sổ tay cdt chuong 17-time v f

8/6/2019 s tay cdt Chuong 17-time v f

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17C s thi gian v tn s

Michael A. Lombardi National Institute of Standards andTechnology

17.1 Gii thiu ........................................................... .1

17.2 Cc php o thi gian v tn s ..........................2

17.3 Chun thi gian v tn s ....................... ........ ....7

17.4 Truyn thi gian v tn s ........................ ........11

17.5 Kt lun .............................................................14

17.1 Gii thiuCc chun thi gian v tn s cung cp ba dng thng tin c bn: thi gian, khong thi gian, v tn s. Thng tin thi

gian c chia thnh gi, pht, giy, nhng cng thng bao gm c ngy thng (thng, ngy v nm). Mt thit b hin thhoc ghi thng tin thi gian c gi l ng h. Nu dng mt ng h nh du thi im s kin din ra th thi im i khi c gi l nhn thi gian hoc tem thi gian. C th dng ngy thng v thi gian m bo cc s kin ngb, hoc xy ra ti mt thi im.

Khong thi gian l mt qung thi gian tri gia hai s kin. n v chun ca khong thi gian l giy (s). Tuy nhin,nhiu ng dng k thut cn vic o lng khong thi gian ngn hn, nh mili giy (1 ms = 10 -3 s), micr giy (1 sm = 10-6),nan giy (1 ns = 10-9), v pic giy (1 ps = 10-12). Thi gian l mt trong by i lng vt l c bn, v giy l mt trong

by n v c bn c nh ngha trong H n v quc t (SI). Cc nh ngha v cc i lng vt l khc da trn nhngha v giy. Giy tng c nh ngha da trn tc quay ca tri t hoc theo nm mt tri. Sau thay i vonm 1967 khi k nguyn nguyn t chnh thc bt u. nh ngha hin nay ca giy theo h n vi quc t SI l:

Khong thi gian ca 9.192.631.770 chu k pht x tng ng vi dch chuyn gia hai cp trng thi c bn ca nguynt xezi 133.

Tn s l tc lp li ca mt s kin. Nu Tl chu k lp li ca mt s kin, th tn s fl nghch o ca n, 1/T.Ngc li, chu k l nghch o ca tn s, T= 1/f. Do chu k l mt khong thi gian biu din bng giy (s), c th d dngthy mt quan h cht ch gia khong thi gian v tn s. n v chun cho tn s l Hc (Hz), c nh ngha l s skin hay chu trnh trong 1 giy. Tn s ca tn hiu in o c thng nhiu Hc, bao gm kil Hc (kHz), mga Hc(MHz), hoc giga Hc (GHz), y 1 kHz bng mt nghn (10 3) s kin trn mt giy, 1 MHz bng mt triu (10 6) s kintrn mt giy, v 1 GHz bng mt t (109) s kin trn mt giy. Thit b to tn s gi l my to dao ng (oscillator). Qutrnh thit lp cc my to dao ng v cng mt tn s gi l s iu hng.

BNG 17.1 Cc sai s thc hin vt l ca cc n v SI c bn

n v c bn SI Lng vt l Sai s

Canla Cng sng 41 10-

Kenvin Nhit 73 10-

Mol Tng s phn t 88 10-

Ampe Dng in 84 10-

Kilgam Khi lng 81 10-

Mt di 121 10-

Giy Khong thi gian 151 10-

Nh vy, ba dng thng tin thi gian, khong thi gian v tn s lin quan mt thit vi nhau. Nh cp, n vchun ca khong thi gian l giy. Bng cch tnh giy, chng ta c th xc nh c ngy thng v thi gian trong ngy.V bng cch tnh s s kin hay chu trnh trong mt giy, chng ta c th tnh c tn s.

Hin nay, khong thi gian v tn s c th o vi t sai s v c phn gii cao hn bt k mt i lng vt l nokhc. Chun thi gian v tn s tt nht thu c trong giy SI c sai s c 121 10- . Vic o lng vt l cc n v SI c

bn khc c sai s ln hn kh nhiu, xem bng 17.1 [1-5].

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S tay C in t

Thi gian quc t c ta ha (Coordinated Universal Time UTC)

Cc phng th nghim o lng chnh trn th gii u n o cc chun thi gian v tn s ca h v gi d liu o tiBurea International des Poids et Measures (BIPM) ti Sevres, Php. BIPM ly trung bnh d liu thu c t hn 200 chuntn s v thi gian nguyn t t ti hn 40 phng th nghim, bao gm c Vin tiu chun v cng ngh quc gia M(NIST). Vi kt qu ca vic ly trung bnh ny, BIPM to ra hai thc o thi gian, thi gian nguyn t quc t(International Atomic Time TAI) v thi gian quc t c ta ha (Coordinated Universal Time UTC). Cc thc othi gian ny cng gn giy SI cng tt.

UTC c tn s ging TAI. Tuy nhin, UTC khc TAI s giy tch phn. S khc nhau ny tng khi xut hin cc giynhun. Khi cn thit, cc giy nhun c cng vo UTC trong ngy 30 thng 6 hoc 31 thng 12. Mc ch ca vic thm

cc giy nhun l gi thi gian nguyn t (UTC) trong khong 0,9s ca mt thc o thi gian c hn gi l UT1 da trn

tc quay ca tri t. Cc giy nhun c thm vo UTC vi t l nh hn mt ln trong nm, bt u t nm 1972[3,5].

Nh rng BIPM gi TAI v UTC nh l cc thc o thi gian giy t. Cc phng th nghim o lng chnh dng ccd liu cng khai t BIPM thay i ng h v b to dao ng ca h v to ra cc phin bn thi gian thc ca UTC.Rt nhiu phng th nghim ny phn pht cc phin bn UTC ca h qua tn hiu radio, s c m t trong phn 17.4.

