Lecture 14: FDM, AM Radio, and the Superheterodyne Receiver. Dr. Mohammed Hawa. Electrical Engineering Department, University of Jordan

Transcription

1 Lecture 14: FDM, AM Radio, and the Superheterodyne Receiver Dr. Mohammed Hawa Electrical Engineering Department University o Jordan EE421: Communications I: Lecture 14. For more inormation read Chapter 4 in your textbook or visit Multiplexing: FDM Frequency Division Multiplexing (FDM) is a process that allows the transmission o several signals over the same channel at the same time. This is achieved by modulating the dierent signals on dierent carriers with dierent carrier requencies. The receiver isolates one signal rom the rest using a tuneable BPF. 2 1

2 TV Broadcasting (FDM) For an FDM system, you need to know: Broadcast requencies or the stations (i.e., allocated spectrum). Bandwidth o each station. Guardband between adjacent stations. 3 TV Broadcasting Terrestrial TV uses broadcast requencies within the ranges: VHF (Very High Frequency): 30 MHz to 300 MHz UHF (Ultra High Frequency): 300 MHz and 3 GHz. 4 2

3 TV Broadcasting Satellite TV uses broadcast requencies within the ranges (Uplink/Downlink): C band: 6/4 GHz Ku band: 14/10-12 GHz Ka band: 27-31/18-20 GHz 5 Uplink/Downlink 6 3

4 AM Radio Broadcasting Each station is an AM modulation o human voice. FDM is used to multiplex signals on the air waves. US: Each station occupies a bandwidth o. Europe: Each station occupies a bandwidth o. 7 HW: Look at Your Radio Dial 8 4

5 The Superheterodyne Receiver Receivers in FDM system require a BPF. It is extremely diicult (expensive) to design highly selective (narrowband) ilters at high center requencies. This is specially true i the ilter is tuneable. Solution: Use a two-stage iltering process, one o which at lower requency. 9 AM Superheterodyne Receiver 10 5

6 Ganged Capacitor 11 Image Station Problem RF Filter RF Filter IF Filter IF Filter

7 Image Station (Part 2) 2 IF IF Filter IF Filter Superheterodyne Why's Why the RF Filter? Eliminates the image station. Reduces the amount (power) o noise that enters the receiver. Why the IF Stage (heterodyning)? With its high-selectivity and lower price, the IF ilter isolates the desired radio station rom all others sent using FDM. Since the IF requency does not change with the tuned station, it is easier to design the E.D. 14 7

8 Superheterodyne Why's Why the sum, not dierence? The sum (as opposed to the dierence) in the receiver results in a smaller tuning range ratio, which requires a smaller tuning capacitor or the local oscillator. Hence, this solution is cheaper. 15 Homework Now design a superheterodyne receiver, but this time using the dierence or L.O.: I you want to listen to the station at 1000 what settings should you choose or the RF BPF, the oscillator, and the IF BPF? Repeat the same problem i you want to listen to the 1020 and 1500 stations. What is the requency o the image station i you are listening to the station at 1000? 16 8

9 Superheterodyne Everywhere! The superheterodyne receiver is much more popular nowadays compared to the homodyne receiver. It is used in many communication systems including: FM Radio, Analog and Digital TV broadcasting, Cellular phones, WiMAX, Satellite and Microwave systems, GPS, etc. Some popular IF requencies: AM radio receivers: FM radio receivers: 10.7 MHz Analogue television receivers: MHz 17 Homework 18 9

10 Solution: Not in the Exam Supply Block Voltage Tone Polarization Frequency band Local oscillator requency Intermediate req. range 13 V 0 Vertical GHz, low 9.75 GHz 950 1,950 MHz 18 V 0 Horizontal GHz, low 9.75 GHz 950 1,950 MHz 13 V 22 Vertical GHz, high GHz 1,100 2,150 MHz 18 V 22 Horizontal GHz, high GHz 1,100 2,150 MHz 19 10