o f a m e a su re m e n t to th e s ta rt o f th e n e x t m e as u re m en t b y using th e sam p le
ra te fu n ctio n . T h u s , th e d ata disp lay tim e c an be alte re d by vary ing th e
sam p le ra te .
rate
Data display tim e = Gate tim e + Sample rate tim e
T im e B ase, In te rn al/E x te rn a l
F re q u en c y c o u n te rs are used fo r m e asu rin g tim e o r c o u n tin g th e n u m b e r o f
pulses d u rin g a c erta in p e rio d o f tim e. T o o b ta in an a c cu ra te m e as ure m e n t
re su lt, a tim e b ase g e n e ra to r is n ee d e d to ge n era te an a cc u ra te tim e. M ost
c o u n te rs in c o rp o ra te a c ry sta l o s cilla to r as th e in te rn a l tim e base g e n era to r.
T h e a cc ura c y o f th is g en e ra to r d e te rm in e s th e acc ura cy o f th e freq u e n cy
c o u n te r.
If a g e n e ra to r w ith g rea te r ac cu ra cy th a n th e b u ilt-in g e n e ra to r is e x te rn a lly
ava ilable, g re a te r m e a su re m en t a cc u ra cy o f th e c o u n te r is o b ta in e d b y re p la c
ing th e in te rn a l g e n e ra to r w ith th e o u tp u t o f th e ex te rn a l g e n era to r. Th e
o u tp u t o f th is e x te rn a l g e n e ra to r is called th e e x te rn a l tim e b ase.
T im e E xp a n d er M eth od
T he ± c o u n t e rro r ca used b y th e re la tio n s h ip b e tw e e n th e ele c tro n ic c o u n te r
in te rn a l tim e b ase (f o r e x a m p le , 10 M H z) a nd th e tim e in terv al to b e m ea su r
ed or 1-cycle tim e is used as a sig n ifica nt tim e v alu e. L et th e d iffere n ce tim e
o cc urrin g a t th e lea d in g edg e o f th e tim e to be m ea su re d be A T j a nd th e
d iffe re n c e tim e o c c u rrin g a t th e trailin g edge be A T 2 , th e n th e tim e to be
m e as ure d T x = N -T 0 ± A T t — A T 2 (w h e re T 0 = in te rn a l tim e b ase, N =
p o sitiv e in teg er). A T X — A T 2 c a n b e re ad a t a b e tte r p rec isio n b y a fa cto r o f
100 o r 1,0 0 0 b y co n v ertin g th e d iffe re n ce tim es to a na lo g vo ltag es b y a h ig h
sp eed tim e-v oltag e c o n v e rte r, th e n A /C co n v e rte d w ith a h igh sp eed an d
high p rec isio n . A ssum in g th e tim e base to be 10 0 x 10*9s, T x is eq u a l to
re so lu tio n 1 x 1 0 '9 s o r 10 0 x 10‘12s. T h is m e th o d o f e x p a n d in g th e d if
feren c e tim e is th e tim e e x p an d e r m e th o d .
T im e In terv al A verage, P eriod A verage
C o u n te rs c a n m e asu re p erio d a n d tim e in te rv a l. W ith a single m ea su re m en t,
th e disp lay is u n stab le a nd d iffic u lt to re ad o r th e m e as u re m e n t valu e is n o t
Gate tim e
Data display
tim e
A—7
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