Bn c th coi UTC l chun c bn cho thi gian trong ngy, khong thi gian v tn s. Cc ng h c ng b haUTC cho gi, pht, v giy ging nhau trn ton th gii (v gi trong vng 1 giy ca UT1). Cc b to dao ng c iuhng vi UTC to cc tn hiu dng nh cc chun tham chiu cho khong thi gian v tn s.

17.2 Cc php o thi gian v tn s

Cc php o thi gian v tn s l cc php thng thng c dng trong cc h thng o lng. Chun tn s hocng h c o bi thit b c kim tra (device under test - DUT). Php o so snh DUT vi mt chun hoc tham chiu.Chun c th lm tt hn DUT bng mt t s ring, gi l t s sai s kim tra (TUR). L tng, TUR c th l 10:1 hoccao hn. T s cng cao, cng cn t php o trung bnh c c cc kt qu o c gi tr.

Tn hiu kim tra cho cc php o thi gian thng l mt xung xut hin mt ln trong mt giy (1 pps). rng v ccxung thay i theo thit b, nhng thng cng cc mc TTL. Tn hiu kim tra cc php o tn s thng l tn s 1 MHzhoc cao hn, thng thng l 5 hoc 10 MHz. Cc tn hiu tn s thng l cc sng hnh sin, nhng cng c th l ccxung hoc sng vung. Nu tn hiu tn s l mt tn hiu hnh sin, n c th ging nh trn hnh 17.1. Tn hiu ny sinh ramt vng (pha 360o hoc 2p raian) trong mt chu k. Bin tn hiu c biu din l vn, v phi tng thch theo thit

b o. Nu bin qu nh, thit b o c th khng o c, nu bin qu ln, tn hiu phi c ngt trnh gy hi

cho thit b o.Phn ny s xem xt hai c tnh chnh ca cc php o thi gian v tn s l chnh xc v tnh n nh. Ngoi ra cn

nu mt s thit b dng o thi gian v tn s.

HNH 17.1 Mt sng dao ng hnh sin

HNH 17.2 o lng s dng b m khong thi gian

chnh xc2


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C s ca thi gian v tn s

chnh xc l mc ng ca mt gi tr o hoc gi tr tnh c so vi nh ngha ca n. chnh xc lin quanvi gi tr b t gi tr l tng. V d, thi gian b l sai khc gia xung thi gian ng o c v xung thi gian ng ltng trng chnh xc vi UTC. Tn s b l sai khc gia tn s o c v tn s l tng vi sai s bng khng. Tn s ltng ny c gi l tn s danh ngha.

Thi gian b thng c o bng mt b m khong thi gian (time interval counter TIC), nh thy trn hnh 17.2.Mt TIC c cc u vo cho hai tn hiu. Mt tn hiu khi ng b m v mt tn hiu dng b m. Khong thi gian giatn hiu khi ng v kt thc c o bng cch m cc chu trnh t b to dao ng thi gian c bn. phn gii ca

TIC gi thp hn ch chu k thi gian c bn ca n. V d, mt TIC vi b to dao ng thi gian c bn 10 MHz s c phn gii 100 ns. Cc TIC tinh vi hn dng cc b ni suy pht hin cc phn ca chu trnh thi gian c bn v cho phn gii cao hn, bnh thng l 1ns v c c 20 ps.

C th o tn s b trong min tn shoc min thi gian. Mt php o trong min tn s n gin gm vic tnh ton vhin th trc tip u ra tn s ca DUT vi mt b m tn s. Tham chiu cho php o ny l b to dao ng thi gian c

bn bn trong hoc bn ngoi b m (Hnh 17.3). phn gii ca b m, hoc s ch s n c th hin th, gii hn khnng o tn s b ca n. V d, mt b m tn s 9 ch s c th pht hin mt tn s b khng nh hn 0.1 Hz ti 10MHz ( 81 10- ). Tn s b c xc nh nh sau:

( ) measured nominal

nominal

f ff offset

f

-=

y measuredf (fo c) l tn s c c t b m tn s, v nominalf (fdanh ngha) l tn s ghi trn nhn ca b to dao ng,

hay gi l tn s ring ca u ra.

HNH 17.3 o lng s dng b m tn s

HNH 17.4 So snh pha s dng my hin sng

Cc php o tn s b trong min thi gian i hi phi so pha gia DUT v tn hiu tham chiu. Vic so pha n gin c

th c thc hin bi mt my hin sng (Hnh 17.4). My hin sng s hin th hai sng hnh sin (Hnh 17.5). Sng hnhsin bn trn biu din tn hiu t DUT, v sng hnh sin bn di biu din tn hiu tham chiu. Nu hai tn s ny ging htnhau, mi quan h pha ca chng s khng thay i v c hai s xut hin n nh trn mn hnh hin sng. Khi hai tn skhng tht s bng nhau, tn hiu tham chiu s xut hin n nh v tn hiu DUT dch chuyn. Bng cch o tc dchchuyn ca tn hiu DUT, chng ta c th xc nh tn s b ca n. Cc ng thng ng c v t cc im ti misng hnh sin chuyn qua im khng. Phn di ca hnh v cho thy cc thanh vi rng ca n biu din lch phagia cc tn hiu. Trong trng hp ny, lch pha s tng, biu th rng DUT c tn s thp hn tham chiu.

