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tr
Page
6
As the
trigger level is
increased
either positive or
negative,
ittenuation of
input
signal
is
introduced
'
The slope
error
increases as attenuation
is intro-
duced. At
a
trigger
level of
I00
volts
the
attenuatiorl
is
approximately
5
to
I,
at
30
volts
about
2'7 to
I'
and
at l0 volts about
I.6
to
l.
Example:
A
measurement
of
time
inte.rval
is
be
ing
made where
the
stalt
voltage has
a
rise of I volt in
120
lrsec
and
the stop voltage has a
rise oI
L
voLt
in
5
usec.
at
the
trigger
point.
If
the
time
measured
is about
500
milliseconds
when
counting
IOO
kc, the Iargest error
would
be
tachometry
measurements.
The @Model 506I}
is
a
photoelectric tachometer
head
and the
@Model
508A
is
a
60 tootJ)
magnetic tachometer,
The
ModeI
5O8A
is
mechanically
connected
to
the
shaft being measured
and
produces
60
output
pul'ses
per
revolution.
An indication
oi rpm is
given
di-
iectly
when
the output frequeocy
ol the tachometer
is measured. With
a
I
second gate
tjme,
an
accu-
racy
of
,I
rpm
rs
obtained.
Usiflg a
l0
second
gate
tim;
-IlI0
rpm accuracy may
be
obtained'
The
Model 5068 photoelectric tachometer
head
is
designed
lor
use
where mechanical linkage
is im-
pract ical.
The
light
source
and
phototube
u""".r0,,
should
be
adjusted
so
that the
light
reflected
from the rotating
member
rvill
enter the phototube. The number of
pulses
or
counts
per revolution
will
depend upon
thc
ilesign oI
tie
rotating
n)ember.
Alternate
bLeck
ancl
reflecting strips may
be
placed
'ln
the
rotaUng
s
halt
or
drsc
and
the
speed
wiII
be:
1.
00I
percent
oJ 500
rms
r
period of I00
kc
+ (I20
usec. +
5
usec,
)
Maximum possitrle
e
rror
= +5.
0 usec.
=
t10
use
c.
= +125 usec.
=
i40
usec.
There is
a
maximum
error
of
140
usec. out of
the
500.00
milliseconds measured, Ulis is 4
maxrnlum
e
rror
of .028
Percent.
2.4
APPLICATIONS
.IIgSuu-!!r_lggggjl9I11t"
Measru
ement
oI frequency
is a
hitel| l.rit
1,,:
.r,
)Jn-
ters.
Such
measurements as
osciiLator ca,lll)ration,
oscillator
drilt
with time, or
drilt
with ljn.-
voltaqo
n1ay
be
made.
Frequency response
characierietlcs
of
tuned
circuits,
band
pass
lilters, etc.,
can
be
plotted
Some
specific frequency
moasurement
.lppli
-
cations
are described belov
.
Repetition Rate
oi
Pulse.
s
The
repetition rate of
pu-Ises
is merely
counting
the
pulses
for
a standard
time.
The input
of the counter
is
direct colple
d 3nd
adjusted
to
triggcr
at
zem volts.
Since a
p,lse ls
essentia-lly
all
positiye
or all
nega-
Live
lrom
a
reference voltage,
it
may
be
necessary
to introdwe
some
dc
bias to
enable t])e
pulse
to
c.ross
the zero
axis.
With
tlis
in
ntind repet
ition fates
clin
be
measured
in the same manner
as anv
r)thor
fre-
quency.
te9-!9l!!1ry
There are a number
of
transducers a\.ailable
for
Speed
(RPM)
Countcr indication
x
{i0
=
FirniE;-i of
rcn,'cilnr iir
ips
:
Gate
tinle in
sec.
Prr rs"
-.
a,.
-",
r-T_11ll
i
ir(,
\'ibrating wirc
tj-pe tr'ansducer
is
a
pressul'e-
s(,nsiti.,,e
elemeDt
n
ii-i)
an
associated ieedback
ampli-
lie..
Its
output
is
a
frequency
which
is detcrmined
by
ilie pressure
exerteC on
ttie Yibr.ating
wire cell
at1d
has
a
resolution oI ailout ,02 percent
flrr
or:tout
of the IccJl)ack
ampIfier
is
connectpd to
ih€ iri,unter
to
read frequencY.
S{ch
triusci!:ccrs
can
ir
used
in rerDote Iocations.
Since
the indication
is
a
singl.'frequency,
no
alter-
.ttion
oI illformation
can
be caused by
transmissio
n.
Srniil;rr units
can
irlso be obtained
fol
ir,dicating
trrt)pe
rature,
Exterldcd Gate Time
It is
sometimes desirable to
me
asure a signal
for
a
Ionger time than prcvided
by
the
counter in
order
to
obtain an average
of fluctuating signal
or
to obtain
greater
accuracy. This
may be
done
on
eitherire-
qucn(
J'
or
period measllrentent.
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