RF Detector
Understanding RF Detectors
RF detectors are the eyes of every wireless system. Without the ability to measure and monitor RF power levels, it would be impossible to control transmit power, implement automatic gain control, measure received signal strength, or calibrate test equipment. The detector converts invisible electromagnetic energy into a measurable electrical quantity that digital control systems can process.
The key challenge is achieving accurate detection across a wide dynamic range with minimal frequency dependence. A simple diode detector provides 25-30 dB of usable dynamic range. A logarithmic detector extends this to 60-80 dB. A thermocouple power sensor achieves the highest accuracy but with a narrower dynamic range. The choice depends on the application: AGC loops need fast response and moderate accuracy, while calibrated power measurement needs high accuracy and frequency flatness.
Detector Equations
Vout = γ × Pin (V/W)
γ = sensitivity (500-2000 mV/mW)
Log detector transfer:
Vout = Vslope × (Pin−Pintercept)
Vslope = 20-25 mV/dB
Pintercept = −60 to −70 dBm
Tangential signal sensitivity:
TSS ≈ −55 + 10log(BW) + NF dBm
BW=1 MHz, NF=6: TSS = −89 dBm
Thermal power:
P = V²/R, thermocouple: ±0.5%
RF Detector Type Comparison
| Type | Dynamic Range | Accuracy | Speed | Application |
|---|---|---|---|---|
| Schottky diode | −60 to 0 dBm | ±1-2 dB | <1 ns | Envelope, fast AGC |
| Log detector | −65 to +15 dBm | ±1 dB | 10-100 ns | RSSI, AGC, monitor |
| RMS detector | −50 to +10 dBm | ±0.5 dB | 1-10 μs | True power, OFDM |
| Thermocouple | −35 to +20 dBm | ±0.5% | ms | Calibrated measurement |
| Diode sensor | −70 to +20 dBm | ±1% | ms | Power meter |
Frequently Asked Questions
What is square-law detection?
V_out ∝ P_in below −20 dBm (quadratic I-V region). Independent of waveform: correctly measures modulated signal average power. Above 0 dBm: linear region (V_out ∝ V_in), needs correction for modulated signals. Square-law ideal for true power. Transition region (−20 to 0 dBm) requires calibration.
What is a log detector?
Cascade of limiting amplifiers with detector taps. V_out ∝ P_in(dBm): linear-in-dB over 60-80 dB. Low power: all stages amplify. High power: early stages saturate. Summed outputs = log response. 20-25 mV/dB slope. Used for RSSI, AGC, TX power control. AD8317: −65 to +15 dBm, DC-10 GHz.
RMS vs. thermal?
RMS: analog computation (translinear square/average/sqrt). True RMS regardless of waveform/crest factor. Thermal: measures heating in matched load (P=V²/R). Thermocouple sensors: ±0.5%, DC-50 GHz, −35 to +20 dBm. Diode sensors extend to −70 dBm with square-law + correction. Gold standard for calibrated measurement.