Vic o cc tn hiu c chnh xc cao vi my hin sng l khng thc t do mi quan h pha gia cc tn hiu thayi rt chm v phn gii ca my hin sng b gii hn. Cc so snh pha chnh xc hn c th c to vi mt TIC,dng mt b tng t nh hnh 17.2. Nu hai tn hiu u vo c tn s ging nhau, khong thi gian s khng thay i. Nuhai tn hiu c tn s khc nhau, khong thi gian s thay i , v tc thay i l tn s b. phn gii ca mt TIC xcnh thay i tn s nh nht m n c th pht hin khi khng ly trung bnh. V d, mt TIC gi r vi phn gii 100 ns

c th pht hin c s thay i tn s l

7

1 10

-

trong 1 s. Gii hn hin nay cho phn gii TIC l 20 ps, c ngha l cth c pht hin c 112 10- ln thay i tn s trong 1 s. Vic ly trung bnh cc khong di hn c th ci thin phn gii n


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S tay C in t

HNH 17.5 Hai sng hnh sin vi mi quan h pha thay i

Do cc tn s chun nh 5 hoc 10 MHz khng kh thi o vi mt TIC, cc b chia tn s(thy trn hnh 17.2) hocb trn tn sc dng bin i tn s kim tra thnh tn s thp hn. Cc h thng chia n gin hn v linh hot hn,

do chng c th d dng c xy dng hoc lp trnh cung cp cc tn s khc nhau. Cc h thng trn tn s t hn,cn nhiu phn cng vi mt b to dao ng tham chiu b sung, v thng ch o c mt tn s u vo (v d 10MHz), nhng chng c t s tn hiu-nhiu ln hn cc h thng chia.

Nu dng cc b chia, cc php o c thc hin t TIC, nhng thay v dng cc trc tip cc php o ny, chng ta xcnh t l thay i t c sang c. T l thay i ny c gi l lch pha. Chng ta c th xc nh tn s b nh sau:

( )t

f offsetT

- D=

y tD l tng lch pha, v Tl chu k o.

thy r, hy xt mt php o lch pha +1 sm trong mt khong thi gian o l 24h. n v dng cho khong thi giano (h) phi c chuyn i sang n v dng cho lch pha ( sm ). Phng trnh tr thnh

( ) 111

1,16 1086.400.000.000

stf offset

T s

m

m-

-- D= = = -

Nh thy, mt thit b tch ly 1 sm lch pha/ngy c tn s b l 111,16 10-- tng ng vi tham chiu. V dn gin ny ch cn c hai khong thi gian, v tD n gin l hiu gia hai ln c. Thng thng, ta ly nhiu ln cv tn s b c c lng bng cch dng hi quy tuyn tnh bnh phng b nht trn tp d liu, v tD thu c t dc ca ng bnh phng b nht. Thng tin ny c biu din di dng th pha, nh thy trn hnh 17.6. Thit bdi kim tra tn s cao chnh xc c 91 10- , c lch pha 1 ns/s [2,7,8].

HNH 17.6 th pha mu

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HNH 17.7 Mi quan h gia chnh xc v n nh

Cc gi tr tn s b khng th nguyn c th c chuyn sang n v tn s (Hz) nu bit tn s danh ngha. minhha cho vic ny, hy xt mt b to dao ng vi tn s danh ngha 5 MHz v tn s b 111,16 10- . tm tn s b dng Hc, ta nhn tn s danh ngha vi tn s b:

( ) ( )6 11 55 10 1,16 10 5,80 10 0,0000580Hz- - + = = +

Sau , cng vi tn s danh ngha c tn s thc:

5.000.000 0,0000580 5.000.000,0000580Hz Hz Hz + =

Tnh n nh

Tnh n nh biu th kh nng mt b to dao ng c th sinh ra tn s hay thi gian b ging nhau trn mt khongthi gian cho trc. Khng th ni rng thi gian hoc tn s l ng hay sai, m ch c th ni n cho kt qu gingnhau. Ngc li, chnh xc biu th mc b to dao ng t ng thi gian hay ng tn s. hiu s khc nhauny, hy xt mt b to dao ng n nh cn iu chnh c th sinh ra mt tn s b ln. Hoc, mt b to dao ngkhng n nh va c iu chnh tm thi c th sinh ra mt tn s gn gi tr danh ngha ca n. Hnh 17.7 cho thymi quan h gia chnh xc v tnh n nh.

Tnh n nh c nh ngha l c lng thng k dao ng thi gian hoc tn s ca mt tn hiu trn mt khong thigian cho trc. Nhng dao ng ny c o kt hp vi thi gian b hoc tn s trung bnh. Tnh n nh ngn hn thngch cc dao ng trong cc khong nh hn 100 s. Tnh n nh di hn c th ch cc khong o lng ln hn 100 s, nhngthng ch cc chu k di hn 1 ngy.

Cc c lng tnh n nh c th c tnh trong min tn s hoc trong min thi gian, v c th c tnh t mt tptn s b hoc cc php o khong thi gian. Trong mt s lnh vc o lng, tnh n nh c xc nh bng cch ly lch chun ca tp d liu. Tuy nhin, lch chun ch thc hin vi d liu n nh m cc kt qu c lp v thigian, v nhiu l trng, c ngha l n phn b u dc theo di tn s o. D liu b to dao ng thng khng n nh, don cha nhiu c lp v thi gian phn b bi dch tn s. Vi d liu n nh, lch chun v trung bnh s hi t n ccgi tr ring khi thc hin nhiu php o hn. Vi d liu khng n nh, lch chun v trung bnh khng bao gi hi tn bt c gi tr ring no. Thay vo , c mt trung bnh ng thay i mi ln chng ta thm mt php o.

Vi nhng nguyn nhn ny, php thng k khng truyn thng thng c dng c lng tnh n nh trong minthi gian. Thng k ny thnh thong c gi l Allan phng sai, nhng do n l cn bc hai ca phng sai, tn thch hpca thng k ny l lch Allan. Phng trnh cho lch Allan ( ( )ys t ) l

( )

( )1

2

1

1

1( )

2 1

M

y i i

i

y yM

s t-

+

=

= -

-

yyi l tp cc o lng tn s b bao gmy1, y2, y3,..., Ml s gi tr trong chui yi, v d liu c t u trong ccon t giy. Hoc

( )( )

22

2 121

1( ) 2

2 2

N

y i i i

i

x x x N

s tt

-

+ +

=

= - +-

y xi l tp cc o lng pha trong n v thi gian bao gm x1, x2, x3,..., Nl s gi tr ca chui xi, v d liu l cckhong t giy u nhau tng ng . Ch rng, trong khi lch chun tr i trung bnh t mi php o trc khi bnh

phng tng ca chng th lch Allan tr i im d liu trc. S sai khc gia cc im d liu lin tc loi b nhiuph thuc thi gian gy ra bi tn s b.

