advantest corporation TR5821 - Page 71

T h e re cip ro c a l sc he m e, w h ic h c alc u late s th e inverse o f th e p e rio d m ea su re 
m e n t re su lt, is v ery ec o n o m ic al an d q u ic k in re sp o n se fo r m e a su rin g low
fre q u en cie s w ith hig h p rec isio n . In th is c o u n te r, th e m ic ro p ro c ess o r, o p e ra t
in g as a c o n tro lle r, p erfo rm s in verse ca lc u la tio n a n d disp lay s th e fre q u e n cy .
T h e disa dv an tag e o f th e re c ip ro c al ta k in g c o u n te rs is th a t n o ise ad d e d to th e
signal d ire ctly affec ts th e ac cu ra cy o f th e m ea su re d signal. A ssum e su p e rim 
po sed no ise as illu stra te d b e lo w , a n d th e er ro r (ca lled th e trig ger e rro r) is
as fo llow s:
rp . _ A P _ E n ^
T rig ger e rro r ~ " y " ........................................................ ^
w h ere: En = N oise v oltage
Es = V oltag e o f th e signal o f in te re st
5-3-2. Frequency Measurement (FREQ. B)
T h e lo w er th e E n/E s ra tio , th e g rea ter th e a cc u ra c y . T h e c o u n te r n a tu ra lly
co n ta in s in te rn a l n oise w hic h m u st also b e c on sid e re d in d e te rm in in g a cc u ra 
cy. T h e in te rn a l n oise in this co u n te r is 1 00 /iV rm s o r less. T h e re fo re , if th e
signal to be m e as ure d do es n o t hav e a n y n oise an d th e in p u t vo ltag e is 100
m V rm s,
T . lOOxlO"6 ^ 4 /57\
Tngger error - TTTooTTF5" - 3-2 * >0 ..................... ©
T o o b tain g re ate r ac cu ra cy , a n average m e as u re m en t is n e e d e d . T h e trigg er
e rro r is av e rag ed b y e x p a n d in g P in th e ab ov e figu re b y 1 0n p erio d s:
T rig ger e rro r = x j g - ........................................................... ®
T h e c o u n te r p e rfo rm s th is a u to m a tic a lly ; th er e fo re , th e h ig h e r th e fre q u e n c y ,
th e g re ate r th e m ea su re m en t a cc u ra c y . T h e rela tio n sh ip b e tw ee n th e fre q u e n 
cy , m e a su re m en t tim e a nd th e n u m b e r o f p e rio d s is sh o w n o n th e n e x t page.
5-8