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RF detector basics

The most common RF detector is a diode detector. It is used to sample amplitude of the input RF signals. These detectors work as simple rectifiers which samples RF peak while charging a capacitance to hold the peak voltage and discharging the capacitance across the load slowly. Hence it is also known as peak detector. Schottky diode is used as broadband sensors with frequency range upto 50 GHz.

The RF detector curve will usually have three regions viz. square law, linear and saturation. Square law region exists at low RF input levels where in output is proportional to square of the input amplitude. When input level reaches to about 1V, the curve will be linear. Saturation region reches when input level becomes larger.

Let us understand the RF detector used as AM demodulator. As we know demodulator is the circuit which converts and recovers modulating signal from the modulated signal input.

RF detector circuit diagram

RF detector

RF detector circuit used as AM demodulator is shown in the figure. As shown, AM signal is transformer coupled to a basic half-wave rectifier circuit made of diode D1 and resistor R1. The diode conducts only when positive half cycles of the AM signal are subjected to the RF detector circuit. During the negative half cycles, diode gets reverse biased. This results into no current flow. Hence voltage across R1 is due to positive pulses only whose amplitude varies with the modulating signal.

In order to retrieve the original modulating signal, a capacitor is connected across resistor R1. The value of this capacitor is carefully chosen so that it has a very low impedance at the carrier frequency and high impedance at modulating signal frequency. This results into filtering of the carrier frequency signal. Hence only original modulating frequency signal will be available at the output.

In order to make this circuit works effectively time constant of C and R1 is chosen to be long compared to the period of the RF carrier signal. As explained this diode detector circuit recovers the envelope of the Amplitude Modulated signal, the RF detector circuit is referred as envelope detector.

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