BASIC LAYOUT OF AN FM RECEIVER

The figure below shows the block diagram of a basic FM receiver. The deemphasis circuit is used only for receivers that pick up commercial FM broadcasts. FM receivers that are used for NBFM reception do not use preemphasis or deemphasis.

This block diagram does not include special circuits required for stereo reception. These will be discussed in a later section. 

The receiver layout is similar to the AM receiver described in Module A, but there are some differences:

1. There is no AGC circuit.

2. There may be a deemphasis network

3. A discriminator is used rather than a diode detector.

AGC is not necessary in an FM receiver, because the limiter circuit provides AGC action. The limiter is a special amplifier whose output is essentially constant over a wide range of inputs. It clips all noise peaks from the IF signal and the output of the linear has a constant amplitude.. Because the limiter’s output is essentially constant over a wide range of input values, it provides AGC action. A limiter cannot be used for this purpose in an AM receiver, because amplitude variations in the signal contain information as well as noise.

Deemphasis is performed to undo the preemphasis used at the transmitter site to improve the signal to noise ratio at higher audio frequencies. This type if signal processing is only done for wideband FM. The audio bandwidth of a NBFM signal is typically limited to 3 KHz and preemphasis/deemphasis is not necessary.

FM signals must be detected using a different type of detector from AM. Typically, an FM receiver will use one of the following types of detectors:

            Foster-Seely discriminator

            ratio detector

            PLL (phase-locked loop)

An RF amplifier is necessary for an FM receiver, for the following reasons:

1.      Since FM has superior noise suppression capabilities, it can be used with very weak signals. It is necessary to amplify these weak signals so that their amplitude is sufficient to drive the mixer.  

2.      Frequencies used for FM communications and broadcasting are generally in the VHF range and above, where the capture area of an antenna is quite small.  Additionally, FM is often used for mobile services, in which small antennas (with very small capture area) are used.

It is general practice to use FET’s as RF amplifiers because they produce less distortion than BJT’s. At extremely high frequencies (~ 1 GHz) RF amplifiers are not used because available devices are too noisy. Better performance can be obtained by connecting the antenna directly to the mixer and downconverting the signal. The noise factors of FET’s and BJT’s are much lower at the IF frequency and as the Friis formula indicates, the overall noise figure is determined primarily by the first stage.