T h ere are tw o in p u t c o u p lin g m e th o d s : th e A C c o u p lin g th a t c u ts o u t th e
DC in p u t signal a n d passes th e A C co m p o n e n t a lo n e , a n d th e DC co u p lin g
to m easu re lo w fre qu e nc ies.
M asking
W ith a m ask in g fu n c tio n , reg ard less o f th e m a g n itu d e o f n o ise , d esire d signal
alon e is m ad e available b y in h ib itin g fo r a req u ired p e rio d o f tim e th e w av e
sh ap ed o u tp u t. B y ad ju stin g th e m ask in g tim e , th is fu n c tio n m a ke s po ssible
th e m e asu re m e n ts o f th e signal in no ise in clu d in g a c h a tte rin g n oise o r th e
m o d u la te d w ave sign al.
O ven Lam p
A lam p th a t in d ica te s a ctiv a tio n o f th e c ry stal-o ven h e a te r a n d th e in te rn a l
re fe re nc e circ uit w hen th e c o u n te r is c o n n e c te d to th e AC p o w e r so u rc e
regardless o f th e PO W E R sw itc h o p era tio n .
Prescaler
T he presc ale r divides th e in p u t freq u e n cie s b y a fa c to r o f n w ith a div ider
fo r c o u n tin g . (S ee th e figu re b elo w .) In th is ca se, th e c o u n tin g re su lt is
1/n o f th e ac tu a l fre q u e n c y ; th e r e fo re , th e tim e base is m u ltip lie d b y n to
d isp lay th e fre q u e n cy m e a s u re m en t. T his re qu ires a gate tim e e q u al to n
tim es th a t re q u ire d fo r th e d ire ct co u n tin g m e th o d ; w ith th e sam e gate tim e ,
th e re so lu tio n is 1/n . In th e presc ale r, th e u p p er lim it o f th e m e asu re d fre
q u en c y is d e te rm in e d b y th e fre q u e n cy re so lu tio n o f th e d iv ide r. T h e g ate
is o p e ra te d by th e 1/n fre q u e n c y , e n ab lin g m ea su re m en t o f h ig h er freq u en cies
th a n b y th e d ire ct c o u n tin g m e th o d . A t p re se n t, a 1.5 G H z pre sca lin g c o u n
te r is available.
Input Coupling
Divider Gate
Sam ple R ate
A fu n ctio n to c o n tin u o u sly vary th e disp lay tim e o f th e m e as u re m e n t re su lt.
As a m a tte r o f fa c t, th e gate tim e is d ete rm in e d b y th e re so lu tio n o f th e
c o u n te r. T h e d isp lay tim e can be ch an g ed b y v ary in g th e tim e fro m th e e nd
A—6
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