Ebook Cơ sở kỹ thuật đo lường điện tử (in lần thứ tư có sửa chữa, bổ sung): Phần 1

VO QUY DIEM (Chu bien)
PHAM VAN TUAN
n g u y In THUY ANH
DO LE PHU
n g u y I n NGOC VAN

o’sd
ky thuat
do luong
dien tu


PGS. Vu Quy Diem ( Chu bien)
Pham Van Tuan, Nguyen Thuy Anh
Do Le Phu, Nguyen Ngoc Van

CO SO KY THUAT
■

DOLlTbNGDIENTUT
■

In Ian thit tit co sua chita va bo sung

NHA X U A T BAN KHO A HOC VA K Y TH UA T
HA NOI - 2007


LCfl N O I D A U
Giao trinh “ Co so ky thuat do luong dien tur” duoc bien soan nhim phuc vu viec
hoc tap ciia sinh vien dai hoc thuoc cac nganh ky thuat Dien tir - Vi£n thong. Cuon sach

nay ciing co the dung lam tai lieu tham khao cho cac nganh ky thuat khac co sir dung ky
thuat Do ludng dien tir nhu la mot phuong phap de nghien curu khoa hoc, sir dung khai
thac ky thuat cua nganh minh.
Do sir phat trien cua nganh Dien tir - Vi6n thong va su phat trien cua ky thuat Dien
tir, ky thuat Do luong dien tir cung thay doi va phat trien rat nhieu. Noi ve sir phat trien
cua ky thuat Do luong dien tu' tru'cfc het phai noi den nhung thay d6i co ban cua cac thiet
bi do co su dung cac bo vi xir ly ( microprocessors). Viec ap dung ky thuat vi xir ly va do
ludng da lam tang tinh nang va thong so cua thiet bi do len rat nhieu, da mo ra cach giai
quyet cac van de ma trudc kia chua dat ra duoc. Cac bo vi xir ly da tro thanh mot bo
phan thiet yeu cua cac thiet bi do. Co bo vi xu ly cac thiet bi do co the da chirc nang hon,
don gian hoa viec dien khien, tir dong di6u chinh, tir dong lay chuan, tir dong kiem tra,
lam tang them do tin cay cua cac thong so phep do, cho phep thuc hien tfnh toan, thong
ke ket qua cua phep do nhanh chong va da dang hon, nhu vay da tao ra duoc thiet bi do
luong lap trinh tu dong. Mot phan cua cuon sach nay dung de trinh bay nhung nguvcn
tdc va kha nang ciia cac thiet bi do co bo vi xir ly.
Tuy nhien, trong thuc te nhifiu khi chi can cac thiet bi do don gian hon, nen rat
nhieu thiet bi do trong ky thuat hien nay van dang dung cac thiet bi do sir dung ky thuat
tuong tu cung nhu ky thuat so. Cuon sach nay con de cap den ca nhung thiet bi do nhu
vay va cac nguyen tac truyen thong ciia ky thuat Do luong dien tir.
Ky thuat Do luong dien tir la mot nganh ky thuat co pham vi rat rong ca v i doi
tuong do, moi truong va dieu kien do cung nhu dai tAn do, lirong trinh do va cau tao
mach do. Tham vong ciia cac tac gia la lam the nao de co the goi gon duoc ca pham vi
rong Ion noi tren vao nhung van de rat co ban va cach trinh bay phai the hien duoc cac
nguyen tile truygn thong cung nhu cap nhat duoc cac nguyen tac moi trong cuon sach
ciia minh. Mac dii da co ging de cuoii sach dat duoc y tuong noi tren, song chdc chdn
khong tranh khoi con sai sot, cac tac gia mong duoc sir gop y va chi dftn ciia ban doc.
Moi y kien xin giri ve: Khoa Dien tii-Vien thong, trmmg dai hoc Bach Khoa H a
Noi, dien thoai 04.8692242.
L&n xuat ban nay co nhieu chinh ly, sira chua va bo sung so voi lan xuat ban truoc
(thang 10 nam 2001) do Thac sy Nguyfin Thuy Anh, Thac sy D6 Le Phu, Thac sy

Nguyfin Ngoc Van, giang vien giang day mon hoc “ Co so kv thuat do luong dien tir”
ciia Khoa Dien tir - Visir chi’ dao ciia tac gia chii bien.

Ha noi, ngay 15 thdng 8 nam 2003
Tac gia chu bien
PG S. Vu Quy Diem



Chiidng I
Gl6l THIEU
■' CHUNG VE KY THUAT
■
0 0 LUONG DIEN TLf
■

MCf DAU
Trong qua trinh phat trien cua khoa hoc ky thuat ma toan bo the gioi dang clnrng
kien, dien tu la mot trong nhung nganh phat trien mui nhon. ung dung cua dien tir, tin
hoc, vien thong dang ngay mot Ion va anh hudng sail sac den cuoc song va cach thuc
lam viec cua loan xa hoi. De phat triem duoc cac ITnh vuc trong mot tong the cluing la
nganh dien tir, till van de do luong la mot van de can duoc quail tarn va phat trien. Noi
dung cua giao trinh “ Do lirong dien tir duoc gioi thieu trong tap sach nay co the noi mot
cach tom tat la: nghien cim cac phuong phap do lirong dien tir co ban, cac bien phap ky
thuat de thuc hien cac phuong phap do va cac tliao tac ky thuat do luong d6 dat duoc
nhung yen can can thiet cua phep do.
Cu the!, noi dung nay bao gom cac van de ve cac phuong phap do luong cac thong
so cua tin hieu va mach dien, cac bien phap cau tao cac mach do cung nhir can true tinh
nang cua may do, cach nang cao do chfnh xac cua phep do cung nhu cach xac dinh, han

che sai so cua ket qua do.
Do luong cac thong so dac tmli cua tin hieu nhu la do cac thong so cirong do cua
tin hieu (V i du nhu cac thong so dong dien, dien ap, cong suat...), nhu quan sat dang cua
tin hieu, do tan so, do di pha, phan tich pho cua tin hieu. Do cac thong so cua mach dien
nhu cac thong so cac linh kien ducfng thing, linh kien khong duong theing (cac linh kien
co so nhu dien tro, tu dien, den dien tu, den ban dan... den cac linh kien nhir 1C, cac loai
mach tich hop...), trong cac mach dien co phan tu tap trung, cac thong so cua cac linh
kien duong thang trong mach sieu cao tan.
Dac diem co ban cua^ ky thuat do luong dien tu la cac phep do duoc thuc hien
trong mot dai ph6 rat rong, tir OHz (tin hieu khong bien doi) den 3.10" Hz (song quang).
Do vay cac phuong phap do, cau true cua may do va ca do cln'nh xac cua phep do cung
deu tuy thuoc vao dai tin qua, doi tuong do luong. V i du 6 tan so thap thi d6 dang do
diroc dong dien va dien ap, nhurng (V sieu cao tan thi cac thong so can xac dinh la dong
dien, dien ap tra nen vd nghTa khi c^n djnh luong thong .6 tren mach, ma phai xac dinh

