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CHNG 2 : Gii thiu chung v tng i in thoi II.1 Tng i in thoai: 11.1.1 inh ngha:

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Tong i in thoi l mt h thng chuyn mch, no c nhim vu kt ni cc cuc lin lc t thit b u cui chu gi (Clling sid) n thit b cui (Clld sid) 11.1.2 Cu trc mang in thoai tng: Cc thnh phn chnh cu mng in thi chuyn mch cng cng c phn cp nh hnh v:

Trung tm min (lp 1) Trung tm vung (lp 2) Trung tm cp 1 (lp 3) Trung tm ng di Trung tm chuyn tip ht Trung tm u cui (tng i ni ht)

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H II.1 Cu truc mng in thoi ----------- ng chn cui .............. Trung k c s dung ca e gii quyt nh tuyn, trng phan lp chuyn mach tuy th cuc gi ni hat, lin tnh trng nc hay ngoi nc m ngi ta qui nh m s my cua ngi s dung, m tng i au cui, m tnh hay m tng nc khc nhau the nhng m s khc nhau: Mt ngi s dung au cui la thu ba nha hc cng s trc tip ni n tng i u cui (ni hat) cua mang in thai cn c gi la tng i lp 5 hc trung tam in thai ni hat (C.O). Cc ngi s dung au cui thng c ni n C.O qua mt i dy xn n hc c gi la ng dy thu ba (C.O:cntral ffic) Cc tram cp 4 (tng i ng di) c th p ng 2 chc nang: + L mt chuyn mach ng di, tng i cp 4 la mt phn cua mang ng di, ni cch khc, mt tng i lp 4 c th hat ng nh mt chuyn mach chuyn tip ni cc tng i lp 5 ny khi c u lu lng thng tin gia cc tng i iu chnh cc trung th trc tip.

+ Cac chuyn mch ca tong i chuyn tip ni ht cng c th chuyn mch iu khin lu thong vt trn trn cc trung k trc tip gia cc tong i u cui. S phn bit gia cc chuyn mach ng di v chuyn mach chuyn tip tr nn c bit quan trong dn n kt qu l phn ly cc ngun thong tin trong mang Vic nh tuyn gia cc C.O phai luon luon m bo s" tong i cng t cng tot giam n toi thiu chi ph tuyn dn lu lng. Tuyn thc c chon s phu thuc vo cc yu to nh khoang cch gia 2 C.O, mc lu lng cua mang hin tai v vo thi gian cua ngy. Nu hai ngi s dung c ni vt ly n cung C.O th cuc gi ch can thong qua mt tong i duy nht. nhng ni m hai thu bao c noi n cc trung tm in thoai noi hat khc nhau v hai tong i lp 5 c noi n mt tong i lp 4 th trung tm ng di o co th thc hin cuoc noi.
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Khi cc C.O cch xa nhau th s dung cc tong i khc, mc du cc tong i lp 5, 4 hoc 3 khong cn luon luon noi qua cp chuyn mach cao hn tip tho. Mot chuyn mach ln co th tao ra tt c cc chc nng chuyn mach ln thp hn. V du mt tong lp 5 co th c p ng bng mt tong i cua lp 4, 3, 2 hoc 1. Hnh II.1 ch ra cu truc tuyn chon cuoi (s cp) c bo sung bng mt cu truc nh tuyn lun phin. toi thiu hoa cc ti lu lng ln cc cp mang cao hn v s suy giam tn hiu khi tuyn gom nhiu trung k co s dung cao gia cc tong i ln bt ky, o hiu chnh mt cch kinh t. II.1.3 Bang thong va o rOng bang thong:
n n
m m

Trc khi phn tch yu cu tuyn dn ting ni cu con ngi, u tin t phi xc nh rng cu bng tn lin qun ng thu bo in thoi. T bit tn s" cu mt tn hiu tng t l s" cc song hnh Sin hon chnh c gi i trong mi giy v c o bng s" chu ky trn giy. Bng thong cu mt knh l khong tn s" c th truyn knh o. rng bng tn n thun l rng bng thong. Ting ni cu con ngi c th to r nhng m trong bng thong khong 5 n 15. Hz (15 kHz) vi rng bng tn 14,95Khz. Ti ngi c th ngh c cc m thnh nm trong bng thong 2 Hz-2.Hz ( rng bng tn l 19,98Khz) Bng thong cu ng thu bo ni ht khong t 3Hz-3.4Hz. iu ny c th lm ngc nhin nu coi rng ting ni cu con ngi to nn cc m thnh gi 5Hz- 15.iHz Trong thc t, ng thu bo khng phi dnh mng chon tn hiu tng t bt ky no m c toi u cho ting ni cu con ngi nm trong bng thong khong 2Hz-35Hz. y l khong tn s" ch phn ln cng sut, nh vy bng thong 3Hz-3.4Hz l thch hp truyn ting ni cu con ngi c cht lng. Ly do chu yu mng in thoi s dung bng tn 3,1Khz hp thch hp hn so vi ton bo bng tn ting noi 13Khz l v bng hp cho php nhiu cuc m thoi c truyn i mt knh vt ly duy nht. y l mt vn thc t qun trong cho cc trung k ni cc tong i chuyn mch in thoi. Cc bo loc v cc cun dy phu ti trong mng s ct cc tn hiu ting noi di 300Hz3.400Hz trn cuoc no! con kh nng truyn cc tn so" co hn nhiu.