Amplitude Modulation Receiver.

AM demodulation is the reverse process of AM modulation. A conventional double-sideband AM receiver simply converts a received amplitude-modulated wave back to the original source information. To do this, a receiver must be capable of receiving , amplifying, and demodulating an AM wave. It must also be capable of band limiting the total radio- frequency spectrum to a specific desired band of frequencies. The selection process is called tuning the receiver.

To completely understand the demodulation process, first it is necessary to have a basic understanding of the terminology commonly used to describe radio receivers and their characteristics. The RF section is the first stage of the receiver and is therefore often called the receiver front end. The primary functions of the RF section are detecting , band limiting and amplifying the receiver RF signals.

The mixer/converter section is the next stage. This section down converts the received RF frequencies to intermediate frequency , which are simply frequency that fall somewhere between the RF and information frequency hence the name intermediate. The primary functions of the IF section are amplification and selectivity. The AM detector demodulates the AM wave and converts it to the original information signal , and the audio section simply amplifies the recovered information.

There are several parameters commonly used to evaluate the ability of a receiver to successfully demodulate a radio signal. The most important parameters are selectivity and sitivity, which are often used to compare the quality of one radio receiver to another.

Selectivity is a receiver parameter that is used to measure the ability of the receiver to accept a given band of frequency and reject all others.For example, with the commercial AM broadcast band , each station's transmitter is allocated a 10-khz bandwidth . Therefore, for a receiver to select only those frequency assigned a single channel, the receiver must limit its bandwidth to 10khz. If the passband is greater than 10khz , more than one channel may be received and demodulated  simultaneously. If the passband of a receiver is less than 10khz ,a portion of the modulating signal information for that channel is rejected or blocked from entering the demodulator and , consequently, lost.

There are several acceptable ways to describe the selectivity of a radio receiver. One common way is to simply given the bandwidth of the receiver at the -3-db points. This bandwidth, however , is not necessarily a good means of determining how well the receiver band width at two levels of attenuation, for example, -3db and -60db. The ratio of these two bandwidths is called the shape factor.

In today's overcrowded radio frequency spectrum the FCC makes adjacent channel assignments as close together as possible, with only 10khz separating commercial broad cast band AM channels. Spacing for adjacent commercial broadcast- band FM channels is 200khz, and commercial television channels are separating by 6 mhz . A radio receiver must be capable of separating the desired channel's signal without allowing interference from an adjacent channel to spill over into the desired channel's passband.

If the bandwidth can be reduced , the noise will also be reduced by the same proportion, thus increasing the signal-to -noise power ratio , improving system performance . There is . of course , a system performance limitation as to how much the bandwidth can be reduced .The bottom line is that the circuit bandwidth must exceed the bandwidth of the information signal; otherwise , the information power and/ or the frequency content of the information signal will be reduced , effectively  degrading system performance. 

When a signal propagates from the antenna through the RF section , mixer converter section. and if section the bandwidth is reduced , thus reducing the noise. The theoretical problem is hoe much the bandwidth should be reduced, and the practical problem is in the difficulty of constructing stable narrow- band filters.

The input signal-to -noise ratio is calculated at a receiver input using the RF bandwidth for the noise power measurement. However the RF bandwidth is generally wider than the bandwidth of the rest of the receiver. Reducing the bandwidth is effectively equivalent to reducing the noise .