3


chung thong qua cong suat. Hay vi du, cung la dai lirong cin do la tro khang cua mach,
ma d tan so thap thi co the dung cac thiet bi do la cac loai cau bon nhanh, o tan so cao
hon thi thiet bi do la cau cong huong dien ap hay dong dien, va d tan so sieu cao tan thi
thiet bi do phai dung la day do hoac do bang ong din song hay day d6ng true.
Do chinh xac cua phep do thudng phu thuoc nhieu vao sir khu bo cac anh lnrdng
ghep ky sinh cua cac thong so cua ban than may do toi mach can do, vi du nhu dien
dung, dien cam, dien tro cua may do, anh hirong nay tang khi tan so cang tang cao. Do
vay khi do cung mot dai luong ma d tan so khac nhau thi khong nhung can co cac
phuong phap khac nhau ma may do duoc dung de do cung phai co cau tao khac nhau.
Khi da chon dung phuong phap do va may do thfch hop roi thi cung can phai chu y toi
thao tac can thiet, each mac do the nao der nang cao hon do chinh xac cua phep do. V i du
nhu can giam toi mire toi thieu dien ap tap tan, dien dung ky sinh cua day noi, cua may

do. Cac anh huong tren thirong tro nen rat dang k6 trong ITnh vuc do lirong dien tii, vi
phep do thirong duoc thuc hien d tan so cao, cong suat be va hay duoc tien hanh 6 trang
thai cong huong.
Luong trinh cua dai luong can do trong ky thuat dien tu cung kha rong va da dang.
Vi' du nhu vdi viec do tan so thi phai thuc hien phep do co luong trinh tir OHz den
10i5H z . D o cong suat thi tu cac thiet bi co cong suat Ion toi 10* W , dudi cac phuong thuc
diSu che tin hieu khac nhau nhu: dieu bien, dieu tan, di£u pha va ca dieu xung, voi do
rong xung toi 10 V
Sir can thiet cua do ludng trong ky thuat dien tu la rat Ion, hiu nhu chung ta phai
sir dung d moi luc, moi chd. Khi nghien ciru thiet ke, di£u chinh khai thac, lip dat van
hanh... cac he thong dien tii, vi£n thong, khong the khong co may do. Cho mot he thong
lam viec, hay di6u chinh mot thiet bi dien tur, la mot qua trinh do ludng cac che do cong
tac, lay dac tinh cua tung khoi, tung khau rieng biet hay toan bo. Do vay. chi vdi cac
may do co do chinh xac cdn thiet thi moi co the didu chinh duoc thiet bi dat duoc cac
yeu cau mong muon. Khi lip rap, che tao cac thiet bi dien tu, cac thiet bi vien thong,
cung rat can do lirong. Vi tinh toan thiet ke chi cho duoc cac so lieu so bo. muon co
duoc che do cong tac thuc te va thong so thich hop nhat thi chi tren co so thuc nghiem
moi co. Vdi cong tac nghien cuu thi viec xay dung phuong phap do va kien toan thiet bi
do lai cang quan trong hon. Khong phai chi co so luong cac ket qua, ma sir phan tich
chat luong cung co ich loi cho cac cong viec lien quan tdi 17nh vuc nghien ciru. Khi khai
thac cac he thong dien tu, can phai luon luon kiem tra phat hien cac hu hong, thuc hien
cac qua trinh cong tac, gifr duoc cac chi tieu ky thuat cao trong qua trinh lam viec. xac
dinh nhanh chong cac nguyen nhan lam mat cac tieu chuan cong tac. Tat ca nhfmg dieu
do dfiu khong thuc hien duoc neu khong co sir to hop phep do va may do.

4


Cung vdi qua trinh phat trien cua khoa hoc cong nghe, ky thuat dien tu va ky thuat
viln thong la nhffng qua trinh tien trien gin chat vdi kha nang thuc hien va hoan thien ky

thuat do ludng. V f du, nhung thanh tuu cua linh vuc nghien ciru khong gian vu tru nhu
ve tinh cua trai dat.... la nhung he thong rat phuc tap cua cac may moc do ludng dien tu.
Qua trinh dieu khien va tu dong bao ham mot so ldn cac phep do cac loai khac nhau vdi
do chinh xac cao. Tren co sd phat hien nhung doan tan so song mdi, nhung phuong phap
do mdi cung xuat hien theo, tao ra them cac yeu cau mdi va dac biet ve che tao cho cac
may do. Ngay nay, vdi sir phat trie'll cua khoa hoc cong nghe ngirdi ta da tir dong hoa
duoc cac qua trinh san xuat, nhung thanh tuu mdi nay da duoc thfch ung vdi cac phuong
phap do va thiet bi do mdi cua ky thuat do ludng tir dong.
Do chfnh xac cua phep do phu thudc nhieu vao chu quan cua ngudi do. Muon ket
qua do chfnh xac, phai chon duoc phep do diing vdi iiliiem vu dat ra, thfch hop vdi doi
tuong can do. Can phai n&m duoc cac phuong phap do khac nhau, biet duoc tinh nang
cac may do, xir ly thfch dang duoc cac nguon goc sai sd do. Khong can than trong qua
trinh do, kh6ng biet day dii dac diem ciia doi tuong do, dac tinh ciia tin hieu can do, kha
nang ciia may do thi khong the; co ket qua do chfnh xac. Sir thong thao ciia ngudi lam ky
thuat khi do ludng co the nang cao duoc do chfnh xac cua phep do, va co the thuc hien
cac phep do mot cach linh hoat. Sir thong thao khong nhung the hien d chd hieu ro cac
phep do va sir dung thanh thao cac may do, ma con the hien d chd biet van dung de! hieu
duoc cac nguyen ly do ludng d cac he thong dien tir hien dai. Vf du nhu he thong Radar
la he thong co yeu cau do klioang thdi gian, co sd ciia mot sd he thong dieu khien la
phep do di pha rdi rac hoa va chi' thi bang sd.
De nghien ciru giao trinh “ Co sd ky thuat do ludng dien tir” nay, yeu cau ngudi
doc da dupe nghien ciru cac giao trinh ky thuat co sd ciia nganh ky thuat dien tir. Cac
giao trinh ky thuat co sd co quan lie true tiep can phai ke den nhu giao trinh dien tir va
ban din, ly thuyet co sd ve tin hieu va mach dien vo tuyen dien, co sd thiet bi khuech
dai, ky thuat xung va sd, ly thuyet xac suat va thong ke. Sd dT nhu vay, vi tat ca cac kien
thire chung cua cac giao trinh tien la ly thuyet co sd can thiet de xay dirng cac phuong
phap do ludng ve cac thong sd ciia tin hieu cung nhu ciia mach dien tir. Do cung la cac
ky thuat co sd de xay dung cac bien phap thuc hien cac phuong phap do nay, tire la cau
true cu the ciia cac mach do va ciia cac may do.
Co sd phat trien ky thuat dien tir la tir ky thuat dien, nen do lirong trong dien tir

