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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
Modul 05 - Siskom I - Receiver FM & AGC 5
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
Modul 05 - Siskom I - Receiver FM & AGC 6
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)
Modul 05 - Siskom I - Receiver FM & AGC 7
Demodulasi Sinyal FM
Keluaran detektor selubung (masukan DC blocking):
Keluaran DC blocking:
)(.2 tmCtmkAtm fc
tmkfAtS fcc 22 selubung dari SFM(t)
DC blocking
Modul 05 - Siskom I - Receiver FM & AGC 8
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
Modul 05 - Siskom I - Receiver FM & AGC 9
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
Modul 05 - Siskom I - Receiver FM & AGC 10
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
Modul 05 - Siskom I - Receiver FM & AGC 11
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
Modul 05 - Siskom I - Receiver FM & AGC 12
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
Modul 05 - Siskom I - Receiver FM & AGC 13
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)
Modul 05 - Siskom I - Receiver FM & AGC 14
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
-
Modul 05 - Siskom I - Receiver FM & AGC 15
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
Modul 05 - Siskom I - Receiver FM & AGC 16
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
Modul 05 - Siskom I - Receiver FM & AGC 17
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
Modul 05 - Siskom I - Receiver FM & AGC 18
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
19Modul 05 - Siskom I - Receiver FM & AGC
AGC (Automatic Gain Control)
V2V1 Vi
Vo
20Modul 05 - Siskom I - Receiver FM & AGC