Mt th lch Allan c thy trn hnh 17.8. N cho thy tnh n nh ca thit b tng ln khi chu k trung bnh ( t) tng, do mt s dng nhiu c th loi b bng cch ly trung bnh. Tuy nhin, ti mt s im, vic ly nhiu trung bnhhn khng ci thin c kt qu. im ny gi l sn nhiu, hay im m nhiu cn li cha cc qu trnh khng n

nh nh nhiu rung hoc bc ngu nhin. Thit b o trn hnh 17.8 c sn nhiu 115 10-: ti 100st = .

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S tay C in t

HNH 17.8 th n nh tn s

Thc t, mt th n nh tn s cng cho chng ta bit thi gian cn ly trung bnh nhm loi b nhiu do h thngtham chiu v o. Sn nhiu cng cung cp mt s du hiu tng trung bnh cn thu c mt TUR ln v gip ta cc tn s b thc ca DUT. Nu DUT l mt b to dao ng nguyn t (phn 17.4) v tham chiu l mt sng v tuyn

c iu khin bi chun truyn (phn 17.5) chng ta phi ly trung bnh trong 24 h hoc lu hn c kt qu o tin cy.Nm dng nhiu thng c ni n trong cc ti liu tn s v thi gian l: nhiu trng, pha rung, tn s trng, tn srung, v tn s bc ngu nhin. dc ca ng lch Allan c th gip vic xc nh tng trung bnh cn loi bcc nhiu ny (Hnh 17.9). Loi nhiu th nht cn loi b bng cch ly trung bnh l nhiu pha, hoc dao ng ngu nhin,nhanh trong pha tn s. V l tng, ch thit b di kim tra mi thm nhiu pha vo kt qu o, nhng thc t, mt snhiu pha t h thng o v tham chiu cn phi loi b thng qua vic ly trung bnh. Ch rng lch Allan khng phn

bit nhiu pha trng v nhiu pha rung. Bng 17.2 cho thy mt vi thng k khc dng c lng tnh n nh v xcnh cc loi nhiu cho cc ng dng khc nhau.

BNG 17.2 Thng k dng c lng tnh n nh v cc dng nhiu thi gian v tn s

Tn K hiu ton hc M t

lch Allan ( )gs t c lng tnh n nh tn s. c bit thch hp cho

cc o lng trung bnh n di hn. lch Allan sai

MOD ( )gs t c lng tnh n nh tn s. Khng ging nh lch Allan thng thng, n c th khc nhau gianhiu pha trng v chp chn, lm cho n thch hp hncho cc c lng tnh n nh ngn hn.

lch thi gian ( )xs t c dng o tnh n nh thi gian. Nhn dng rrng c nhiu pha trng ln chp chn, cc dng nhiucn quan tm nht khi o thi gian hoc pha.

lch tng ( ),TOTALgs t c lng tnh n nh tn s. c bit thch hp chocc c lng di hn khi t vt qu 10% mu d liutng cng.

HNH 17.9 S dng th n nh tn s nhn dng dng nhiu

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Vic xc nh v loi tr ngun nhiu ca b to dao ng c th l mt vn phc tp, nhng vic v cc sai phn bcnht ca tp cc o lng min thi gian c th to ra mt c s ca phng php loi b nhiu bng cch ly trung bnh.Hnh 17.10 c to ra bng cch dng mt on d liu t th n nh trong hnh 17.8. N cho thy th pha chi phi

bi nhiu pha trng (1 s trung bnh), nhiu tn s trng (64 s trung bnh), nhiu tn s rung (256 s trung bnh), v tn s bcngu nhin (1024 s trung bnh). Ch rng th nhiu pha trng c t l 2 ns, v cc th khc dng t l 100 ps [8-12].

HNH 17.10 Cc th pha ca bn dng nhiu

17.3 Chun thi gian v tn s

Tt c cc chun thi gian v tn s u da trn mt s kin tun hon lp li vi mt t l khng i. Thit b sinh ra skin ny c gi l b cng hng. Trong trng hp n gin l ng h qu lc, qu lc l b cng hng. D nhin, mt

b cng hng cn mt ngun nng lng trc khi n c th chuyn ng qua li. Ngun nng lng v b cng hng to

thnh mt b to dao ng. B to dao ng chy theo mt t l c gi l tn s cng hng. V d, qu lc ng h c thc t quay i quay li vi t l mt ln trong mt giy. Vic m s ln qu lc dao ng hon ton sinh ra khong thigian 1 s. Vic m tng s ln dao ng to ra mt thc o thi gian trong cc khong thi gian di hn, nh pht, gi, vngy. Thit b thc hin vic tnh v hin th hoc ghi li cc kt qu c gi l ng h. Bng 17.3 cho thy sai s tn sca b cng hng ng h tng ng vi sai s thi gian ca ng h.

Trong lch s, cc nh thit k ng h tm ra nhiu b cng hng n nh hn, v s thay i ca cc chun thi gianv tn s c tng hp trong bng 17.4. Cc sai s c lit k theo cc chun mi nht i din cho cc thit b hin nay(nm 2001), v khng ging cc nguyn mu ban u. Ch rng cc chun thi gian v tn s ci thin 13 bc bin sovi 700 nm trc v khong 9 bc bin so vi 100 nm trc.