cung xuat phat tir cac co sd ciia ky thuat do ludng dien. Tuy co quan he gin bo nhu vay,
nhung hai moil hoc nay co quan diem co ban khac nhau. Nhiem vu cua ky thuat dien la
tao ra, truyen dan va bien doi nang luong dien tir. Con nhiem vu ciia ky thuat dien tir la
truyen lan va gia cong tin tire nlid dao dong dien tir. Do vay, hai nganh dien va dien tir
phai dupe nghien ciru theo hai quan diem khac nhau, quan diem nang lupng va quan
diem thong tin.

5


Nhu vay, khi do ludng trong lTnh vuc dien tu. ngudi ta thudng it quan tam tdi khfa
canh nang luong cua qua trinh. Dieu chu trong nhieu hon la cac thong sd va dac tfnh dac
trung cho mach va tfn hieu ve mat thong tin, vf du nhu tan sd, pha, tro khang dac tfnli. he
sd truyen dat va cac thong so dang cua tfn hieu...
Cac phan sau day, chung ta se xem xet tdi cac doi tuong va phuong phap do luong
dien tu mot cach chi tiet.

1.1 DOI TUONG CUA DO LUONG DIEN TLf
1.1.1 Cac dac tinh va thong so ciia tin hieu
De: mo ta tfn hieu trong linh vuc dien tu, ngudi ta sir dung cac bieiu thuc toan hoc
sau day:
s(t)=s(t, a„ a2,...,a„)
hoac

s(f)=s(f, t>,, b2,...,bn)

Tu cac bieiu thuc tren day, ta thay rang, tfn hieu s(t) khong nhung phu thuoc vao
thdi gian va s(f) khong chi phu thuoc tan sd ma chung con phu thuoc vao nhieu dai
!uong khac la a,, a2,..., a„ va b,, b:,...,b„. Cac dai luong do duoc goi chung la cac thong so
cua tfn hieu.

Tfn hieu s co rat nhieu dang khac nhau, tuy theo nine dfch sir dung turc la tuy thuoc
vao loai tin tufc ma tfn hieu nay phan anh.
De nghien cuu nhung bien phap truyen dan va bien doi tfn hieu, chung ta can phai
tien hanh do ludng cac thong sd cua no. Ngudi ta khong xet tdi thong sd cua tat ca cac
loai tfn hieu, bdi vl ro rang tren thuc te la khong the lam nhu vay duoc, va thuc ra la
khong can thiet. Sd luong tfn hieu dupe dung de quy dinh lam doi tuong do ludng la rat
ft so vdi sd luong tfn hieu tren thuc te va dupe goi la nhung tfn hieu mau. Sd tfn hieu
mdu nay la tdi thieu nhung ve mat do ludng, chung da thoa man duoc yeu cau la bie’u
dien dupe mo hlnh don gian cua cac tfn hieu tren thuc te.
Khi do ludng cac thong sd va dac tinh cua cac m;ich dien, ngudi ta cung dun" cac
tfn hieu mSu nay. Biet dupe phan ung cua mach vdi cac dang cua tin hieu ay. thi cuna co
the suy ra phan ung cua mach vdi cac dang tfn hieu khac.
Cac tfn hieu trong dien tu thudng dupe bieu dien theo ham ciia thdi gian hoac theo
ham theo tan sd.
Dang ciia cac tin hieu co ban duoc khao sat thong so, bao gdm:
-Tfn hieu dieu hoa

6


-Tfn hieu tiutn hoan
-Tfn hieu xung
-Tin hieu so.

1. Cach bieu d iin tin hieu theo ham so cua thdi gian va theo ham so cua tan so
a. Ham so theo thdi gian
Ham so theo thdi gian la ham sd dang:
s=f(t)
Ngoai tham sd la thdi gian ra, nhir da trinh bay d phan tren, con co cac tham sd
khac trong bi£u thuc cua f(t), nen bidu thuc cua s co the dirpc bi£u di£n thanh dang nhu

sau:
S=f(t, a,, a2,..., a„)
Cac thong sd a,, a2,..., an cua tin hieu se xac dinh dang cua tin hieu, do do vdi mdi
tin hieu khac nhau, ta se co nhung tham sd khac nhau va cac ham sd khac nhau. Vdi
nhung loai tin hieu khac nhau do, d£ do cac thong sd tin hieu ciia chung, ngudi ta phai
co nhung phuong phap phu hop nham dua ra ket qua gdn vdi thuc te nhat.
Vi du khi sir dung O-xi-lo de hi£n thi mot tin hieu theo thdi gian, ta co the thay
dupe cac tham sd v6 dang
cua tin hieu nhu cudng do,
chu ky, do di pha.
Cac thiet bi trong do
ludng cung dupe thiet ke cho
viec do dac mot vai thong sd
nao do nen tuy theo thong so
nao can do, ta phai chon cac
loai may do thich hop, vf du
Hinh 1-1 Tin hieu hien thj tren O-xi-lo

nhu de do cac thong sd v£

cudng do ta co the dung von-met de do dien ap, ampe-met de do dong dien, oat-met de
do cong suat. Ngoai ra vdi mdi giai lupng trinh khac nhau, ngirdi ta cung phai sir dung
nhung phirong phap va thiet bi do phu hop, vi du nhu vdi cac giai tan sd khac nhau,
ngudi ta phai co nhung phirong phap va thiet bi do khac nhau.

