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Theory of Operation-PG 506 (SN 8040000
&
up)
(00)
--.1--
I
COUNTER
I
FULL
I
PULSE
PIN 9 U400C
V
I
I
I
RAMP
I
(00)
(00)
1000115
I
10 00 115
I
- -
200
COUNTS------.~
-.1------
200 COUNTS
------i.~
I
Y
Y
I
I
PIN 3
U470
LATCH
PULSE
PIN 13
U400D
PIN 8
U400C
I~~
_
_
-+-b
I
I ..
I
Y
~I
MEASUREMENT
MEASUREMENT
INTERVAL
INTERVAL
3
3
8
3·
10
Fig.
3-1. DVM timing diagram for a negative input to the integrator U460.
referenc
e
current source is used to discharge C462. For
positive
ramps,
the Latch Pu lse
occ
uring at zero crossing
is transmitted through CSOO and CRSOO to pin 13 of
U400D.
Referen ce currents from both reference sources track
with changes on the 100 V bus and
i
n the same
direction,
all
owing the instrument to be cal ibrated directly for
osci II oscope deflection error rat her than out put am plit ude
from the PG
S06.
The adjustment of R415 calibrates the
HIGH indicat ion, while the adjustment or R425 calibrates
the LOW indication.
CR395 and CR397 protect components in the DVM
circuitry if the
+
and
-
16.5 V input connections are
accidentally reversed
.
Period Generator and Display
0
The Period Generator circuit consists of six transistors
(0575,
OS80,
0585,
0595,
0605,
and 0610) and timing
3·4
capacitor C580 in series with main timing resistors R587,
R590,
and RS93. These components set a basic 0.5 {.Js
period (2 MHz square-wave signal) for the entire instru
ment.
The timing capacitor and resistor(s) have a common
connection at the base of 0580. Th
e
Signal on the base of
0580 is basically a linear ramp (with switching transients)
that causes the output l evel at the collector of 0605 to go
high and low when the ramp crosses the hysteresis limits
of the circuit.
Assume that when power is applied, OS75 turns on and
0580 turns oft
.
This action also turns off 0595, setting one
end of the timing resistors to the saturated level at the
collector of 059S. The base of OS80 now begins a ramp
rundown toward the lower limit of the hysteresis
window.
C580 is charging toward a S V supply through RS82 and
the timing
resistors.
When the ramp at the base of OS80
crosses the lower limit of the hysteresis
window,
OS80
turns on and 0575 turns off
.
This action turns OS85 on and
@
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