Amplitude Modulation (AM) is a method of transmitting information by varying the amplitude of a carrier wave in proportion to the message signal. However, in order to recover the original information from the received AM signal, we need to demodulate it. One of the most common methods of demodulating an AM signal is through the use of an single diode based envelope detector circuit.
Circuit Diagram and Operation
The following shows circuit diagram of envelope detector circuit diagram for AM demodulation.
Working Principle:
During positive input cycles, the diode conducts, charging capacitor C1 through the source internal resistance RS, to the peak value of the input signal. When the input signal falls below this value, during the negative cycle, the diode becomes reverse biased and the capacitor discharges through resistor RL. This cycle of charging and discharging repeats.This charge-discharge cycle reconstructs the original message signal.
The component values of the RC filter depends upon the modulating signal frequency. For audio signal the maximum audio frequency can be taken to be 22KHz. Considering the diode to be ideal with zero impedance when forward biased and infinite impedance when reversed biased we have two criteria for the component values selection for the capacitor C and the resistor RLwhich is as follows.
The first condition is that the charging time \(R_SC\) should be short compared to the carrier signal period, that is,
\[R_SC<<\frac{1}{f_c}\]
where, \(f_c\) is the carrier signal frequency
This second condition is that the discharging time constant \(R_LC\) must be long enough so that the capacitor discharges slowly through the resistor RL in time interval between each positive peak of the carrier signal while at the same, the discharge time constant should not be so long that the capacitor is not discharged at a rate lower than the modulating signal. This condition can be expressed as follows.
\[\frac{1}{f_c}<<R_LC<<\frac{1}{B.W}\]
where, \(B.W\) is the bandwidth of the message signal.
Envelope Detector LPF calculator
Here is a calculator that can be used to determine the value of the resistor conditioned on the above two criteria.
The above calculator helps to calculate the source voltage internal impedance Rs and the resistor RL for the low pass filter, provided the carrier frequency, the signal bandwidth and the capacitor values are given.
Video Demonstration
See the following video which demonstrates demodulation of AM signal using the envelope detector circuit.
In conclusion, designing an envelope detector circuit for AM detector demodulation is not a complex task, but it's important to choose the correct type of circuit based on the specific requirements of the application. The peak detector circuit is the simplest, the balanced modulator circuit has a faster response time, and the synchronous detector circuit has the fastest response time and can be used for high-frequency signals.
If you are interested in learning more about AM modulator circuits, AM demodulator circuits then see the following tutorials.
- Standard AM with AD633 Analog Multiplier IC