h. Ham so theo tan so
Ham theo tan so la ham co dang
s=cp(f)

7

Ham so theo tan so thudng dupe dung de bieu dien cho cac tin hieu tuan hoan
hoac cho mot tin hieu trong mot khoang thdi gian huu han.
Khi bidu dien mot ham so theo tin sd, iru diem cua no la ngudi ta co the thay duoc
dai tin cua tin hieu, tu do ngudi ta se co nhung phuong phap phu hop cho viec gia cong

Hinh 1-2 Bieu dien tin hieu

V f du khi can lay m&u cua mot tin hieu, ngudi ta phai biet dupe dai tan cua no va
se lay mlu trong cac khoang thdi gian phu hop vdi tin sd cua tin hieu theo dinh ly lay
mlu:

u

a, Tin hieu tn/dc khi lay mau

Hinh 1-3 Lay mau tin hieu

“

MAX

Vdi T,mla chu ky lay mau;
F max la tan so Ion nhat ciia tin hieu.

8

b, Tin hieu sau khi lay mau

2*. Cac thong so cua cac dang tin hieu

a. Tin hieu dieu hoa
Dao ddng dieu hoa dung de m6 phong tiS'ng noi, am nhac,..., va co bi^u thuc toan
hoc dU(ri dang hinh sin (hoac cos):
s(t)= A msin(27cf0t-Kp0)

Do thi ciia no nhu trong hinh 1-4.
Ngoai thdi gian t con co cac thdng sd' Am, f0, va cp0tham gia anh hudng vao tin hieu
nay.
Am: BiSn do cua dao dong, co thir nguyen la von (V ) neu s(t) la dien ap, hoac co
thu' nguyen la ampe (A ) neu s(t) la dong dien.
f0: Tan sd cua dao dong, do bang hec
( Hz); Tii tan sd f0, con co cac thong sd dSn
xuat sau day:
co0: Tan sd goc, do bang radian/s;
CD0=27Cf0

T„: Chu ky, do bang gi&y va ta co:
T„ = —

Hinh 1-4 Tin hieu dao dong dieu hoa

"

f (|

A.,,: Budc song, do bang met va

trong do c«3.108tn/s, la van toe anh sang.
(p„: Goc pha dau cua dao dong, do bang do hoac radian. Goc pha ddu tinh tCr mot
thdi diCm bat ky dupe chon lam goc. Vi goc thdi gian la tuy y nen khi ngudi ta noi do
pha, thi khong phai la do pha dau cua mot dao dong ma la do sir dich pha giira hai dao
dong di6u hoa cung tan so (hinh l -5).
Bien do A„, do bang vdn-met
neu s(t) la dien ap, hoac bang ampemet neu s(t) la dong dien. Tren thang

t

dt) ciia cac dung cu nay, ngudi ta
khong khac dp theo gia tri bien do cua

eo0
Hinh 1-5 Si/lech pha cua 2 tin hieu

9


dao dong ma khac do theo gia tri hieu dung. Giua gia trj hieu dung A va gia trj bien do
A mco quan he sau:
A

=

* 0,707A m

Tan sd f0 hoac budc song X() do bang may do tan so (tan so-met) hay may do song.
That ra, trong hai dai luong nay co the chi can do mot dai hrpng roi suy ra dai lucmg kia.
*

Djch pha giua hai dao ddng di£u hoa duoc do bang may do pha (pha-met)
Trong ky thuat do ludng dien tu, dao dong di6u hoa duoc tao ra bang cac bp tao
song (am tan, cao tan va sieu cao tan) dac biet. Nhd co nhung co cau dieu chinh va cac
bo hien thj kiem tra nen bien do va tan sd cua dao dong tao ra ta co the bien doi duoc
trong mot pham vi nao do, thudng la kha rong. Trong sd cac may phat tin hieu do ludng,
thi cac may phat tin hieu dao ddng di6u hoa la pho bien nhat.

b. Tin hieu tudn hoan
Tin hieu loai nay co dang tuy y va vi vay v£ mat nao do, no la tong quat hon cac
trudng hop tren. Do tinh tuan hoan nen co the bieu dien no dudi dang sau:
s(t)= s(t+nT)

khi -oc < t < +0 0 ;

trong do T la chu ky lap lai cua tin hieu ( T = —, vdi F la tan sd lap lai).
F
Xet mot dang mau cua tin hieu nay d hinh l-6a.
Vi dang la bat ky nen de dac trung cho loai nay, chung ta phai dung kha nhidu
thdng sd, ta se lan lupt neu ra dudi day.
Trong trudng hop tong quat, dao dong co the co thanh phan mot chieu (hinh 1-6b)
va do do ta co the xem no nhu tong cua thanh phan mot chieu nay vdi thanh phan xoay
chi£u (hinh l-6c):
s(t)=s_ + s _(t)
Trong do thanh phan mot chi6u:
1 ft" +t

s_ = - }.

10

s(t)dt


t* la thdi difem tuy chon, neu chon t -0 thi
1 fT

s_ = f j;s(,)d ,

Hinh 1-6 Phan tich tin hieu tuan hoan

Tu day ta tha> ring s_ chinh la chieu cao (bien do) cua mot xung vuong co do
rong la T va dupe tmh bing dien tich phan mat phing gidi han bdi phan dudng cong s(t)
nim trong khoang T va true thdi gian t. Neu ke trong mot chu ky T thi phan dien tich
nim giua thanh phan xoay chieu s_(t) va muc mot chieu s_ dupe phan bo deu tren va
dudi muc nay.
Do lech cuc dai cua s(t) tmh tir mire mot chieu ve hai phia A llin va A dlllJi co the khac
, nhau nen d day khong dung khai niem bien do chung dupe. Tong cua hai dai lirong nay
xac dinh khoang bien thien cua thanh phan xoay chieu:
A =A

-i-A

Cong suat tire thdi cua dien ap hoac ddng dien tieu thu tren mot dien trd R van tinh
nhu thudng le:


Tuy nhien trong tinh toan va do ludng. ngudi ta hay dung khai niem cong suat
trung binh hem. Neu dien trd tai R bang IQ thi cong suat trung binh se tinh nhu sau:

P = I f PCKIt =

u;(l)dt = i £ i2(t)dt

(I»

