reliable if th e in p u t signal is in te rfe re d w ith no ise o r u n sta b le . T o solve th is
p ro b le m , c o u n te rs have a fe atu re to average 10 o r 10 0 m e as u re m e n t valu es to
re d u ce th e in flu en c e o f no ise an d in p u t v a ria tio n . T h is fu n c tio n is called
th e tim e in te rv a l average a n d p e rio d average to d istin g u ish fro m a single
m e as u re m en t. T im e re q u ire d fo r average m ea su re m en t is as m an y tim es
lo ng er as th e n u m b e r o f averages ta k e n .
T ra h et M eth od
A D V A N T E S T ’s p a te n te d te c h n iq u e (U S P A T . N o. 3 9 3 2 8 1 4 ).
T his m e th o d uses a Y IG tu n in g o s cillato r w ith ex c e lle n t lin e a rity , ta k in g
ad van tag e o f th e tra n s fe r o scilla to r a n d th e h e te ro d y n e c on v ersio n te c h n i
q u es.
T rigger Level
W hen a signal is in p u t to a fre q u en c y c o u n te r, th e in p u t signal m u s t cross a
ce rtain lev el (a lso called th e th re sh o ld valu e) fo r th e c o u n te r to sense it as a
signal a n d m ea su re it. T h is level is called th e trigg er level. T he level can
u su ally be varied w ith a p o te n tio m e te r, e tc .
T rigger M o n ito r O u tp u t
A signal o u tp u t fro m th e trig g er m o n ito r circ u it as an au x ilia ry m ean s w hen
a c o u n te r is m e asu rin g tim e in te rv a l. A n o scilo sco p e (w ith Z-axis m o d u la tio n
te rm in a l) sh ow s in te n sity m o d u la tio n o n th e w av efo rm fo r ea ch gate tim e .
T he m e as u re d p o rtio n o n th e trac e is in te n sifie d as sh o w n be lo w .
Gate tim e
T rigg er S lo p e
F o r a fre q u e n cy c o u n te r to sense an in p u t sig nal, th e in p u t sign al sh o u ld
m e et th e tw o re q u irem e n ts. O ne is th a t th e sig nal m u st cross th e trig ger
level, a nd th e o th e r is th a t th e slo p e o f th e in p u t sign al m u s t m a tc h th e p rese t
trig ger slo p e . W ith th e trig ger slo p e set to p lu s (+ ), th e c o u n te r senses th e
in p u t sign al w he n th e in p u t signal cro sses th e trig ger level fro m m in u s ( - )
to p lu s (+ ).
A—8
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