Sabtu, 26 Januari 2013

Frequency Modulation (FM)

Modulasi Frekuensi (Frequency Modulation = FM ) adalah proses menumpangkan sinyal informasi pada sinyal pembawa (carrier) sehingga frekuensi gelombang pembawa (carrier) berubah sesuai dengan perubahan simpangan (tegangan) gelombang sinyal informasi. Jadi sinyal informasi yang dimodulasikan (ditumpangkan) pada gelombang pembawa menyebabkan perubahan frekuensi gelombang pembawa sesuai dengan perubahan tegangan (simpangan) sinyal informasi. Pada modulasi frekuensi sinyal informasi mengubah-ubah frekuensi gelombang pembawa, sedangkan amplitudanya konstan selama proses modulasi. Proses modulasi frekuensi digambarkan sebagai berikut: 

Proses Modulasi Frekuensi (Frequency Modulation, FM)
Besar perubahan frekuensi (deviasi), δ atau fd, dari sinyal pembawa sebanding dengan amplituda sesaat sinyal pemodulasi, sedangkan laju perubahan frekuensinya sama dengan frekuensi sinyal pemodulasi. Persamaan sinyal FM dapat dituliskan sebagai berikut: 
eFM = Nilai sesaat sinyal FM 
Ec = amplituda maksimum sinyal pembawa 
ωc = 2π fc dengan fc adalah frekuensi sinyal pembawa 
ωm = 2π fm dengan fm atau fs adalah frekuensi sinyal pemodulasi 
mf = indeks modulasi frekuensi 

Pada modulasi frekuensi kita mengenal istilah indeks modulasi (mf). Indeks modulasi ini didefinisikan sebagai berikut: 

Spektrum Sinyal FM
Lebar bandwidth sinyal FM adalah tak berhingga. Namun pada praktek biasanya hanya diambil bandwith dari jumlah sideband yang signifikan. Jumlah sideband signifikan ditentukan oleh besar indeks modulasinya seperti dalam fungsi tabel besel berikut. 

Tabel Fungsi Besel Untuk FM
Ji : nilai amplituda komponen frekuensi sideband ke i (i≠0) 
Jo : nilai amplituda komponen frekuensi sinyal pembawa (bukan sideband) 
β = mf : indeks modulasi 

Lebar bandwidth pada modulasi FM dapat ditentukan menggunakan teorema carson sebagai berikut : 
fd = frekuensi deviasi 
fm = frekuensi maksimum sinyal pemodulasi 

Karakter dari transmisi modulasi frekuensi (Frequency Modulation, FM) adalah : 
Tidak dapat dipantulkannya gelombang elektromagnetic dari modulasi frekuensi sehingga jarak pancaran adalah line of sight dan terbatas pada daya pancar. 
Ketahanan modulasi terhadap noise pada transmisi modulasi frekuensi, sehingga kualitas sinyal informasi yang diterima jernih seperti aslinya.

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