Tnh n nh ca cc chun thi gian v tn s lin quan mt thit vi h s cht lng ca chng, Q. Q ca mt b todao ng l tn s cng hng ca n chia cho rng di cng hng. Tn s cng hng l tn s ring ca b to daong. rng di cng hng l khong tn s c th m b to dao ng s dao ng. Mt b cng hng Q cao skhng dao ng tr khi gn tn s cng hng ca n. Hin nhin, mt tn s cng hng cao v di rng cng hng

hp u l li th khi tm mt Q cao. Ni chung, Q cng cao, b to dao ng cng n nh, v mt Q cao c ngha l b todao ng s nm gn tn s cng hng ring ca n.

BNG 17.3Mi quan h ca sai s tn s v sai s thi gianSai s tn s Khong o lng Sai s tn s

31,00 10- 1 s 1ms

61,00 10- 1 s 1 sm

91,00 10- 1 s 1ns

72,78 10- 1 h 1ms

102,78 10- 1 h 1 sm

132,78 10- 1 h 1ns

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S tay C in t

81,16 10- 1 ngy 1ms

111,16 10- 1 ngy 1 sm

141,16 10- 1 ngy 1ns

BNG 17.4 S tin ha ca cc chun thi gian v tn s

Chun B cng hng Thi im Sai s thigian (24 h)

Sai s tn s(24 h)

ng h mt tri Chuyn ng hin nhinca mt tri

3500 trcCN

Khng bit Khng bit

Con nga dngthanh

C cu dng thanh vfoliet

Th k 14 15 pht 21 10-

Qu lc Qu lc 1656 10s 41 10-

ng h bm giHarrison

L xo v bnh xe cnbng

1759 350 ms 64 10-

Qu lc Shortt Hai l xo, t v ch 1921 10 ms 71 10-

Tinh th thch anh Tinh th thch anh 1927 10 sm 101 10-

T bo kh Rubii Cng hng 87Rb(6834682608 Hz)

1958 100 ns 121 10-

Tn hiu Xezi Cng hng 133Cs(9192631770 Hz)

1952 1 ns 141 10-

Maze Hyr Cng hng Hyr(1420405752 Hz)

1960 1 ns 141 10-

Ngun Xezi Cng hng 133Cs(9192631770 Hz)

1991 100 ps 151 10-

Phn ny s trnh by v b to dao ng thch anh, thit b thu c Q cao nht trong tt c cc thit b c kh. Sau stho lun v cc b to dao ng c h s Q cao hn, da trn s cng hng ca nguyn t rubii v xezi. Cc b to daong nguyn t dng cc mc nng lng lng t ha trong nguyn t v phn t nh l ngun cng hng ca chng. Ccqui lut ca c cu lng t pht biu rng nng lng ca mt h kn, nh mt nguyn t, c cc gi tr ri rc nht nh no. Mt trng in t ti mt tn s ring c th y mt nguyn t t mt mc nng lng ny ln mt mc cao hn.Hoc, mt nguyn t mc nng lng cao c th chuyn xung mc nng lng thp hn bng cch gii phng nnglng. Tn s cng hng (f) ca mt b to dao ng nguyn t l hiu gia hai mc nng lng chia cho hng s Planck(h):

2 1E Efh

-=

Nguyn l c bn ca b to dao ng nguyn t l khi tt c nguyn t ca mt cht no ng nht, chng c th sinhra chnh xc mt tn s ging nhau khi chng hp th hay gii phng nng lng. V l thuyt, nguyn t l mt con lc

hon ho m vic m cc dao ng ca n dng o khong thi gian. Vic tho lun cc b to dao ng nguyn t chgii hn cc thit b c sn trn th trng, v loi tr cc chun s cp v thc nghim c tm trong cc phng thnghim nh NIST. Bng 17.5 s a ra mt tng kt [1,4,8].

Cc b to dao ng thch anh

Cc b to dao ng tinh th thch anh l cc chun thi gian v tn s thng dng nht. Khong hai t ( 92 10 ) b todao ng thch anh c sn xut hng nm. Hu ht l cc thit b nh dng cho ng h eo tay, ng h, v cc mchin t. Tuy nhin, chng cng c trong cc thit b o lng v kim tra, nh b m, b to tn hiu, my hin sng; v ttnhin trong tt c b to dao ng nguyn t.

BNG 17.5 Tng kt cc dng b dao ng

Dng bdao ng Thch anh(TCXO) Thch anh(OCXO) Rubii Tia Xezithng mi Maze Hyr

Q 104 n 106 63,2 10 107 108 109

8


9/14


Dng bdao ng

Thch anh(TCXO)

Thch anh(OCXO)

Rubii Tia Xezithng mi

Maze Hyr

Tn s cnghng

Khc nhau Khc nhau 6,834682608GHz

9,192631770GHz

1,420405752GHz

Nguynnhn chnh

gy hng

Khng Khng n rubii(Thi gian

mong i >15nm)

ng tia Xezi(Thi gian

mong i >3n 15 nm)

X ht hyr(Thi gian

mong i >7nm)

n nh,( )

gs t , t = 1s

81 10- n

91 10-

121 10- 115 10- n125 10-

115 10- n125 10-

121 10-

Sn nhiu( )

gs t

91 10- ( t = 1 n 102

s)

121 10- ( t = 1 n 102

s)

121 10- ( t =103 n 105 s)

141 10- ( t =105 n 107 s)

151 10- ( t =103 n 105 s)

S hagi/nm

75 10- 95 10- 101 10- Khng 131 10-:

Dch tn ssau khi khing

6

1 10-

8

1 10-

n101 10-

11

5 10-

n125 10-

12

5 10-

n145 10-

12

1 10-

n131 10-

Chu k khing


10/14

S tay C in t

HNH 17.11 B to dao ng Rubii

Do s lo ha v cc nh hng mi trng nh nhit v rung ng nn kh gi cc b dao ng thch anh tt nhttrong 101 10- tn s danh ngha ca chng khi khng c s iu chnh c nh. Vi l do ny, cn cc b to dao ngnguyn t dng cho cc ng dng cn chnh xc v tnh n nh di hn tt hn [4,13,14].

