X [dB] + Y [dB] = Z [dB]; X [dB] - Y [dB] = Z [dB]

X [dBm] + Y [dB] = Z [dBm]; X [dBm] - Y [dB] = Z [ dBm]

X [dBm] + Y [dBm] = Z [ ]; X [dBm] - Y [dBm] = Z [dB]

dBm l decibell tnh so vi mt miliwatt, con c goi l [di-bi-em]

Tuong t? v?y, dBi d?c l [di-bi-ai], l tang ch (hay d? l?i) c?a m?t ang-ten so v?i ang-ten d?ng hu?ng (isotropic).

dBd, d?c l [di-bi-di], l tang ch c?a m?t ang-ten so v?i m?t ang-ten lu?ng c?c (ch? d sau dB ?y l vi?t t?t c?a t? dipole, nghia l ang-ten lu?ng c?c) n?a sng (half wave dipole).

C? dBi l?n dBd d?u cng d? ch? tang ch (G) c?a ang-ten, v?n d? l so v?i ang-ten no, d?ng hu?ng hay lu?ng c?c n?a sng.

G [dBi] = G [dBd] + 2.15 [dB].

http://vntelecom.org/diendan/showthread.php?t=2145

n V Thng Dng Trong ngnh Vin thng (dB, dBm, dBw dBd, dBi, dBc)1. dBm, dBwdBm l n v cng sut thun ty (khng so snh). Cch tnh 1dBm=10log(PmW/1mW)V d: Cng sut pht ca thit b l 10W, biu din dng dBm l 10lg(10000/1)=40dBm.Tng t, dBW cng l n v cng sut thun ty, nhng c chuyn i t W sang. Vd: 1W > 10lg(1) = 0dBW; 2W = 3dBW.2. dBi, dBddBi v dBd u l cc n v biu din li cng sut (power gain) ca antenna, nhng c tham chiu khc nhau.dBi so snh power gain ca antenna vi antenna ng hng (omni-antenna/isotropic antenna: antenna im c cng sut pht i cc hng bng nhau)dBd so snh power gain ca antenna vi antenna lng cc i xng (half-wave dipole)0dBd = 2.15dBiXdBd = (X + 2.15)dBi

Anten 3G (UMTS) thng c li khong 18dBi (dual: 2 x 18dBi)3. dBdB l n v so snh v mnh (intensity), cng sut (power).i vi in p (V), dng (I) v trng E (in trng, t trng), cng thc tnh l 20lgX (dB)i vi cng sut (P), li (G), cng thc tnh l 10lgX (dB)Vd: Cng sut A l XW tng ng XdBm, Cng sut B l YW tng ng YdBm. Khi so snh A ln hn (nh hn) B bao nhiu dB, tnh bng 10g(X/Y)dB hoc (X-Y)dB.Antenna A c li 20dBd, B l 14dBd, vy A c li ln hn B 6dB4. dBcdBc c phng php tnh ging vi dB v cng l mt n v tng i, c lin h n mt i lng khc. dBc thng c dng m t kh nng ca cc RF components, vd: carrier power c m t bi mc interference, coupling, scattering u dng dBc, c th thay bng dB. dBc t c dng n.----------------------------------+ dB l deci Bell (vi Bell l tn ngi do n c vit hoa)+ dB th n l 10lg(P2/P1).+ dBm th lc ny l 10lg(P/1mW).mi quan h gia dB v dBm l nh sau:

X[dB] +Y [dB] = Z[dB]X[dB]- Y[dB] = Z[dB]X[dBm] + Y[dB] = Z[dBm]X[dBm] - Y[dB] = Z[dBm]X[dBm]- Y[dBm] = Z[dB]X[dBm] + Y[dBm] = Z []

+ Vi dBi th i l vit tt ca t ng hng (isotropic). N l n v ca h s tng ch ca anten pht x ng hng. Ngoi ra vi anten pht x c hng th n v ca h s tng ch l dBd. V trong lc tnh ton th dBi c coi nh dB.------------------------------------------------ chuyn gia cc i lng tnh theo dB, dBm, ... bn nn ch l cc gi tr u l gi tr so snh theo 1 i lng chun (reference), v d dB ly chun l 1W, dBm ly chun l 1mW. V vy chuyn i qua li th cch d dng nht l chuyn v dng tuyn tnh ri i ngc li. V d 1.5dB = ? dBm. Ta chuyn 1.5dB sang W bng 10^(1.5/10) = 1.4125W, i sang mW c 1412.5 mW, i ngc li dBm 10*log(1412.5) = 31.5 dBm

Cch lm nh vy tuy di nhng khng s sai, iu ct yu l bn hiu c nhng ci reference khc nhau (tu mi lnh vc ngi ta s chn ra reference tng ng)

Nhng bn cng c th lm cch khc nhanh hn l da vo tng quan ca cc reference v tnh cht ca vic ly logarithm, cc bi ton nhn chia trong h tuyn tnh tr thnh bi ton cng tr trong h logarithm. V d A = B*1000 trong h tuyn tnh th trong h logarithm log(A) = log(B) + log(1000) nu tnh theo dB th 10log(A) = 10log(B) + 10log(1000) = 10log(B) + 30.Tr li bi ton trn reference ca dB l 1W, ca dBm l 1mW cho ta thy khi mun chuyn t dB sang dBm bn ch cn cng thm 30.V vy khi bn hiu tng quan gia cc reference th khng cn i sang h tuyn tnh na m c th trc tip lm trn h log.

1 ch nh l dB cn c dng th hin tng quan gia 2 i lng cng sut A, B ch khng nht thit gia A vi 1. Lc ny n ch th hin tng quan t l ch khng c n v. V d A = 4mW, B = 2mW -> ta ni cng sut A gp 2 ln B hoc A ln hn B 3dB -> 3dB khng c th nguyn.

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