Khai niem cong suat trung binh co lien quan den cac gia tri hieu dung cua dao
dong:

a>

Shd =

Hinh l-6c bieu didn duong cong s_(t) tuong urng vdi hinh l-6a. Tu do, de thay rang
cong suat trung binh chfnh la mire mot chieu cua s 'Jt) boi vi tir (1) va (2) ta co:
P=s=w
De do ludng cac thdng
sd k6 tren, ngudi ta dung
nhieu dung cu do khac nhau.
Von-met

(hoac

ampe-met)

mot chidu de; do s_. Thanh
phan xoay chieu co the tach
rieng ra de do cac thdng so
ciia no bang cach cho tfn hieu
s(t) di qua tu dien hoac bien
ap. Cac thdng so A,,,,,, A (tU(I1 do

bang von-met dinh (neu s(t) la
dien ap). Thdng thudng de do cac gia tri tuc thdi va nghien cuu dang cua tin hieu dao
dong, ngudi ta dung dao dong ky. Cong suat trung binh P do bang oat-met.
Cac gia tri dinh, trung binh, hieu dung, cong suat cua dao dong cung nhu cac gia
tri tuc thdi thudng dupe goi chung la “ cac thdng sd cudng do".
Ngoai phirong phap do true tiep bang cac dung du ke tren, ngudi ta con co the do
cac thong so va dac tinh ciia loai tin hieu nay dua vao nguyen ly dupe neu ra sau day.
Moi dao ddng tuan hoan co dang bat ky ddu co the; phan tich thanh tong cua vo so
dao ddng di6u hoa vdi thanh phan mot chieu:


s(t)=s + A,sin(Qt+(p, ) + A :sin(2Qt-Kp2)+ ... + A nsin(nQt+trong do, cac dao dong hinh sin thanh phan goi la cac song hai, Q =2nF goi la tan
so co ban. Cac song hai co tan sd bang boi so nguyen Ian cua tSn so co ban: nQ; n=l,
2,... goi la bAc cua song hai. Bien do A n va pha ban dau (p„ phu thuoc bac cua song hai.

a)
A4

zn.

.in.

JL

ASL

CO

Tir day, chung ta thay rang

s(t) khong nhung co the bieu diln
theo thofi gian o dang hinh 1-6 ma
con co the bi6u dien theo tan so
nua. Khi do, tach s(t) lam hai
thanh phan: bien dp va pha. Do
thi bieu dien su phu thuoc ciia
a, Do thj pho bien do-tan so
b, Oo thi pho pha-tan so
c, Lap lai tin hieu tCr cac thanh phan hai

bien do cac hai vao tan so:
A=A(io) goi la d6 thi pho bien dptan so (vi du nhu tren hinh 1.8a).

Con do thi bieu dien sir phu thuoc cua pha dau cac hai vao tan so (p=ip(o>) goi la
do thi pho pha-tan so (vf du nhu hinh 1-8b).
Dung cong thuc (4) va tir hai do thi nay co the lap lai dupe dang cua s(t) ban dau
(hinh l-8c). Dang va gia tri ciia pho bien do quan sat va do dupe bang may phan tich
pho bieu thi bang ong tia dien tir.
Con mot thong so nua lien quan den dao dong, la b£ rong pho Af, do bang Hz. Do
la dai tan so tren do co phan bo cac vach ph6 cua dao dong. Viec bo tri cac kenh thong
tin trong mot dircmg thong tin duy nhat va tmh toan dai thong cua cac thiet bi trong he
thong thong tin nhat thiet yeu cau phai biet b£ rpng ph6 ciia tin hieu chon dung. Khi d6
thi pho bien do cua dao dong da hien tren man hien song cua may phan tich pho, ta dd
dang do dupe be rong pho Af (xem chuong 7).

13


c. Tin hieu xung
Tin hieu xung duoc sir dung dac biet rong rai trong ky thuat vA tuyen: Thong tin

xung, rada, di£u khien, truyen hinh vo tuyen v.v... V i vay do ludng cac thong so cua
xung chiem mot vi tri kha quan trong.
Tin hieu xung co nhidu loai va voi m6i loai lai co mot nhom thong so dac trung.
Ngudi ta phan biet nhung loai tin hieu xung sau:
-Xung don
-Nhom xung
-Nhom xung cot
-Day xung tuan hoan

Xung dan : la dong dien hoac dien ap ma gia tri cua no chi khac khong trong

*

mot khoang thdi gian huu han, co the so sanh duoc voi thdi gian qua do cua mot mach
dien nao do khi xung nay tac dong vao.
Rieng xung don lai co nhieu loai phan biet nhau theo dang va m6i loai gan li6n vdi
nhung thong so khac nhau.
Thong dung nhat la xung vuong (dung ra phai goi la xung vuong goc), hinh l-9a.
Trong do:
A m: Bien do;
t

:

do rong xung ; doan ab la sudn truoc, doan be la dinh va doan cd la sudn sau
c
b
b
c

t
a


t

c)

0
Hinh 1-9 Cac dang xung
14


cua xung
Xung rang cua (hinh 1-9b) duoc dac trung boi hanh trinh thuan (doan ab) va hanh
trinh nguoc (doan be). Hinh chieu ciia cac doan nay tren true thdi gian, tuong ung la thdi
han (do rong) hanh trinh thuan x lh va thdi gian (do rong) hanh trinh ngiroc x,lg. Gia tri
dinh cua xung goi la bien do, ky hieu la A m.
Xung hinh thang (l-9c) co sudn trudc la doan ab, khi duoc chieu len true thdi gian
thi dirge do long sudn trudc x

sudn sau cd chieu len true thdi gian se duoc do rong

sudn sau x2. Bien do A mtinh tir dinh xung. Do rong xung x la khoang thdi gian giua hai
A
diem sudn trudc va sudn sau dat tdi --- . Do do:
X, + X 2

Neu giam t , va x2tdi khong thi xung hinh thang se tro thanh xung vuong. Neu t,=

x2= xn va xcl=0 thi xung hinh thang se trd thanh xung tarn giac (hinh l-9d) vdi do rong x
= x0. That ra, day cung la trudng hop rieng cua xung rang cua khi hanh trinh thuan va
hanh trinh nguoc bang nhau.
Xung ham sd mu (hinh 1-9e) co sudn trudc thang dung sau guim theo ham sd mu,
tuc la:

0
s(t) =

-I
ex

khi
khi

t <0
t >0

Trong do, A mla bien do, con x la hang sd thdi gian cua xung.
x tinh bang cach ke mot tiep tuyen vdi dudng cong d mot diem bat ky, hoanh do
cua no ky hieu la t,, tiep tuyen nay cat true hoanh tai die!m t2, ta co :
x =t2-1,
Ngoai nam dang tren, ngudi ta con dung xung don co cac dang khac nhau nua,
nhu xung hinh chuong, xung hinh sin binh phuong.... Tuy nhien, cac dang xung nay
khong diein hinh lam.