B to dao ng rubii

Cc b to dao ng rubii l b c gi thp nht trong h cc b to dao ng dng nguyn t. Chng hot ng ti6.834.682.608 Hz, tn s cng hng ca nguyn t rubii (87Rb), v dng tn s rubii iu khin tn s ca b to daong thch anh. Mt tn hiu vi sng pht ra t b to dao ng thch anh c dng cho hi 87Rb trong mt nh, anguyn t n mt trng thi nng lng ring. Sau , mt tia sng c bm vo trong nh v c hp th bi ccnguyn t a chng n mt trng thi nng lng ring. Mt b t bo quang tch sng o s lng tia sng b hp th, vu ra ca n c dng iu chnh b to dao ng thch anh ti mt tn s sao cho ti a ha tng s nh sng c hpth. Tip theo, b to dao ng thch anh c hm ti tn s cng hng ca rubii, v cc tn s chun c nhn t bto dao ng thch anh v c cung cp nh l u ra (hnh 17.11).

Cc b to dao ng rubii ngy cng nh hn v r hn, v c th c t s gi/kh nng tt nht trong cc b to daong. Tnh n nh di hn ca chng tt hn nhiu so vi b to dao ng thch anh v chng cng nh hn, tin cy hn, rhn b to dao ng xezi.

Q ca mt b to dao ng rubii l khong 107. Cc dch chuyn trong tn s cng hng phn ln do va chm ca cc

nguyn t rubii vi cc phn t kh khc. Nhng dch chuyn ny hn ch tnh n nh di hn. Tnh n nh ( ( )ys t , ti1st = ) thng l 111 10- , v khong 121 10- mt ngy. Tn s b ca mt b to dao ng rubii c khong t

105 10- n 125 10- sau khi chu k lm nng l mt vi pht hoc gi, nn b to dao ng ny p ng yu cu chnhxc ca hu ht cc ng dng m khng cn hiu chnh.

B to dao ng xezi

Cc b to dao ng xezi l cc chun tn s cn bn do giy SI c nh ngha t tn s cng hng ca nguyn txezi (133Cs), vi tn s 9.192.631.770 Hz. Mt b to dao ng xezi hot ng bnh thng s gn vi tn s danh ngha can m khng cn iu chnh, v s khng c thay i tn s do s lo ha.

HNH 17.12 B to dao ng dng tia Xezi

10


11/14


Cc b to dao ng sn c trn th trng dng cng ngh tia xezi. Bn trong b to dao ng xezi, cc nguyn t 133Csc nung nng thnh kh trong mt ci l. Cc nguyn t kh ri l trong cc tia tc cao chuyn ng trong ng chnkhng ti b nam chm. Cc nam chm hot ng nh l mt cng ch cho php cc nguyn t ca mt mc nng lngring i qua vo trong mt l hng vi sng, chng pht ra mt tn s vi sng thu c t b to dao ng thch anh. Nutn s vi sng ph hp vi tn s cng hng ca xezi, cc nguyn t xezi s thay i trng thi nng lng t ca chng.

Sau , tia nguyn t s i qua mt cng t khc gn pha cui ca ng. Nhng nguyn t b thay i trng thi nnglng ny khi i l hng vi sng s c php i n mt b tch sng pha cui ng. Cc nguyn t khng thay i trng

thi s b lm trch khi b tch sng . B tch sng sinh ra tn hiu phn hi ph hp mt cch lin tc vi b to dao ngthch anh theo cch ti a ha s ln thay i trng thi sao cho s nguyn t ln nht ti c b tch sng . Cc tn s ura chun thu c t b to dao ng thch anh b kha (hnh 17.12).

Q ca mt chun xezi thng mi l mt phn ca 108. ng tia thng c chiu di 100 m/s bn trong ng. iu ny hn ch thi gian quan st xung cn mt vi mili giy, v rng cng

hng xung cn mt vi trm hc. Tnh n nh ( ( )ys t , ti 1st = ) thng c 125 10- v dn ti sn nhiu gn 141 10-

trong khong 1 ngy, m rng n hng tun hoc thng. Tn s b thng gn 121 10- sau chu k lm nng 30 pht.

17.4 Truyn thi gian v tn s

Nhiu ng dng cn ng h hoc b to dao ng cc v tr khc nhau c t thi gian ging nhau (ng b ha)hoc tn s ging nhau (iu hng). Cc k thut truyn thi gian v tn sc dng so snh v iu chnh cc ng h

v b to dao ng ti cc v tr khc nhau. Truyn thi gian v tn s c th n gin l t ng h eo tay ca bn theomt tn hiu thi gian m thanh, hoc phc tp hn nh l iu khin tn s ca b to dao ng trong mt mng ti cc phnca 1013.

Truyn thi gian v tn s c th dng cc tn hiu pht thng qua nhiu phng tin khc nhau nh cp ng trc, cpquang, tn hiu sng v tuyn (ti nhiu vng trong ph sng v tuyn), ng in thoi, v Internet. Vic ng b ha cnc xung ng gi v m thi gian. Vic iu hng cn chnh xc mt tn s n nh t i pht. Tn s c th mang trnchnh sng mang hoc t m thi gian hay thng tin khc iu ch t sng mang.

Phn ny s tho lun v cc vn c bn ca c truyn thi gian v tn s v cc tn hiu sng v tuyn c dng nhl tham chiu chun. Cc vn ny c tng kt trong Bng 17.6.

