Modul #05 TE3113 SISTEM KOMUNIKASI 1
RECEIVER FM & AGC AGC: Superheterodyne, p y Demodulator FM, FM Stereo, AGC Kelas TE-29-02 Program Studi S1 Teknik Telekomunikasi Departemen p Teknik Elektro - Sekolah Tinggi gg Teknologi g Telkom Bandung – 2007
FM receiver
FM receiver is similar to the superheterodyning (down converting) layout:
BPF-RF
mixer
LO
SFM(t)-IF
BPF-IF
limiter
S’FM(t)-IF
Discriminator
Detektor selubung
DC Blocking
Modul 05 - Siskom I - Receiver FM & AGC
2
Up Converter (di Pemancar)
fosc—IF’ fosc+IF’
MIXER
BPF IF
Filter RF
IF’ IF fosc
OSCILLATOR
Modul 05 - Siskom I - Receiver FM & AGC
3
Down Converter (di Penerima)
RF’ — fosc RF’+ fosc
MIXER
Filter RF
Filter RF’ RF
IF fosc
OSCILLATOR
Modul 05 - Siskom I 4- Receiver FM & AGC
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 p variations of the remove unwanted amplitude FM signal
Limiter
Modul 05 - Siskom I - Receiver FM & AGC
5
Demodulasi Sinyal FM
Dengan menggunakan diskriminator/differensiator Pada sinyal FM, informasi terkandung pada frekuensi sinyal FM t ⎡ ⎤ S FM (t ) = Ac cos ⎢2πf IF t + 2πk f ∫ m(t )dt ⎥ 0 ⎣ ⎦
Jika dilakukan diferensiasi terhadap SFM(t) (⇒keluaran discriminator) didapat : t ⎛ ⎞ ' S FM (t ) = Ac 2πf IF + 2πk f m(t ) sin ⎜⎜ 2πf IF t + 2πk f ∫ m(t )dt ⎟⎟ −∞ ⎠ ⎝
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Informasi terkandung pada bagian selubung dari S’FM(t) Modul 05 - Siskom I - Receiver FM & AGC
6
Demodulasi Sinyal FM
Keluaran detektor selubung (masukan DC blocking):
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]
selubung dari S’FM(t)
S (t ) = Ac 2πf c + 2πk f m(t )
DC blocking
Keluaran DC blocking:
mˆ (t ) = Ac 2πk f m(t ) = C.m(t )
Modul 05 - Siskom I - Receiver FM & AGC
7
Discriminator
The heart of FM is this relationship fi(t)=fc+kfm(t)
What we need is a device that linearly follows inst inst. frequency fcarrier is at the IF frequency Of 10.7 MHz Disc output Disc.output -75 KHz
fcarrier
+75 KHz
f
Deviation limits Modul 05 - Siskom I - Receiver FM & AGC
8
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
output
fc
fo
Modul 05 - Siskom I - Receiver FM & AGC
f
9
Commercial FM
Commercial FM broadcasting uses the following parameters
Baseband:15KHz
= W = fm
Deviation
ratio:5
Peak freq. Deviation=75KHz BFM=2(β+1)W=2x6x15=180KHz (β )
Modul 05 - Siskom I - Receiver FM & AGC
10
Commercial FM spectrum
The FM landscape looks like this
carrier i
FM station A
25KHz gguardband
FM station B
FM station C
150 KHz 200 KHz Modul 05 - Siskom I - Receiver FM & AGC
11
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
12
Subcarrier modulation
The mono signal is left alone but the difference channel is amplitude modulated with a 38 KHz carrier L ft channel Left h l +
Composite baseband
mono
+
+
Right channel +
DSB-SC fsc=38 kHz
fsc=
38KHz
freq divider
Modul 05 - Siskom I - Receiver FM & AGC
13
Stereo signal
Composite baseband signal is then frequency modulated Composite baseband
Left channel
+
mono
+
FM transmitter
+
Right channel +
DSB-SC fsc=38 kHz fsc= 38KHz
ffreq divider
Modul 05 - Siskom I - Receiver FM & AGC
14
Stereo spectrum
Baseband spectrum holds all the information. It consists of composite baseband, pilot tone and DSB-SC spectrum
L f i h Left+right
DSB-SC
19 KHz
38 KHz
15 KHz
Modul 05 - Siskom I - Receiver FM & AGC
15
Stereo receiver First, FM is stripped, i.e. demodulated Second,, composite p baseband is lowpass p filtered to recover the left+right and in parallel p demodulated to recover the leftamplitude right signal
Left+right
DSB-SC
19 KHz 15 KHz Modul 05 - Siskom I - Receiver FM & AGC
38 KHz
16
Stereo receiver diagram
lowpass filter(15K)
Left+right
+
+ left
+
coherent detector 15 KHz 19 KHz 38 KHz
bandpass at 38KHz
X
lowpass
right
- + +
FM receiver PLL X
lowepass p Divide 2
VCO
Modul 05 - Siskom I - Receiver FM & AGC
17
Zero crossing detector FM
Hard
limiter
Zero Crossing detector
Multivibrator
Averaging circuot
Output
FM input more frequent ZC’ means ZC’s higher inst freq in turn means Larger message amplitudes p
Hard limiter
ZC detector multiV Averaging g g circuit
Modul 05 - Siskom I - Receiver FM & AGC
18
AGC (Automatic Gain Control)
Vi
Vo
Penguat Variabel Detektor Penguat g Diferensial
Vr
LPF
Modul 05 - Siskom I - Receiver FM & AGC
19
AGC (Automatic Gain Control)
Vo
V1
V2
Modul 05 - Siskom I - Receiver FM & AGC
Vi 20
