Subject: Principles of Communication Engineering

Difficulty : Medium

Marks : 10

Pratical Diode Detector :

• A number of additions have been made to the simple detector, and its practical version is shown(in diagram). The diode has been reversed, so that now the negative envelope is demodulated.

• This has no effect on detection, but it does ensure that a negative AGC voltage will be available.

• The resistor R of the basic circuit has been split into two parts R1 and R2 to ensure that there is a series DC path to ground for the diode, but at the same time a low pass filter has been added, in the form of R1-C1.

• This has the function of removing any RF ripple that might still be present.

• Capacitor C2 is coupling capacitor, which is used to prevent the diode dc output from reaching the volume control R4.

• The combination R3-C3 is a low pass filter to the carrier strength, and which may be used for automatic gain control.

• It can be seen that the DC diode load is equal to R1+R2, whereas the audio load impedance Zeq is equal to R1 in series with the parallel combination of R2, R3 and R4, assuming all capacitors have reactance which may be ignored.

• This is true at medium frequencies, but at high and low audio frequencies Zeq may have reactive component, causing phase shift and uneven frequency response.