SISTEM KOMUNIKASI

RECEIVER FM & AGC: Superheterodyne, Demodulator FM,

FM Stereo, AGC

FM receiver

FM receiver is similar to the superheterodyning

(down converting) layout:

BPF-RF

mixer

LO

BPF-IF limiter Discrimi-

nator

Detektor

selubung

DC

Blocking

SFM(t)-IF S’FM(t)-IF

A

A’

B

C D

Up Converter (di Pemancar)

Filter

BPF-RF

MIXER

OSCILLATOR

RF IF

fosc

fosc—IF

fosc+IF

Down Converter (di Penerima)

Filter

BPF-IF

MIXER

OSCILLATOR

IF RF

fosc

fosc — RF

fosc + RF

Limiter

A limiter is a circuit whose output is constant

for all input amplitudes above a threshold

Limiter’s function in an FM receiver is to

remove unwanted amplitude variations of the

FM signal

Limiter

Demodulasi Sinyal FM

Dengan menggunakan diskriminator/differensiator

Pada sinyal FM, informasi terkandung pada frekuensi sinyal

FM

Jika dilakukan diferensiasi terhadap SFM(t) (keluaran

discriminator) didapat :

t

fIFcFM dttmktfAtS0

)(22cos

t

fIFfIFcFM dttmktftmkfAtS )(22sin22'

Informasi terkandung pada bagian selubung dari S’FM(t)

Demodulasi Sinyal FM

Keluaran detektor selubung (masukan DC blocking):

Keluaran DC blocking:

)(.2ˆ tmCtmkAtm fc

tmkfAtS fcc 22

selubung dari S’FM(t)

DC blocking

Discriminator

The heart of FM is this relationship

What we need is a device that linearly

follows inst. frequency

fi(t)=fc+kfm(t)

Disc.output

f

Deviation limits

+75 KHz -75 KHz

fcarrier

fcarrier is at the IF frequency

Of 10.7 MHz

Examples of discriminators

Slope detector - simple LC tank circuit

operated at its most linear response curve

This setup turns an FM signal

into an AM

fc fo

output

f

Zero crossing detector

Hard

limiter

Zero

Crossing

detector

Multi-

vibrator

Averaging

circuit

FM Output

FM input

Hard limiter

ZC detector

multiV

more frequent

ZC’s means

higher inst freq

in turn means

Larger message

amplitudes

Averaging circuit

Commercial FM

Commercial FM broadcasting uses the

following parameters

Baseband:15KHz = W = fm

Deviation ratio:5 (index modulasi)

Peak freq. Deviation=75KHz

BFM=2(+1)W=2x6x15=180KHz

Commercial FM spectrum

The FM landscape looks like this

FM station B FM station A FM station C

25KHz guardband

150 KHz

200 KHz

carrier

FM stereo:multiplexing

First, two channels are created; (left+right)

and (left-right)

Left+right is useable by monaural receivers

-

Left channel

Right channel

+

+

+

mono

(left+right)

(left-right)

Subcarrier modulation

The mono signal is left alone but the

difference channel is amplitude modulated

with a 38 KHz carrier

Left channel

Right channel

+

+

+

mono

DSB-SC

fsc=38 kHz

+

fsc=

38KHz

freq

divider

Composite baseband

-

Stereo signal

Composite baseband signal is then

frequency modulated

Left channel

Right channel

+

+

+

mono

DSB-SC

fsc=38 kHz

+

fsc=

38KHz

(:2) freq

divider

Composite baseband

FM

transmitter

-

A

B

C

D

E

F

G

H I

FM stereo multiplexing

Stereo spectrum

Baseband spectrum holds all the information.

It consists of composite baseband, pilot tone

and DSB-SC spectrum

38 KHz 19 KHz

15 KHz

Left+right DSB-SC

Stereo receiver

First, FM is stripped, i.e. demodulated

Second, composite baseband is lowpass

filtered to recover the left+right and in parallel

amplitude demodulated to recover the left-

right signal

38 KHz 19 KHz

15 KHz

Left+right DSB-SC

Stereo receiver diagram

FM

receiver

lowpass

filter(15K)

bandpass

at 38KHz

X lowpass

VCO Divide 2

X lowpass

+

+ -

+

+ +

Left+right left

right

PLL

coherent detector A

B

C

D

E F

38 KHz 19 KHz 15 KHz

SA(f)

FM stereo demultiplexing

G

H

AGC (Automatic Gain Control)

Penguat

Variabel

Detektor

Penguat

Diferensial

LPF

Vi Vo

Vr

AGC (Automatic Gain Control)

V2 V1 Vi

Vo

MATERI UTS: MODUL 1 – 6

SIFAT UJIAN : TUTUP BUKU

LAMA UJIAN : 2 JAM (4 SOAL)

TIPE SOAL : ESSAY