HP 214B - SERVICE BLOCK 3 TIMING BOARD A2

Model 2148
Service
SERVICE BLOCK 3
TIMING BOARD A2
THEORY OF OPERATION
Rate Generator
As can be seen from Figure 8-3-1, the rate generator is essentially a voltage-controlled oscillator,
comprising
the following functional blocks:
-
adjustable constant voltage source U 12a
- constant current source U12b / 029
- constant current sources U13a/ 030 (negative) and
U13b / 031 (positive)
-
ramp capacitors C36/C31 / C38 /C39/ C40 /C41 / C45
-- transistor switch 032 / 033
- Schmitt trigger 034 / 036
POSIT I VE CURRENT
CONTROL
PO S ITIVE CURRENT
SOU RCE
2I
034/036
L-----I U 13 A
all 1EXTERNAL TRIGGER
SWITCH FOR
DISABLING
SCHMITT TRIGGER
NEGATIVE CURRENT
CONTROL
Figure 8- 3-1. Functional diagram of rate generator
NEGATIVE CURRENT
SOURCE
+V
Voltage control is achieved via RATE VE R N I ERR 1 07 which adjusts the constant voltage source U 1 2a.
In accordance with the output voltage at U12a/pin 7, the current through 029 is varied and generates
changing voltages across resistors R 118 / R 113 (Note : an equal voltage drop across each resistor). These
voltages then control current sources U 13a/ 030 and U 13b / 031.
The positive current source is switched on and off the ra mp node (034 base) via 032 / 033 wired as a
current switch. With the positive source delivering a current 21, a current I charges the ramp capacitors
(switched in by pushbutton selection at S2a) and a current I flows into the negative source. (The 2: 1
ratio
of positive to negative current is determined by resistors R 120 and R 117). When the voltage at the
ramp node reaches the Schmitt trigger threshold voltage, the Schmitt trigger changes state and switches
8-
15
Scans by Artekmedia => 2009