Character Generator - Tektronix 213 DMM Instruction Manual

Theory of Operation-213 Service
+15 V INTEGRATOR
REFERENCE
VOLTAGE
VR217, CR217A
THRESHOLD
DETECTOR
CLOCK
0240
0245
0247
+15 V
REFERENCE
CURRENT
R234
R235
R236
CR217B
SWITCH
0254
C227
0230
COUNT
INPUT
TRANSFER
CONVERTER
LATCH
U250C & D
INPUT COUNTER
BCD OUTPUT
U260
• ---TO CHARACTER
GENERATOR
COUNT
EXTEND
_ _ _ _ _ _ __.OUTPUT
1481-11
Fig. 3-3. A/D Converter detailed block diagram.
that the unknown potential is applied at all times and not
switched off during capacitor discharge. A current directly
related to the unknown input is applied to an integrator
which causes a ramp. When the counter reaches the trailing
edge of the 9999 count, a reference current of the opposite
polarity causes a ramp in the opposite direction. When the
opposite ramp begins, the counter is at 0000. When the in-
tegrated waveform reaches a negative threshold, the number
in the counter is stored as BCD data, and the reference cur-
rent is turned off. Upon receiving a scan input command,
this stored BCD count is forwarded to the Character Gener-
ator and subsequently displayed as the value of the measur-
ed input.
The DMM reference consists of VR217, CR217A,
CR217B, R235, and R236. Temperature-stable zener diode
VR217 generates a reference voltage (with respect to the
+15 V power supply), which is filtered and applied to the
positive input of the integrator U220. This voltage also
appears at the negative input of U220 and across resistor
string R235 and R236, which causes current flow through
them and CR217B when 0254 is not conducting. 0254
acts as a switch to turn the reference on and off. When
0254 is conducting, CR217B is reverse biased and the cur-
rent flow is through 0254 instead of CR217B, which essen-
tially disconnects the reference from U220. Rectifier
CR217 A is used to temperature compensate CR217B.
During the DC Zero calibration adjustment, R234 increases
the A/D Converter sensitivity by a ratio of about 10 to 1.
Refer to the A/D Converter timing diagram, Figure 3-4 and
the schematic diagrams for the remainder of the A/D Con-
verter description.
Emitter coupled multivibrator 0240 and 0245, generates a
clock pulse of approximately 70 kilohertz which is applied
@
through buffer 0247 to pin 7 of the four decade counter
U260. During the count, C227 is ramping in the positive
direction in response to the unknown input. After the coun-
ter reaches a count of 9999, pin 8 of U260 is pulsed HI and
then falls LO on the trailing edge of the next clock pulse at
which time the counter returns to count 0000. The trailing
edge of the pulse from pin 8 is differentiated by 0258,
C258, R258, R252, and R253, and appears as a LO pulse
at pin 9 of U250C. A LO at pin 9 of U250C causes a HI
on pin 8 which turns 0254 off, and a HI on pin 13 of U250D
causes pin 11 to go LO. Pin 9 of U250C returns HI at the
end of the differentiated pulse from pin 8 of U260 which
causes no change. With 0254 turned off, the reference cur-
rent causes U220 to integrate both the input and reference
currents as a negative ramp. When the negative ramp goes
low enough to turn on CR230 and 0230, pin 12 of U250D
goes LO. This LO at pin 12 causes a HI at pin 11, a HI at
pin 10 of U250C, and a LO at pin 8 of U250C, which turns
0254 on. Also, this LO is differentiated by C255 and
R255 and applied to pin 5 of U260 which causes the BCD
count of the counter to be transferred to a storage register
(latch) where it remains for display until the next transfer
pulse. Whenever pin 9 of U260 receives a negative pulse
from pin 1 of U270, the BCD data is transferred serially to
the Character Generator one character at a time. As each
character is transferred, the output from pins 1, 2, 3, and
4 of U260 provides spacing current to the horizontal am -
plifier through 0260, 0262, 0264, and 0266. This spacing
current establishes the horizontal position on the crt where
each character is displayed.
Character Generator
The Character Generator is a Tektronix integrated circuit
device that provides X, Y, and Z signals to the crt deflection
system to form the numerical readout display . The Charac-
ter Generator provides X and Y signals to form the seven
3-5