T he least sig n ific an t digit on th e re a d o u t. C o u n tin g re so lu tio n d iffe rs w ith
gate tim e . A t a g ate tim e o f 1 sec o n d , th e re so lu tio n is 1 H z w ith a ty p ic al
c o u n te r.
D irect C ou n tin g
T he d ire c t c o u n tin g is th e m o st fu n d a m e n ta l m e th o d to m easu re th e fre q u e n
cy . (S ee th e fig ure b elo w .) T his sc hem e is w id ely used fro m th e au d io fre
q u e n c y b a n d to th e U H F b a n d . In th e d ire c t c o u n tin g m e th o d , th e u p p e r
lim it o f fre q u e n c y m e a su re m e n t is d ete rm in e d b y th e gate tim e a n d th e
fre q u e n cy re s o lu tio n . E n h a n c e m e n t o f th e p erfo rm an c e o f th e se m ic o n d u c
to r devices a nd a dv a n ce d c irc u it b o ard te c h n o lo g y hav e realized a c o u n te r o f
1 G H z utilizin g th e d ire c t c o u n tin g te ch n iq u e s.
Counting Resolution
E x p an din g R ecip ro cal M e tho d
T he m e th o d used b y e le c tro n ic c o u n te rs to m ea su re a p e rio d , e x e cu te inverse
c alc u la tio n (1 /p e rio d ), an d disp la y th e fre q u en c y is called th e recip ro cal
m e th o d .
T h e m ain fe a tu re o f th is m e th o d is th a t, in p e rio d m ea su re m e n t, it en ab les
fr eq u e n c y m ea su re m e n t o f h ig h -re s o lu tio n an d h ig h-p re cisio n u p to th e ord e r
o f th e in te rn a l tim e b a se. F o r ex a m p le , le t th e tim e base be 100 x 10‘9s, th e n
7-d igit disp lay is alw ays p o ssib le w h en a fre q u e n cy (1 0 M H z or less) is m e a
su re d a t a gate tim e o f 1 se c o n d . T o o b ta in a 10-digit disp lay at a gate tim e o f
1 se co n d w ith th is m e th o d , th e in te rn a l tim e base m u s t be 100 x 10’12s
(e q u iv a le nt to 10 G H z). T o re alize a 10 G H z tim e b ase, th e tim e e x p a n d e r
m e th o d is used to g eth e r w ith th e re cip ro cal m e th o d , th u s en ab lin g a high-
re so lu tio n hig h -p re cisio n fre q u en c y m e as u re m e n t. T his m eth o d is called th e
e x p a n d in g rec ip ro c a l m e th o d . (* S ee T im e E x p an d e r M eth o d .)
G ate T im e
T he tim e d u rin g w h ic h a c o u n te r m e asu re s th e in p u t signal. D urin g th is tim e,
th e G A T E lam p usu ally go es on to n o tify th e u ser th a t th e in p u t sign al is
b ein g m e asu re d .
Gate
Measurement in p u t o-
Tim e base o-
A—5
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