Envelope Detection of amplitude Modulated Signals

Posted on 9:09 PM Under 0 comments

Demodulation is the act of extracting the original information-bearing signal from a modulated carrier wave. A demodulator is an electronic circuit (or computer program in a software defined radio) that is used to recover the information content from the modulated carrier wave. These terms are traditionally used in connection with radio receivers, but many other systems use many kinds of demodulators. Another common one is in a modem, which is a contraction of the terms modulator/demodulator.

An AM signal encodes the information onto the carrier wave by varying its amplitude in direct sympathy with the analogue signal to be sent. There are two methods used to demodulate AM signals.The envelope detector is a very simple method of demodulation. It consists of anything that will pass current in one direction only, that is, a rectifier. This may be in the form of a single diode, or may be more complex. Many natural substances exhibit this rectification behavior, which is why it was the earliest modulation and demodulation technique used in radio. The crystal set exploits the simplicity of the modulation to produce an AM receiver with very few parts.


The product detector multiplies the incoming signal by the signal of a local oscillator with the same frequency and phase as the carrier of the incoming signal. After filtering the original audio signal will result. This method will decode both AM and SSB, although if the phase cannot be determined a more complex setup is required.

An AM signal can be rectified without requiring a coherent demodulator. For example, the signal can be passed through an envelope detector (a diode rectifier). The output will follow the same curve as the input baseband signal. There are forms of AM in which the carrier is reduced or suppressed entirely, which require coherent demodulation.

Frequency modulation or FM is more complex. It has numerous advantages over AM, such as better fidelity and noise immunity. However, it is much more complex to both modulate and demodulate a carrier wave with FM, and AM predates it by several decades.

There are several common types of FM demodulator:
  • The quadrature detector, which phase shifts the signal by 90 degrees and multiplies it with the unshifted version. One of the terms that drops out from this operation is the original information signal, which is selected and amplified.
  • The signal is fed into a PLL and the error signal is used as the demodulated signal.
  • The most common is a Foster-Seeley discriminator. This is composed of an electronic filter which decreases the amplitude of some frequencies relative to others, followed by an AM demodulator. If the filter response changes linearly with frequency, the final analog output will be proportional to the input frequency, as desired.
  • Another method uses two AM demodulators; one tuned to the high end of the band and the other to the low end, and feed the outputs into a difference amp.
  • Ratio detector
  • Using a digital signal processor, as used in software-defined radio.


About the author

Paul Ryan A. Dedumo is a blogger and an Electronics and Communication Engineering student at University of San Carlos. Learn and inspire as you follow him in his links, blog and other social media accounts.


0 comments