e In Sections 2.1.1-2.1.3, we started with the necessity receivers for modulation and demodulation and you were in- troduced to the basic principles of modulation, de- A block diagram of a double conversion superhet used for modulation and radio propagation. You should now receiving direct broadcast signals (DBSs) from satellites understand the meaning of terms such as AM, FM, PM is shown in Figure 2.1-36. Direct broadcasting satellites and digital modulation. for the United Kingdom region transmit in the 11.6-12.4 In Section 2.1.4, you were introduced to radio prop- GHz band. Each TV channel uses a 26 MHz bandwidth. agation, wave polarisation, field strength and power The double superhet is basically a superhet receiver density of radio waves. with two i.f. sections. The first i.f. section operates at In Sections 2.1.5 and 2.1.6, you learned about the a much higher i.f. frequency than the second i.f. section. properties of several antennas. These included 2/2 This choice is deliberate because a higher 1st in- dipole, folded dipole, monopole, loop antennas and the termediate frequency gives better image channel re- Yagi-Uda array. jection. The 2nd i.f. section is made to operate at a lower Section 2.1.7 dealt with various antenna distributions frequency because it gives better adjacent channel and matching systems. selectivity. In Section 2.1.8, you encountered some basic con- In this receiver, the input signal, f1, is selected, mixed cepts concerning the reception of radio signals. You with a local oscillator carrier, fx, and frequency translated should now be able to carry out simple calculations with to form the first intermediate frequency (fif1). This regard to selectivity, adjacent channel selectivity, sensi- signal is applied to the 1st i.f. amplifier section, then tivity, S/N ratio and noise figure ratio as applied to radio mixed with fo to produce a second intermediate fre- receivers. quency (fif2) and amplified prior to detection and low Sections 2.1.9 and 2.1.10 described the main func- frequency ampli