measurement artifact, not real gain.
How do I measure insertion loss of a very low-loss device?
Measuring IL < 0.5 dB requires care: (1) Use the best available calibration (SOLT with precision standards or TRL calibration for the lowest uncertainty). (2) Minimize connector effects: use the same connector type on the DUT as the calibration reference (avoid adapters that add their own IL). (3) Repeat the measurement multiple times and average: the statistical uncertainty decreases with the number of measurements. (4) Control temperature: a 0.1 dB drift in cable loss equals the entire DUT loss for a 0.1 dB device. Use phase-stable cables and allow thermal equilibrium. (5) Use high IF bandwidth averaging on the VNA (reduce IF BW to 100 Hz or lower) to improve trace noise. (6) Consider TRL calibration (which uses airlines instead of lumped loads) for the highest accuracy at the reference plane.