Power Meter
Understanding Power Meters
Power meters are the accuracy reference for RF power measurement. When a spectrum analyzer reading must be verified or a transmitter output must be characterized precisely, the power meter provides the most accurate measurement available.
Power Sensor Types
- Thermocouple sensor: Measures average power by converting RF to heat. True RMS measurement independent of waveform. Typical range: -30 to +20 dBm.
- Diode sensor: Rectifies the RF signal. Faster response. Can measure peak and average. Typical range: -70 to +20 dBm. Above -20 dBm, correction tables needed for complex waveforms.
- Thermistor sensor: Highest accuracy (primary standard). Self-calibrating via DC substitution. Limited to -20 to +10 dBm. Used for metrology.
Measurement Considerations
- Average vs peak: Thermocouple measures true average (correct for any waveform). Diode sensor in square-law range measures average; above that, measures peak envelope.
- Mismatch uncertainty: Power meter readings are affected by the mismatch between sensor and source. Mismatch error = +/- 2 x |Gamma_s x Gamma_l| x 100%.
Frequently Asked Questions
What is a power meter?
A power meter accurately measures RF power using a calibrated sensor. It provides the most accurate power measurement (+/- 0.1-0.5 dB), better than spectrum analyzers. Sensor types include thermocouple, diode, and thermistor.
Power meter vs spectrum analyzer for power measurement?
Power meters: higher accuracy (+/- 0.2 dB vs +/- 1 dB), true RMS measurement, simpler operation. Spectrum analyzers: show frequency spectrum, identify individual signals, detect spurs. Use power meter for absolute accuracy; spectrum analyzer for spectral analysis.
What is the most accurate power measurement?
Thermistor-based (bolometric) power meters using DC substitution provide the highest accuracy, traceable to national standards. They are used as transfer standards for calibrating other instruments. Typical uncertainty: +/- 0.1 dB at the sensor reference plane.