*Nhom xung: la tap hop cua mot sd xac dinh cac xung don cung dang cach d£u
nhau. Cac thong sd cua loai nay la: sd xung k, chu ky lap tai T, thdi han nhom xung T„.
Nhom xung ve d hinh 1-10, vdi gia thiet cac xung thuoc nhom la xung vuong, co do
rong x. Tir hinh ve ta tha'y:

T„ =(k-l)T+ x

15


I ______

?

*

3

AfT)

T

TT
®

—

Hinh 1-10 Nhom xung

*Nhdm xung cot. khac voi nhom xung 6 cho cac xung trong nhom co the khong
cung mot dang va khoang each giua chung la tuy y. Tuy nhien, so xung trong nhom,
dang va gia tri cac thong so cua mdi xung da biet mot cach chinh xac. Khi tmh khoang
cach giua hai xung trong nhom. ngudi ta tmh tu diem cac sudn trudc dat toi mot nua
bien do (hinh l -11).

*Ddy xung tuan hoan : la su lap lai mot cach ddu dan (vdi chu ky T) xung don co
•T
dang nao do. Doi vdi loai tin hieu nay, cac thdng sd la do rong xung t, ty sd — = q goi
T
la do trong (do rdng) cua day xung, con nghich dao cua no d = — = — goi la he sd lap
q T
day.
Doi khi con dung ca nhung nhom xung tuan hoan hoac nhom cot xung tuan hoan.
De' do fudng cac thdng sd xung, ngudi ta dung cac von-met dinh (von-met xung),
may do thdi gian. may do tan so lap lai. Nhung trong do ludng xung. thdng thudng hon
ca la dung cac dao dong ky xung co trang bi cac khoi chuan thdi gian. chuan bien do
theo ca hai true X va Y.
Do yeu cAu tao ra tin hieu xung vdi gia tri cac thdng so cho trudc, ngudi ta thiet ke
cac may phat xung do ludng cung co nhung co cau di6u chinh nhu cac may phat sons
dieu hoa da noi d tren. Mdi may phat xung thudng co nhi£u dau ra rieng biet cho phep
lay ra nhung xung co dang khac nhau.

16


cl. Tin liieu so
Tin hieu sd co ban chat la tfn hieu mang thdng tin dang sd nhi phan bao gdm cac
tfn hieu bieu thi cho so 0 va so 1. De djnh nghTa cac bit 0 va 1, ngudi ta co the sir dung
rat nhidu each. Vf du nhu ngudi ta co the' dung mure dien ap, cu the thdng thudng ngudi
ta thudng djnh nghTa bit 0 irng vdi mure dien ap OV (0-0,5V) va bit 1 urng vdi muc dien
ap 5V (4,5-5,5V). Ngoai ra ngudi ta cung co the quy dinh nguoc lai, mure dien ap thap
urng vdi bit 1 va mire dien ap cao ung vdi bit 0. Ta se xet cu the theo dang djnh nghTa
trudc.
Tfn hieu sd thudng duoc bieu thj dudi dang chudi xung vuong. Neu tin hieu la bit
1, ta se co mot xung vuong co mure dien ap cao, neu tin hieu la bit 0 thi dien ap d muc

thap. Do do, tfn hieu dien ap du la miic cao hay mure thap thi no deu co y nghTa mang tin
tire. Khoang cach giua cac bit cach deu nhau va do rong cua mot bit tin hieu thi hoan
toan phu thuoc vao viec ngudi dung djnh nghTa tfn hieu do. Hinh 1-12 la mot vf du ve tfn
hieu so.

u

Hinh 1-12 Tin hieu so
Khi tfn hieu sd duoc truyen di, ngudi ta co the dung cac phuong phap ma hoa khac
nhau. Tuy theo phuong phap ma hoa cua tfn hieu ma dang cua tfn hieu sd da duoc dieu
che' nay se khac nhau.
1.1.2 Cac tham so va dac tinh cua mach dien tii
Trong qua trinh thdng tin, dieu khien trong dien tu , tfn hieu luon luon duoc bien
doi tu dang nay sang dang khac. To hop cac linh kien theo mot each nao do nhim thuc
hien viec bien doi noi tren goi la mot mach dien. Cac linh kien cua mach dien bao gdm
dien trd, tu dien, dien cam, cac loai den dien tu va ban dUn, IC.. va cac phu kien khac
nua.
Tuy thuoc vao tmh chat cac phan tu duoc su dung, mach dien co the chia thanh
mach tuyen tfnh va mach phi tuyen. V I ban chat hai loai mach nay khac nhau nen cac
thdng sd va dac tfnh cua chung cung khac nhau.

17


/. Mach tuyen tinh
Mach tuyen tinh tao thanh tu nhfrng phan tu co gia tri khong phu thuoc vao dong
dien chay qua no. Mot thuoc tinh quan trong cua mach tuyen tmh la doi vdi no. co the
ap dung nguyen ly xep chong.
Phuong phap do ludng cung nhu thiet ke cau tao cac may do thong so va dac tinh
cua mach tuyen tfnh dua vao hai dac diem tren. Viec do ludng cac thong so va dac tmh

