I have studied AM modulation and demodulation. But i can't understand the purpose of envelope detector in demodulation. So my question is: Why, where and how we use envelope detector in AM modulation?
I don't understand how you could have studied AM demodulation and not understand the purpose of an envelope detector. Something is lacking in your studies. In any case, in AM (the M stands for modulation) all of the information is contained in the amplitude variations of the carrier. That amplitude variation is usually called the envelope of the resultant AM signal due to its appearance on an oscilloscope. Hence, to recover the signal information, the envelope of the AM signal must be extracted. This is the job of the envelope detector, hence the name. For analog AM, such as ordinary broadcast radio, an envelope detector is as simple as a diode and a low pass filter. Any communications textbook can provide more information.
An envelope detector is the cheapest and fastest way to demodulate an AM signal.
The purpose of the diode is to only allow the positive alternations of your input signal (or negative if you flip the diode) to pass to the RC circuit, and detect either the positive (or negative) envelope of your signal.
Then the capacitor will gradually charge up until the voltage across it reach the maximum value of the rectified voltage. The time constant of the RC circuit must be well choose so that the discharging time of the capacitor is not too short or not too long. In fact, the time constant must be big compare to the carrier period and short compare to the modulating signal period.
i can't understand the purpose of envelope detector in demodulation
Think about feeding the output of an AC power variac to a half wave rectifier with a filter capacitor and load. The peaks of the AC voltage would produce a dc voltage on the output of the rectifier. Now slowly move the variac output producing a bigger then a smaller AC voltage and note that the output DC rises and falls with the changes in the AC voltage from the variac.
The diode, capacitor and load resistor ARE an envelope detector.
Now speed this scenario up with the variac control being replaced by an audio signal modulating an RF carrier and you still have an envelope detector that demodulates the audio. Previously the power AC was the carrier and the movement of the variac wiper was a slow moving sub-audio signal.
Here's the concept represented graphically, below, and if you left-click the image it'll probably be displayed full-screen.
MCW is a 1MHz carrier modulated by a 1kHz signal, D1,C1, and R1 comprise the envelope detector, and AF is the detected 1kHz modulating signal.
Note that AF is pulsating DC.
AM modulation is of two types
- AM with large carrier (AM-c)
- AM with suppressed carrier (AM-SC)
the suppressed carrier type AM has also has two types AM-DSB(double sideband) and AM-SSB(single sideband). the large carrier one is also double sideband but it is called AM-C that is AM with carrier. Now coming to the question AM with suppressed carrier is difficult to demodulate because carrier is suppressed(not present) and to demodulate we need to multiply signal with carrier, therefore comes to the picture synchronous detection technique in which a carrier is generated at the receiver side which is then multiplied with the receiving signal. In case of AM-C which employs envelope detection technique a carrier is already present in the signal. so baseband signal (message signal) and carrier signal form an envelope which is AM-C and this envelope is detected by envelope detector. envelope detector is nothing but a diode followed by low pass filter. when you will see the diagram of AM-SC and AM-C the difference you will see is that in AM-SC there is a phase reversal at zero crossing. the phase of the envelope changes to 180 while in AM-C there is no phase reversal. so that is another reason to use different demodulation techniques and also the concept of modulation index. it is very wide topic first you need to study different modulation demodulation techniques