written 5.5 years ago by | • modified 5.4 years ago |
Subject: Principles of Communication Engineering
Difficulty : Medium
Marks : 10
written 5.5 years ago by | • modified 5.4 years ago |
Subject: Principles of Communication Engineering
Difficulty : Medium
Marks : 10
written 5.4 years ago by | • modified 5.4 years ago |
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.