What is the proper method for measuring the reverse isolation of an amplifier at high power?
Reverse Isolation Measurement
Reverse isolation is important for: cascaded amplifier chains (poor reverse isolation allows output reflections to propagate backward, causing gain ripple and potential instability), frequency converters (reverse isolation determines the LO-to-RF leakage in a mixer-amplifier chain), and power amplifier protection (the PA must withstand reflected power from antenna mismatch).
| 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
Why does S12 change with power?
At high power levels: the active device's S12 can change because: the transistor's internal feedback capacitance (C_gd for FETs, C_bc for BJTs) is voltage-dependent, and at high output swing: the capacitance varies, changing the reverse coupling. The device may enter compression or saturation: the large-signal operating point changes the device's S-parameters, including S12. For FET and GaN PAs: S12 can degrade by 5-10 dB at output powers near P_sat compared to small-signal. This degradation can reduce the amplifier's stability margin, potentially causing oscillation at high power levels.
How do I protect the equipment?
Protection measures for high-power reverse isolation measurement: at the output (where high power is applied): use a power source with output limiting or shutdown. A circulator between the high-power source and the amplifier output protects both. At the input (where the reverse-leaked signal is measured): use a calibrated coupler (20-30 dB coupling) plus additional attenuators (10-20 dB) to reduce the signal to a safe level for the power meter or spectrum analyzer. Total attenuation: ensure the maximum expected reverse-leaked power (P_applied - S12_estimate) minus the coupler and attenuator attenuation results in a safe power level at the instrument (less than +20 dBm for most power meters).
What is a typical measurement?
Example: measuring S12 of a GaN PA at P_out = +43 dBm: apply +43 dBm to the output using a high-power signal source (through a circulator for protection). Expected S12 approximately -20 dB: anticipated power at the input port approximately +43 - 20 = +23 dBm. Use a 20 dB coupler + 10 dB attenuator at the input: signal to power meter approximately +23 - 20 - 10 = -7 dBm (safe for the power meter). Measure the power at the power meter and correct for the coupler and attenuator losses: S12 = P_measured + 30 dB (coupler + atten) - P_applied.