C s ca truyn thi gian v tn s

Cc tn hiu dng cho truyn thi gian v tn s thng c tham chiu ti cc b to dao ng nguyn t c dn

cng ph hp vi UTC cng tt. Thng tin c gi t mt b pht (A) ti b nhn (B) v tr bi abt , thng c gi ltr ngdn (hnh 17.13).

minh ha tr ng dn, hy xem xt mt tn hiu sng v tuyn pht trn mt qung ng 1000 km. Do cc tn hiu

sng v tuyn truyn vi vn tc nh sng ( 3, 3 /s kmm: ), chng ta c th chun ng i bng vic a mt hiu chnh

3,3-ms vo php o. D nhin, vi nhiu ng dng c th loi b tr ng i mt cch n gin. V d, nu mc ch cachng ta n gin l ng b ha ng h my tnh trong 1 s vi UTC th s khng cn bn tm v mt tr ng dn 100-ms trong mng. V d nhin, tr ng dn khng quan trng trong h thng truyn tn s, do khng cn xung ng gi.Thay vo , truyn tn s ch cn ng dn n nh trong cc tr gi s khng i tng i.

BNG 17.6 Tng kt cc tn hiu v phng php truyn thi gian v tn s

Tn hiu hoc lin kt Thit b nhn Sai s thi gian(24 h)

Sai s tn s (24 h)

Dch v thi gian mytnh quay s

My tnh, phn mm khch th,modem, v ng in thoi


12/14

S tay C in t

HNH 17.13 Truyn thi gian v tn s mt ng

Cc h thng truyn phc tp hn c lng v loi b tt c hoc tng phn tr ng dn. N thng c lm bngmt trong hai cch sau. Cch u tin l c lng abt v gi sm mt khong thi gian ny. V d, nu abt t nht l 20 mscho tt c ngi dng, c th gi trc 20 ms. u im ny ca tn hiu thi gian s loi b t nht mt s tr cho tt c ngidng.

Mt k thut tt hn l tnh ton abt v hiu chnh tn hiu nhn c. C th tnh c s hiu chnh abt nu bit v trca c b truyn v nhn. Nu b truyn ng yn, c th dng mt hng s cho v tr b truyn. Nu b truyn di chuyn (vd nh mt v tinh) th cn phi truyn thm v tr ca n vo thi gian truyn. H thng nh v ton cu (GPS) cung cp chai th gii tt nht mi v tinh GPS pht v tr ca n v b nhn c th dng nh v t nhiu v tinh tnh v tr cachnh n.

Thng tin c truyn thng bao gm c m thi gian sao cho c th t ng h ng thi gian trong ngy. Hu ht mthi gian cha gi, pht, v giy UTC; thng, ngy v nm; v cnh bo trc thi gian tit kim nh sng ban ngy v cc

giy nhy qung.Cc tn hiu truyn thi gian v tn s sng v tuyn

C nhiu dng b nhn sng v tuyn c thit k nhn cc tn hiu thi gian v tn s. Mt s c thit k ban u to ra thng tin thi gian trong ngy hoc xung ng gi, mt s khc c thit k a ra cc tn s chun, v c thdng mt s cho c truyn thi gian v tn s. Phn tip theo xem xt ba dng tn hiu raio thi gian v tn s c phnloi theo UTC l tn s cao (HF), tn s thp (LF), v cc tn hiu v tinh GPS.

Tn hiu sng v tuyn HF (gm WWV v WWVH)

Cc i pht sng v tuyn tn s cao (HF) chim ph sng v tuyn t 3 n 30 MHz. Cc tn hiu ny thng cdng cho vic truyn thi gian v tn s ti cc cp hiu sut va phi. Mt s i pht HF sinh ra cc thng bo thi gian mthanh v cc m thi gian s. Cc i pht khc n gin ch sinh ra cc tn s sng mang dng nh mt tham chiu.

Cc trm thi gian v tn s HF gm cc trm sng v tuyn NIST ca M l WWV v WWVH. WWV c t gnFort Collins, Colorado, v WWVH nm trn o Kauai, Hawaii. Cc hai trm pht cc tn hiu thi gian v tn s 2,5; 5; 10v 15 MHz, v WWV cng pht 20 MHz. Tt c cc tn s pht chng trnh ging nhau, v t nht mt tn s s khng nnh trong tt c cc ln. Cc trm ny cng c th nghe bng in thoi, gi (303) 499-7111 cho WWV hoc (808)335-4363cho WWVH.

Cc tn hiu WWV v WWVH c th c dng mt trong cc dng sau:

Phn m thanh ca i pht gm cc xung c di vi giy hoc vi khong khc, cc tn s m chun, v ccthng bo ging ni v gi v pht UTC. WWV dng ging ni nam v WWVH dng ging ni n.

B m thi gian nh phn c gi trn sng mang con 100 Hz vi tc 1 bit/s. M thi gian gm gi, pht,giy, ngy trong nm, giy ngt ngng v b ch thi gian tit kim nh sng ban ngy (Daylight Saving Time DST), cng mt hiu chnh UT1. M ny c th c c v hin th bi ng h sng v tuyn.

Tn s mang c th c dng nh l mt tham chiu cho chun ca b to dao ng. N hu ht c thc hin

vi cc tn hiu sng mang 5 hoc 10 MHz, do chng thch hp vi cc tn s u ra ca cc b to tn hiu chun.Vic pht thi gian bng WWV v WWVH s b chm khi n ti v tr s dng. Thi gian b ph thuc khong cch t

b truyn n b nhn, nhng nn


13/14


Nhiu trm thi gian v tn s hot ng trong di LF t 30 n 300 kHz (bng 17.7). Hiu sut ca tn hiu nhn b nhhng bi chiu di ng dn v mnh tn hiu. Chiu di ng dn quan trng bi v tn hiu c chia thnh sngmt t v sng khng trung. Tn hiu sng mt t n nh hn. Do n truyn theo ng i ngn nht gia b truyn v bnhn, n n trc v tr ng dn ca n d c lng hn. Sng khng trung phn hi t tng in ly v to ra cc ktqu tng t vi cc kt qu nhn c t vic thu HF. Cc ng dn ngn cc trm thu pht c th bm theo sng mt tmt cch lin tc. Cc ng dn di hn trn hn hp sng khng trung v sng mt t. V vi cc ng dn rt di, chc th thu sng khng trung.

mnh tn hiu cng quan trng. Nu tn hiu yu, b nhn c th tm kim chu trnh mi ca sng mang bm. Mithi im b nhn iu chnh im bm ca n bng mt vng qut, n a ra mt bc pha bng chu k ca sng mang. Vd, mt vng trt vi sng mang 60 kHz a ra mt bc pha 16,67 sm . Tuy nhin, vi mt tn hiu sng mt t mnh cth cho cc kt qu rt tt. Mt b nhn LF bm lin tc trong cng mt chu trnh ca mt tn hiu sng mt t c th truyntn s trong di 121 10- khi ly trung bnh trong 1 ngy.

