Communication Systems

Transcription

1 AM/FM Receiver

2 Communication Systems We have studied the basic blocks o any communication system Modulator Demodulator Modulation Schemes: Linear Modulation (DSB, AM, SSB, VSB) Angle Modulation (FM, PM)

3 AM/FM Radio System Principles: Frequency Spectrum Sharing (many transmitters using one medium) Demodulating desired signal and rejecting other signals transmitted at the same time

4 AM/FM Radio System The source signal is audio Dierent sources have dierent spectrum Voice (speech) Music Hybrid signals (music, voice, singing)

5 AM/FM Radio System Dierent audio sources have dierent bandwidth W Speech- 4kHz High quality music- 15kHz AM radio limits baseband bandwidth W to 5kHz FM radio uses baseband bandwidth W to 15kHz

6 AM/FM Radio System Radio system should be able to receive any type o audio source simultaneously. Dierent stations with dierent sources transmit signals simultaneously. Dierent listeners tune to dierent stations simultaneously.

7 AM/FM Radio System The dierent radio stations share the requency spectrum over the air through AM and FM modulation. Each radio station, within a certain geographical region, is designated a carrier requency around which it has to transmit Sharing the AM/FM radio spectrum is achieved through Frequency Division Multiplexing (FDM)

8 Example o AM Radio Spectrum Dierent radio stations, dierent source signals Fc0 Fc1 Fc2 F Carrier spacing- 10kHz (AM) Bandwidth (3-5kHz)

9 AM/FM Radio System For AM radio, each station occupies a maximum bandwidth o 10 khz Carrier spacing is 10 khz For FM radio, each station occupies a bandwidth o 200 khz, and thereore the carrier spacing is 200 khz

10 AM/FM Radio System Transmission Bandwidth: B T B T is the bandwidth occupied by a message signal in the radio requency spectrum B T is also the carrier spacing AM: BT = 2W FM: BT = 2 ( D + 1) W (Carson s Rule)

11 Design o AM/FM radio receiver The radio receiver has to be cost eective Requirements: Has to work with both AM and FM signals Tune to and ampliy desired radio station Filter out all other stations Demodulator has to work with all radio stations regardless o carrier requency

12 For the demodulator to work with any radio signal, we convert the carrier requency o any radio signal to Intermediate Frequency (IF) Radio receiver design can be optimized or that requency IF ilter and a demodulator or IF requency

13 AM/FM Radio Spectrum Recall that AM and FM have dierent radio requency (RF) spectrum ranges: AM: 540 khz 1600 khz FM: 88 MHz 108 MHz Thereore, two IF requencies AM: 455 khz FM: 10.7 MHz

14 A radio receiver consists o the ollowing: A Radio Frequency (RF) section An RF-to-IF converter (mixer) An Intermediate Frequency (IF) section Demodulator Audio ampliier R F Tuner IF Filter Demodulator Audio Ampliier

15 This is known as the Superheterodyne receiver Two stages: RF and IF (iltering and ampliication) The receiver was designed by Armstrong

16 RF Section Tunes to the desired RF requency, c Includes RF bandpass ilter centered around The bandwidth BRF Usually not narrowband, passes the desired radio station and adjacent stations c

17 The minimum bandwidth o RF ilter: B RF > BT Passes the desired radio channel, and adjacent channels

18 RF-IF converter: Converts carrier requency IF requency How can we convert signals with dierent RF requencies to the same IF requency?

19 Local oscillator with a center requency LO is a unction o RF carrier requency LO LO = c + IF R F Tuner IF Filter Demodulator Audio Ampliier

20 RF-to-IF receiver includes: An oscillator with a variable requency LO (varies with RF carrier requency) By tuning to the channel, you are tuning the local oscillator and RF tunable ilter at the same time.

21 All stations are translated to a ixed carrier requency or adequate selectivity. Fc X FIF FLO

22 Two requencies are generated at the output o product modulator: LO+ c = 2c + LO c = The higher requency component is eliminated through iltering We are let with IF requency IF IF

23 One problem with this receiver: Image Signal Image signal has a center requency: i = c + 2 IF

24 I an image signal exists at the input o the RF-to-IF converter, then the output o the converter will include the desired signal + image signal Fc X FIF FLO LO + i = ( c + IF ) + ( c + 2 IF ) = 2 c + 3 IF LO i = ( c + IF ) ( c + 2 IF ) = IF

25 Example: Incoming carrier requency 1000 khz, Local oscillator = =1455 khz Consider another carrier at 1910 khz I this is passed through the same oscillator, will have a =455 khz component Thereore, both carriers will be passed through RF-to-IF converter

26 Thereore, RF ilter should be designed to eliminate image signals The requency dierence between a carrier and its image signal is: 2 IF RF ilter doesn t have to be selective or adjacent stations, have to be selective or image signals Thereore, BT < BRF < 2 IF

27 IF ilter: Center requency IF Bandwidth approximately same as transmission bandwidth, B T For AM: BT = 2W For FM: BT =2 ( D+ 1) W

28 Depending on the type o the received signal, the output o IF ilter is demodulated using AM or FM demodulators. For AM: envelope detector For FM: requency discriminator