2.2 Detector (Diode) RF Power Sensors

Diode sensors use high-frequency semiconductor diodes to detect the RF voltage developed across a terminating load resistor. The diodes directly perform an AC to DC conversion, and the DC voltage is measured by the power meter and scaled to produce a power readout. In the strictest sense these are not power detectors, but rather voltage detectors, so termination impedance variations can cause more error in the reading due to mismatch than would be seen using thermal sensors. Early devices were simple crystal detectors using galena and a cat-whisker to form a crude diode junction.

In a diode type RF power sensor, one or more diodes perform a rectification (peak detection) function at high levels and act as a nonlinear resistor at lower levels, conducting more current in the forward direction than reverse. This is shown in Figure 2.2.1. Usually a smoothing capacitor is connected to the output of the diode to convert the pulsating DC to a steady DC voltage. Often, two diodes are used so both the positive and negative carrier cycles are detected; this makes the sensor relatively insensitive to even harmonic distortion. A diode detector’s DC output voltage is proportional to power at low signal levels and proportional to the peak RF voltage at higher levels. To achieve high sensitivities, the load resistance driven by the diode’s output is typically several megohms.

Below about -20 dBm (30mV peak carrier voltage), the RF input is not high enough to cause the diodes to fully conduct in the forward direction. Instead, they behave as non-linear resistors as shown in Figure 2.2.2 below, and produce a DC output that is closely proportional to the square of the applied RF voltage. This is referred to as the “square-law” region of the diode sensor. When operated in this region, the average DC output voltage will be proportional to average RF power, even if modulation is present. This means a diode sensor can be used to measure the average power of a modulated signal, provided the instantaneous (peak) power remains within the square-law region of the diodes at all times.

Figure 2.2.1. A balanced, dual-diode sensor diagram

Above about 0 dBm (300mV peak input voltage), the diodes go into forward conduction on each cycle of the carrier, and the peak RF voltage is held by the smoothing capacitors. In this region, the sensor is behaving as a peak detector (also called an envelope detector), and the DC output voltage will be equal to the peak-to-peak RF input voltage minus two diode drops. This is known as the “peak detecting” region of the diode sensor. When operated in this region, the average DC output voltage will be proportional to the peak RF voltage.

While the dynamic range of diode detectors is very large, operation in these two regions is quite different and the sensor’s response is not linear across its entire dynamic range. The square-law and peak-detecting regions, as well as the “transition region” between them (typically from about –20 dBm to 0 dBm), must be linearized in the power meter. This linearization process does not present any difficulties for modern power meters.

Figure 2.2.2. (Top) I-V curve showing “non-linear resistor” characteristic and (Bottom) Diode I-V characteristic in low level “square-law” region (RIGHT) and high-level “peak detecting region (LEFT)

Although very sensitive and easily linearized with digital techniques, diode sensors are challenged by modulation when the signal’s peak amplitude exceeds the upper boundary of the square-law region. When high-level modulation is present, RF amplitude enters the peak detecting region of the diode detector. In this situation, the detector’s output voltage will rapidly slew towards the highest peaks, then slowly decay once the signal drops. Since the input signal could be at any amplitude during the time the capacitor voltage is decaying, it is no longer possible to deduce the actual average power of a modulated signal once the peak RF power gets into this peak-detecting region of the diode.

Graph illustrating Square-Law, Linear, and Compression Region of a Detector Circuit

One solution to this problem is to load the diode detector in such a way that the output voltage decays more quickly, and follows the envelope fluctuations of the modulation. This is normally done by reducing the load resistance and capacitance that follows the diodes (RL and CL in Figure 2.2.1). If the sensor’s output accurately tracks the signal’s envelope without significant time lag or filtering effect, then it is generally possible to properly linearize the output in real time and perform any necessary filtering on this linearized signal (see Figure 2.2.3). This allows a sufficiently fast diode sensor to accurately measure both the instantaneous and average power of modulated signals at any power level within the sensor’s dynamic range. This type of sensor is commonly referred to as a Peak Power Sensor, and is discussed in greater detail in Section 4.2.

Figure 2.2.3. A Wide Bandwidth Detector Correctly Tracks a Pulse Envelope

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