elsiskom modulasi fm 2

TE3113

SISTEM KOMUNIKASI 1

RECEIVER FM & AGC:Superheterodyne, Demodulator FM,

FM Stereo, AGC

oleh: Budi Prasetya

Modul #05

Program Studi S1 Teknik Telekomunikasi

Departemen Teknik Elektro - Sekolah Tinggi Teknologi Telkom

Bandung 2008

Modul 05 - Siskom I - Receiver FM & AGC 2

FM receiver

FM receiver is similar to the superheterodyning

(down converting) layout:

BPF-RF

mixer

LO

BPF-IF limiterDiscrimi-

natorDetektorselubung

DC Blocking

SFM(t)-IF SFM(t)-IF

A

A

B

C D

Up Converter (di Pemancar)

Filter

BPF-RF

MIXER

OSCILLATOR

RFIF

fosc

foscIF

fosc+IF

3Modul 05 - Siskom I - Receiver FM & AGC

4Down Converter (di Penerima)

Filter

BPF-IF

MIXER

OSCILLATOR

IFRF

fosc

fosc RF

fosc + RF

Modul 05 - Siskom I - Receiver FM & AGC


Limiter

A limiter is a circuit whose output is constant

for all input amplitudes above a threshold

Limiters 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 SFM(t)


Demodulasi Sinyal FM

Keluaran detektor selubung (masukan DC blocking):

Keluaran DC blocking:

)(.2 tmCtmkAtm fc

tmkfAtS fcc 22 selubung dari SFM(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 frequencyOf 10.7 MHz


Examples of discriminators

Slope detector - simple LC tank circuit

operated at its most linear response curve

This setup turns an FM signalinto an AM

fc fo

output

f


Zero crossing detector

Hard

limiter

Zero Crossingdetector

Multi-vibrator

Averagingcircuit

FM Output

FM input

Hard limiter

ZC detector

multiV

more frequentZCs meanshigher inst freqin turn meansLarger messageamplitudes

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 BFM station A FM station C

2 X 10 KHz guardband

180 KHz

200 KHz

carrier

10 KHz


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-SCfsc=38 kHz

+

fsc=38KHz

freqdivider

Composite baseband

-


Stereo signal

Composite baseband signal is then

frequency modulated

Left channel

Right channel

+

+

+

mono

DSB-SCfsc=38 kHz

+

fsc=38KHz

(:2) freqdivider

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 KHz19 KHz

15 KHz

Left+rightDSB-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 KHz19 KHz

15 KHz

Left+rightDSB-SC


Stereo receiver diagram

FMreceiver

lowpassfilter(15K)

bandpassat 38KHz

X lowpass

VCODivide 2

X lowpass

+

+-

+

++

Left+right left

right

PLL

coherent detectorJ

K

L

M

N O

38 KHz19 KHz15 KHz

SJ(f)=SH(f)

FM stereo demultiplexing

P

Q

AGC (Automatic Gain Control)

Penguat

Variabel

Detektor

Penguat

Diferensial

LPF

ViVo

Vr


AGC (Automatic Gain Control)

V2V1 Vi

Vo


elsiskom modulasi fm 2

Documents