cua mach tuyen tinh la rat pho bien vi trong ky thuat dien tu, ly thuyet mach tuyen tinh
duoc phat trien manh va duoc su dung dac biet rong rai.
Cac phan tu cua mach tuyen tmh thudng la: dien tro, tu dien, cuon cam khong loi
sat; den dien tir va ban din lam vice d tren doan dirdng thing cua dac tuyen. Tuy theo
tan so cua tin hieu can thong qua mach, ma cau tao cua cac phan tu cua mach co khac
nhau va do do mach tuyen tmh lai con duoc chia thanh hai nhom: mach co phan tu tap
trung va mach co phin tu phan bo.
Mach co phan tu tap trung dung de bien doi tin hieu co tan sd khong ldn lam (nho
hon vai chuc megaHz). Nhu da noi d tren, gia tri cua cac phin tu tuyen tfnh la khong
phu thuoc vao dong chay qua no, va do do, co the xem do la hang sd. Ngudi ta lay ngay
cac gia tri nay lam thong sd dac trung cho cac phan tu cua mach. Ban than mdi phan tu
R, C hoac L rieng biet co the: xem nhu mot mang hai cue, cho nen cac thdng sd cua
mang hai cue la gia tri cua dien trd R, dien dung cua tu dien C va dien cam cua cuon day
L.
Neu nhu m ic noi tiep hoac song song cac phan tir R, C va L hoac h6n hop ca hai
cach mic ay, ta se duoc cac mang bon cuc.
Vi mang bon cuc co the gom ca dien tro R tieu thu nang lirong va ca cac thanh
phan cam khang C va L nen de dac trung cho mang bon cue, ngudi ta dung thong sd trd
khang toan phan Z. Dung khang X c va cam khang X L phu thuoc tan sd tin hieu thong
qua mach, nen trd khang toan phan Z cung la mot ham cua tan sd:
Z(o>) = R ld +jXul
Trong do R ld va X,d la dien trd va dien khang tuong duong, gia tri cua chung phu
thuoc cach noi cac phan tii trong mach (tat nhien X td phu thuoc ca vao tan sd).
Mot trudng hop dac biet cua mang bon cue la cac mach dao dong (con goi la mach
cong hudng). Vdi cac mach nay, con co hai thong sd nua, do la: he sd phaim chat Q va
trd khang dac tmh p:

IS

De clo cac thong so vita ke d tren cua mach co phan tu tap trung, ngudi ta dung:
may do dien tro' (om-met), may do dien cam va dien dung, may do tro' khang toan phan
va may do he so pham chat (Q-met).
Ngoai cac thong so tren, de’ dac trung cho phan ung cua mach vdi nhung tac dong
dpt bien ngudi ta con dung dac tmh thdi gian (dac tmh qua do) va cho phan ting cua
mach doi vdi nhung tac dong dieu hoa (hinh sin), thi dung dac tinh tan so.
Khi tim dac tinh thdi gian, ngudi la cho tac
dong vao mach ham don vi l(t) nhu hinh 1-13.

Uvoo <*>

Ham l(t) duoc bieu dien toan hoc nhu sau:
0
1(0 =

khi

t <0

khi

t >0

Dac tmh nay co the: quan sat true tiep tren
Hinh 1-13 Ham don vi

man ong tia dien tu ciia nhung thiet bi dac biet:
nhung may do dac tmh thdi gian. Vdi mot vai loai

mang bon cue don gian, dang cua dac tmh qua do chi ra d bang l . I . Tir dac tmh qua do

nay, co the xac dinh mot vai thong sd ciia mach nhu hiing sd thdi gian, he sd pham chat
Q va tan sd cong huong ciia mach.
Khi tim dac tmh tan sd, or dau vao ciia mach, ngudi ta dat tin hieu dieu hoa.
Vi du:
u,(t)=Amlsin(27rft+Vi mach la tuyen tinh nen no khong sinh ra tan so mdi va tin hieu o dau ra se co
dang nhu d dau vao:
u2(t)=Am;sin(27tft+(p;)
nghTa la khi thdng qua mach dien, chi co bien do A mva pha dan cp ciia tin hieu la
bi thay doi.
Lap cac ti sd

92. . Khi thay doi tan sd ciia tin hieu vao, gia tri cua cac
A m i va —

ty sd nay cung thay doi theo. Goi
mi

=A(f) la dac tmh bien do-tan sd, — =(Pi

tinh pha-tan so.
D6 thi A (f) va (p(f) vdi f bien thien trong mot dai rong cho ta khai niem v6 dac tmh
thdng sd ciia mang. Thdng thudng, ca A (t) va (p(l) bieu dien chung trong lie toa do cuc
(he toa do doc cue). De ve dac tuyen tan sd, ngudi ta dung mot bo tao dao dong dieu hoa

19

Bang 1-1

Loai mang bon circ

Dac tinh qua do va cac thong so
g(t)=A( l-eal )
a) a =
b) u

RC
2L

b)

a)

l

x=\/a
g(t)=A e
a) a =

]
RC

R
b) a = —
2L

b)

o)

x=l/a
g(t)=Ae

sino>,,t

R

a =—

C

2L
1

co,

vLC

o)

b)
. 4 --------------

g(t)= A (l-ae ulcoso)„t)

a =
C±L

R
2L

0)

co tan SO thay doi duoc trong mot pham vi rong, cac von-met va pha-met. Moi khi lay
mot gia tri tdn so moi. ta lai do cac gia tri A :, A : va (timg diem ta se co dac tinh tan so. Trong kv thuat do luong dien tu hien dai. ngudi ta
dung mot dung cu dac biet. cho phep quan sat true tiep dang dac tmh tan so tren man
cua ong tia dien tu. Dung cu do la may tu dong \e dac tmh tan so, nguyen ly cua no se
trinh bay o trong chuong tu dong ve dac tuyen tan so.

20


2. Mach phi tuyen
Nguoc lai vdi mach tuyen tinh, d mach phi tuyen gia tri cac linh kien cua mach
phu thuoc vao cudng do ddng dien chay qua no. VI vay vdi cac linh kien phi tuyen nay,
khong dung khai niem thdng so cua ban than no.
Theo djnh luat Om, dien ap roi tren cac phiin tu phi tuyen se khong ty le thuan vdi
dong dien chay qua no. Tuy co kho khan v6 mat tfnh toan va trong nhidu trudng hop con
bat Ipi ca v6 mat sir dung nua nhung mach phi tuyen khong the thieu duoc trong ky thuat
dien tu. Dira vao tmh khong duong thang cua dac tuyen Von-Ampe cua mach phi tuyen,
ngudi ta dung no de thuc hien mot loat cac bien doi: dieu che, tach song, nhan va chia
tin sd, tao dao dong va han che,.... Tuy timg muc dfch bien doi, phai chon nhung doan
thfch hop cua dac tuyen Von-Ampe, chon cang chfnh xac thi hieu qua bien doi cang cao.
Trong ca hai trudng hop, ro rang phai biet dupe chinh xac dang dac tuyen VonAmpe cua phan tu va cua mach. Quan sat va nghien ciru dac tuyen Von-Ampe cua cac
phan tu phi tuyen dupe thuc hien nhd nhung may ve dac tuyen, mot vf du thong dung
cua loai nay la may ve dac tuyen den dien tu va ban din (d chuong 3, quan sat va do

ludng dang tfn hieu, co d6 cap den van d£ nay).

