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 limiter Discrimi- nator Detektor selubung DC Blocking S FM (t)-IF S ’ FM (t)-IF A A’ B C D

Up Converter (di Pemancar) BPF-IF Filter BPF-RF Filter BPF-RF MIXER OSCILLATOR RF IF IF’ f osc f osc —IF’ f osc +IF’ 3 Modul 05 - Siskom I - Receiver FM & AGC

4 Down Converter (di Penerima) Filter BPF-RF Filter BPF-RF Filter BPF-IF Filter BPF-IF MIXER OSCILLATOR IF RF RF’ f osc RF’ — f osc RF’+ f osc 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 Limiter’s 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 S FM (t) (  keluaran discriminator) didapat : Informasi terkandung pada bagian selubung dari S ’ FM (t)

Modul 05 - Siskom I - Receiver FM & AGC 7 Demodulasi Sinyal FM Keluaran detektor selubung (masukan DC blocking): Keluaran DC blocking: selubung dari S ’ FM (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 f i (t)=f c +k f m(t) Disc.output f Deviation limits +75 KHz -75 KHz f carrier f carrier is at the IF frequency Of 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 signal into an AM f c f o output f

Modul 05 - Siskom I - Receiver FM & AGC 10 Zero crossing detector Hard limiter Zero Crossing detector Multi- vibrator Averaging circuit FMOutput FM input Hard limiter ZC detector multiV more frequent ZC’s means higher inst freq in turn means Larger message amplitudes 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 B FM =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 25KHz guardband 150 KHz 200 KHz carrier

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-SC f sc =38 kHz + fsc= 38KHz freq divider 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-SC f sc =38 kHz + fsc= 38KHz (:2) freq divider Composite baseband FM transmitter - A B C D E F G H I  

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+right DSB-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+right DSB-SC

Modul 05 - Siskom I - Receiver FM & AGC 18 Stereo receiver diagram FM receiver lowpass filter(15K) bandpass at 38KHz Xlowpass VCO Divide 2 Xlowpass Left+right left right PLL coherent detector A B C D E F 38 KHz19 KHz 15 KHz S A (f) FM stereo demultiplexing G H

AGC (Automatic Gain Control) Penguat Variabel Penguat Variabel Detektor Penguat Diferensial Penguat Diferensial LPF Vi Vo Vr 19 Modul 05 - Siskom I - Receiver FM & AGC

AGC (Automatic Gain Control) V2V2 V1V1 ViVi VoVo 20 Modul 05 - Siskom I - Receiver FM & AGC

MATERI UTS: MODUL 1 – 6 SIFAT UJIAN : TUTUP BUKU LAMA UJIAN :  2 JAM (4-5 SOAL) TIPE SOAL : ESSAY Modul 05 - Siskom I - Receiver FM & AGC 21

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