Antennas, FM Receivers, and Cellular Communication Systems
Antenna Principles
Antennas sense electromagnetic waves and capture a part of them for processing. According to the standard definition by IEEE, an antenna or aerial may be defined as a “means for radiating or receiving radio waves.”
Fig. 1: Source, Transmission Line, and Antenna
Fig. 1 shows the position of an antenna in a transmission system. The source produces a radio frequency (RF) current. The antenna consists of a conductor split at the middle. The RF current is fed into the antenna by a twin wire or coaxial cable. The antenna converts the RF current into electromagnetic (EM) waves. The time-varying magnetic field produces a time-varying electric field and vice versa. This process continues in space as long as there is a time-varying current in the antenna rod. Thus, there is an EM wave motion that advances in space with the speed of light. These are governed by Maxwell’s equations.
Super Heterodyne FM Receiver
Fig. 2: FM Super Heterodyne Receiver
IF Amplifier
It is a fixed frequency double-tuned amplifier. In radio receivers, most of the selectivity is due to the sharp cut-off frequency of IF amplifier filters. In FM receivers, the intermediate frequency is 10.7 MHz.
Amplitude Limiter
It limits the variations in amplitude of the modulated signal occurring due to the interaction of noise from the channel.
Discriminator
It is the FM demodulator, but it also demodulates the variations in amplitude. So, the input to the discriminator must be an FM signal without any variation in amplitude.
Audio Amplifier
The detected message signal is amplified by the AF and power amplifiers. The amplified signal is supplied to the loudspeaker.
Cellular Communication
The introduction of cellular systems helped to improve the user capacity of mobile communication in the allowed spectrum. In cellular systems, high-power transmitters are replaced by a large number of low-power transmitters. Each of these transmitters constitutes a cell that can accommodate a large number of users over a geographical area. The frequency used in one cell can be reused in another cell which is at a sufficient distance. This property helps in communication between a large number of stations using a limited spectrum. This is the basic principle behind cellular communication.
Basic Cellular System
A cellular system consists of 4 basic components:
- Mobile Unit: Mobile handset used by the user.
- Cell Site: Cellular service areas are divided into small regions called cell sites. Each of these has a control unit, radio cabinet, antennas, power plant, and data terminals.
- Base Station: It is a fixed point of communication which is connected to the antenna that receives and transmits the signals on cellular networks to the customer phones.
- Mobile Telephone Switching Office (MTSO): The base stations are controlled by a mobile switching center. It coordinates all the cell sites. It contains a cellular processor and cellular switch. It controls call processing and handles billing services.
Cellular Communication
The introduction of cellular systems helped to improve the user capacity of mobile communication in the allowed spectrum. In cellular systems, high-power transmitters are replaced by a large number of low-power transmitters. Each of these transmitters constitutes a cell that can accommodate a large number of users over a geographical area. The frequency used in one cell can be reused in another cell which is at a sufficient distance. This property helps in communication between a large number of stations using a limited spectrum. This is the basic principle behind cellular communication.
Basic Cellular System
A cellular system consists of 4 basic components:
- Mobile Unit: Mobile handset used by the user.
- Cell Site: Cellular service areas are divided into small regions called cell sites. Each of these has a control unit, radio cabinet, antennas, power plant, and data terminals.
- Base Station: It is a fixed point of communication which is connected to the antenna that receives and transmits the signals on cellular networks to the customer phones.
- Mobile Telephone Switching Office (MTSO): The base stations are controlled by a mobile switching center. It coordinates all the cell sites. It contains a cellular processor and cellular switch. It controls call processing and handles billing services.