1.2 CAC KHAI NIEM CO BAN VE DO LUONG DIEN TLf
1.2.1 Khai niem do ludng
Do luong la khoa hoc vd cac phep do, cac phuong phap va cac cong cu dam bao
cho chung, cac phuong phap de dat dupe do chfnh xac mong muon.
Cac hu'dng nghien ciru chfnh cua do ludng bao gdm:
-Cac ly thuyet chung v£ phep do.
-Cac don vi vat ly va he thdng cua chung
-Cac phuong phap va cong cu do
-Phuong phap xac dinh do chfnh xac ciia phep do
-Co sd bao dam cho viec thdng nhat giua phep do va rat nhidu cong cu thuc hien
no
-Cong cu do chuan va barem
-Cac phuong phap de' chuydn don vj do tir cong cu chuain hoac goc ra cong cu lam
viec.

21


Phep do la cong viec thuc hien chinh cua do lirong, do la viec tim ra gia tri vat ly
bang cach thf nghiem voi sir tro giup ca cac cong cu ky thuat dac biet. Gia tri tint duoc
goi la ket qua cua phep do. Hanh dong thuc hien trong qua trinh do de cho ta ket qua l;l
mot dai luong vat ly goi la qua trinh ghi nhan ket qua. Tuy thuoc vao doi tuong nghien
ciru, vao tmh chat cua cong cu do va ngudi ta can thuc hien phep do ghi nhan mot Ian
hay nhieu Ian. Neu nhu co mot loat ghi nhan thi ket qua phep do nhan duoc la ket qua
khi xu ly cac ket qua tu cac ghi nhan do.
Phep do co ban chat la qua trinh so sanh dai luong vat ly can do voi mot dai lirong
vat ly duoc dung lam don vi. Ket qua cua phep do duoc bieu dien bang mot so' la ty le
cua dai luong can do voi don vi do. Nhu vay de thuc hien phep do. ta can thiet lap don vi

do. so sanh gia tri cua dai luong can do voi don vi va ghi nhan ket qua so sanh dupe.
Thong thudng ngudi ta thudng bien doi tin hieu den dang thuan tien nhat cho viec so
sanh.
Nhu vay, ta co the tom tat lai thanh bon budc chfnh cua phep do la: thiet lap don vi
vat ly. bieu didn tin hieu do. so sanh tfn hieu do vdi don vi dupe lay lam chuan va ghi
nhan ket qua so sanh.
1.2.2 Cac phirong phap va bien phap do ludng co ban

I. Cac phuong phap do
Cac phuong phap co ban cua ky thuat do ludng thudng dupe chia thanh:
-Ph.rong phap do true tiep
-Phuong phap do gian tiep
-Phirong phap do tuong quan

Do true tiep la phuong phap dung cac may do hay cac mau do (cac chuan) de
danh gia sd luong cua dai luong do dupe. Ket qua do duoc chfnh la tri sd cua dai luong
ciin do, ma khong phai tfnh toan thong qua mot phirong trinh vat ly nao lien quan cifra
cac dai luong. Neu khong tfnh den sai so. thi tri sd dung cua dai luong can do X se bang
ket qua do dupe a:
X =a
Cac vf du ve phirong phap do true tiep nhu: do dien ap bang von-met: do tan so
bang tiin so-met ... Do true tiep thi phep do thuc hien don gian \'6 bien phap k\ tluiat,
tien hanh do duoc nhanh chong va loai tru duoc cac sai sd do tfnh toan.


Do gian tiep la phirong phap do ma ket qua do dupe khong phai la tri so cua dai
lupng can do, ma la cac so lieu co sd d6 tinh ra tri sd cua dai lupng nay. NghTa la d day,
X=F(a,, a-,,..., an).
Cac vf du vd phuong phap do gian tiep nhu: do cong suat bang von-met va ampemet; do he sd song chay bang day do....
Trong ky thuat do ludng, thong thudng ngudi ta muon tranh phuong phap do gian

tiep,

v 'i

trudc het no yeu cau tien hanh nhieu phep do (ft nhat la hai phep do) va thudng la

khong nhan biet ngay dupe ket qua do. Song trong mot sd trudng hop thi khong the
tranh dupe phuong phap nay.
Hien nay, ky thuat do ludng da phat trien nhieu ve phuong phap do tuong quan.
No la mot phuong phap rieng, khong nam trong phuong phap do true tiep hay phuong
phap do gian tiep. Phirong phap tuong quan dung trong nhung trudng hop can do cac qua
trinh phuc tap, ma d day khong the thiet lap mot quan he ham sd nao giua cac dai lupng
la cac thong sd cua mot qua trinh nghien ciru. Vf du: tfn hieu dau vao va tfn hieu dau ra
cua mot lie thdng nao do.
Khi do mot thong sd cua tin hieu nao bang phuong phap do tuong quan, thi can ft
nhat la hai phep do ma cac thdng sd tir ket qua do ciia chung khong phu thuoc lan nhau.
Phep do nay duoc thuc hien bdi cach xac dinh khoang thdi gian va ket qua cua mot sd
thuat toan co kha nang dinh diroc tri sd ciia dai lupng thfch hop. Do chfnh xac ciia phep
do tuong quan dupe xac dinh bang do dai khoang thdi gian ciia qua trinh xet. Khi do
true tiep, that ra la ngudi do da phai gia thiet he sd tuong quan giua dai luong do va ket
qua rat gan 1, mac dii co sai sd do quy luat ngau nhien ciia qua trinh bien doi gay nen.
Ngoai cac phirong phap do co ban noi tren, con mot so cac phuong phap do khac
thudng dupe thuc hien trong qua trinh tien hanh do ludng nhu sau:
-Phuong phap do thay the: Phep do dupe tien hanh hai lan, mot lan vdi dai lupng
can do va mot lan vdi dai lupng do m5u. Dieu chinh de hai trudng hop do co ket qua chi
thi nhu nhau.
-Phuong phap hieu so: Phep do dupe tien hanh bang cach danh gia hieu so tri sd
ciia dai lupng can do va dai lupng mau.
-Phuong phap vi sai, phuong phap chi thi khong, phuong phap bii, cung la nhiing
trudng hop rieng ciia phuong phap hieu so. Chung thudng duoc diing trong cac mach cau

do hay trong cac mach bii.
-Phuong phap do thdng: ket qua do dupe dinh lupng true tiep tren thang do ciia
thiet bi chi thi. Tat nhien sir khac do ciia cac thang do nay la da dupe lay chuan trudc vdi
dai lupng mau cimg loai vdi dai lupng do.

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