Advertisement
ll
3
i.
".T
I
E
i.
Maximum Possible
erl
or-
(per.ccltt)
=
+1
count +
accur'acy
()1
standard.
Sect.
II
Page
5
The
leriod
accLrril(.y
Ior
a
sine
wave
is
as follows:
Max.
Elror
in percent
-
23
x
Peak
noise l'oltage
No. of Periods
x
RMS
signal voltage.
Both
intcmal
and
externtrl noise
l,olttges cortribute
to the
errors.
Th.'
internal
noise voltage
is
approxi-
matclv
5
Inv
peak.
Therefore, during
PERIOD mea-
surentorts
lu
accuracy
oI .3
percent can
be expected
for
a
1
volt
rms sine
waye
with
10
mv
noise. This
acculac!'
can
bc improved bv greatcr amplitude
of
input
sign:rl
or.
l)v nteasuring a greater nurnber
of
periods.
Thus
Ior..r
I
volt rnts
sine wave on
10
pE-
iiIOD AVERAGE the
accuracy expected would be .03
percent.
On
10 PE1UOD
AI!.EnAGE wit}t
a
10
volt rms
sigral
containing
1
per.cent
noise
the maximum
pos_
siblc
eIroI
would
tiren be .003 percent.
The
line rrarked
1
Period
-
CouDting
100
kc on
Figur.e
2-3
is also
for
10
Period
-
Counting
l0 kc.
Simtlarlv
tlr.
ilr.xl I)wFr line tdotted lin,.pu6nir* fronr
I
kc
to
.0l
pcrcent)
is
1
Period
-
Counting
10
kc arrl also
10
periol -
Colnrting
1
kc.
The
ne\t line
is
1
period
-
Cour)ting
1
kr:
rrnd
10
period
-
Counting
100
cps. This
chart
couid be
extencled dorvn and
to the left to
a
o)eirsurcrl ir.equency
oI
.0001
cps.
It will
bc noted
that the period curves
and
the
lre-
quency
curves cl.oss at
316
cycles n]easured
Ire-
quency.
Thus,
Iol
lrequencies
abol,e
316
cycles
diIect
measuremcnt of
frequcncl.gives the greatest accu-
racy.
Rclow
31fj
cvcles period me&surements
should
be
used.
Til,ile
Interl,tl
Measu
r.e
ments
There
ar-e
tllt'ee facto.rs contributing to the accuracy
oi
I
in)e
rntcL.,l
mc!sLlrclllcnts
a.
'I'he
stal)ilitt
oI the
100
kc standard which is
+.
001
pe
tcent.
b.
-I'he
aml)igirilv of
+
1
count.
c.
A tinte'
errol
due
to
input slope.
Where
ttle S'I'AIiT
and
STOp
inputs
are sloping
waves,
the
largest
erro[
is usua]ly
due
to the
inability of
the
input
circuits
to
trigger
consistently
at
the
same
lx)int.
Wilh ir setting o[ zero
lolts tligger level,
the
overall
aucuracv should
be:
MiixiDlurn
eIr()1
a.001 percent + perlod
of
counted
lrcq. ,
sum
ot-Iirncs requrrcd fur
start
and stop
voltages
to
change
1r'olt.
,"{
I
..,
i
Figure
2-3.
Accu
ra(jv
CI1at1
The mlL\inrunt possiblc
er'ror Iot'thc
20
kc
exaDtl)le
is
,.0005 percent
+.
001
percent.
ti)e
L!l,(,
l).,ir)g
addi-
tiYc in the worst
case. Tl]ercfore,
the
grcatest
p.Js-
sii)le
eIrol'
would
be
.
0015
percclt.
Due
to the possible
err.or oI
+1 coLrnt. as the
llteas-
ured frequency
ltcornes
lotvei
t]re
percertlrtac
crr.or
lrc(
on)os
hjSlrer. For
I
l0-r v,
lL,
!v.r.c
JI
I
l.ri(,c,,rtrl
gate
the
accuracy is
+10
percpri. For
Ioiv frequt_,n(ics
the
period of the
wal
efornr
shoulil lrc
rrrcasiirecl.
PeIiod
Measu
rements
On
ipth
PERIOD and 10 PERIOD
AVERAGE the
"I.IMF
UNTI switch
will
sel{lct
the counted
lreqLrctrcies
.rhich
range
fi'oln
100
kc
to
1 cps.
Ileferring
again
to the chart oI Figure
2-3. it
will
be
loted that the
gr.eatest
accuracy of
ptriorl
nreas-
urement
is
obtaile<l
oD 10
period averagc
lvhen
coutlt-
ing
100
kc.
This of course
gives the
grealest t.)tal
nuntber-
of
counts.
It
should be
stressed at this point that the greatest
accuracy
for
peri(xl measur.,ment is
ol)tainecl
fr.om
wavclorms having lr sharp
rise
tiDte
or.hrgh
slopr_.
as
they pass through the zero
axis.
Squar.e
*,avi,s
or
pulses
usually
meet
this
requirenlent.
On
a
sine
wave
or other low slope lvaveforlt]
th.
accu_
racy of period measurenlent
is
limited
b,,,
the
atrilitv
of the input
circuits to
triggcl
at the sarnt, poiirt
on
consecutive cycles,
Advertisement
