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26
Circuit description
C. TIME BASE GENERATOR (unit D, Fig. 31)
1. Principle
The sawtooth voltage is obtained by means of a Schmitt trigger and a
"bootstrap" integrator. The working of the Schmitt trigger is the same as
that of the trigger described in chapter B "Trigger pulse shaper". By
means of potentiometer R3 ("STAB.") the multivibrator is adjusted in
such a way that it can operate both free running and triggered. R3,
however, must be so adjusted that valve B503' is cut-off and B503 con-
ductive (in that case no time-base line is visible on the screen). In this
condition, the control grid potential of B503 is equal to the cathode
potential of this valve, because the control grid is connected to the positive
supply voltage via R530-R53I and GR503. The working principle of the
"bootstrap" integrator is shown in the figure below.
R
r,
1
_______
1
Fig. 12. Block diagram of the "bootstrap" integrator
PEM 292
E126
This circuit ensures that the charging current of capacitor C remains
constant, independent of the charge of capacitor C, so that the charge
curve is of good linearity.
A voltage variation across capacitor C is applied to resistor R via an
amplifier with an amplification factor A & I and battery E.
The voltage across resistor R - and thus the charging current through the
capacitor - therefore remains constant, which results in a linear charge
curve of the capacitor.
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