A receiver has the functions of selecting the desired signal from all the other available signals, amplifying and demodulating it, and displaying it in the desired manner. in the Amplitude Modulation (AM) receiver, the incoming signal voltage is combined with a signal generated in the receiver to produce a lower fixed frequency. The signal at this intermediate frequency contains the same modulation as the carrier, and it is now amplified and detected to reproduce the original information.
A constant frequency difference is maintained between the local oscillator and the Radio Frequency (RF) circuits, normally through capacitance tuning, in which all the capacitors are ganged together and operated in unison by one control knob. The IF (Intermediate Frequency) amplifier generally uses two or more transformers, each consisiting of a pair of mutually coupled tuned circuits.
It provides most of the gain and therefore sensitivity and bandwidth requirements of the receiver. The selectivity and sensitivity of the superhet are usually fairly uniform throughout its tuning range and not subject to the variations that affect the TRF receiver.
The IF signal output is an amplified composite of the modulated RF from the transmitter in combination with RF from the local oscillator. Neither of these signals is useable without further processing. The next process is in the detector stage, which eliminates one of the sidebands still presents and separates the RF from the audio components of other sideband. The RF is filtered to ground, and audio is supplied or fed to the audio stages for amplification and then to the speakers, etc.
Because of its narrow bandwidth, the IF amplifier rejects all other frequencies but 455 KHz. This rejection process is the key to the superheterodyne's exceptional performance, which is why it is widely accepted. The process of tuning the local oscillator to a predetermined frequency for each station throughout the AM band is known as tracking and will be discussed later.
This project will cover AM receivers in general, showing why their format has been to a certain extent standardized. Each block of the receiver will be discussed in detail, as will its functions and design limitations.