NIST iu hnh trm sng v tuyn LF WWVB t Fort Collins, Colorado vi tn s truyn 60 kHz. Trm hot ng24/24h vi cng sut u ra c ch 50 kW. M thi gian WWVB ng b vi sng mang 60 kHz v cha nm, ngy ca nm,gi, pht, giy, v c ch trng thi thi gian tit kim nh sng ban ngy, nm ngt qung, v giy ngt ngng. M thi gianc nhn v hin th bng ng h eo tay, ng h bo thc, ng h treo tng, v cc thit b in t dn dng khc[2,7,15].

BNG 17.7 Cc trm truyn pht thi gian v tn s LF

K hiu gi Quc gia Tn s (kHz) Lun lun bt?DCF77 c 77,5 C

DGI c 177 C

HBG Thy S 75 C

JG2AS Nht Bn 40 C

MSF Anh 60 C

RBU Nga 66,666 Khng

RTZ Nga 50 C

TDF Php 162 C

WWVB M 60 C

H thng nh v ton cu (GPS)

GPS l h thng dn hng c pht trin v iu hnh bi B quc phng M (DoD) v c s dng hu khp trntri t. H thng bao gm mt chm t nht 24 v tinh c qu o cao 20.200 km so vi mt t trong su mt phng cnh nghing 55o so vi ng xch o. Chu k qu o l 11 gi 58 pht, c ngha l mi v tinh bng qua mt phng gingnhau trn tri t hai ln mt ngy. Bng vic x l cc tn hiu nhn t cc v tinh, mt b nhn GPS c th xc nh v trca n vi sai s


14/14

S tay C in t

HNH 17.14 n nh tn s ca b nhn GPS

17.5 Kt lun

Nh lu trn, cc chun v o lng thi gian v tn s c ci tin bi chn bc bin trong 100 nm qua.S tin b nhanh chng ny to ra rt nhiu sn phm v cng ngh kh thi mi. D khng th on trc v tng li,chng ta cng c th chc chn rng cc b to dao ng Quarts s tip tc tng, cc sai s o lng s tip tc gim v cccng ngh mi s tip tc c tm ra.

Tham kho

[1] Jespersen, J., and Fitz-Randolph, J., From Sundials to Atomic Clocks: Understanding Time and Frequency, 2nd ed.,Dover, Mineola, New York, 1999.

[2] Kamas, G., and Lombardi, M. A., Time and Frequency Users Manual, NIST Special Publication 559, U.S.Government Printing Office, Washington, DC, 1990.

[3] Levine, J.,Introduction to time and frequency metrology, Rev. Sci. Instrum., 70, 2567, 1999.

[4] Hackman, C., and Sullivan, D. B., Eds., Time and Frequency Measurement, American Association of PhysicsTeachers, College Park, Maryland, 1996.

[5] ITU Radiocommunication Study Group 7, Selection and Use of Precise Frequency and Time Systems, InternationalTelecommunications Union, Geneva, Switzerland, 1997.

[6] Novick, A. N., Lombardi, M. A., Zhang, V. S., and Carpentier, A., A high performance multi-channel time intervalcounter with an integrated GPS receiver, in Proc. 31st Annu. Precise Time and Time Interval (PTTI) Meeting, DanaPoint, California, p. 561, 1999.

[7] Lombardi, M. A., Time measurement and frequency measurement, in The Measurement, Instrumentation, and SensorsHandbook, Webster, J. G., Eds., CRC Press, Boca Raton, Florida, 1999, chap. 1819.

[8] Sullivan, D. B., Allan, D. W., Howe, D. A., and Walls, F. L., Eds., Characterization of Clocks and Oscillators, NISTTechnical Note 1337, U.S. Government Printing Office, Washington, DC, 1990.

[9] Jespersen, J., Introduction to the time domain characterization of frequency standards, in Proc. 23rd Annu. Precise

Time and Time Interval (PTTI) Meeting, Pasadena, California, p. 83, 1991.[10] IEEE Standards Coordinating Committee 27, IEEE Standard Definitions of Physical Quantities for Fundamental

Frequency and Time MetrologyRandom Instabilities, Institute of Electrical and Electronics Engineers, New York,1999.

[11] Walls, F. L., and Ferre-Pikal, E. S., Measurement of frequency, phase noise, and amplitude noise, in WileyEncyclopedia of Electrical and Electronics Engineering, John Wiley and Sons, New York, 1999,

[12] Howe, D. A., An extension of the Allan variance with increased confidence at long term, IEEE Int. Freq. ControlSymp., 321, 1995.

[13] Vig, J. R., Introduction to quartz frequency standards, Army Research and Development Technical Report, SLCET-TR-92-1, October 1992.

[14] Hewlett-Packard Company,Fundamentals of Quartz Oscillators, HP Application Note 200-2, 1997.

[15] Carr, J. J.,Elements of Electronic Instrumentation and Measurement, 3rd ed., Prentice-Hall, NJ, 1996.[16] Hoffmann-Wellenhof, B., Lichtenegger, H., and Collins, J., GPS: Theory and Practice, 3rd ed., Springer-Verlag, NewYork, 1994.

14

sổ tay cdt chuong 17-time v f

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