| 摘要 |
A position reference voltage for controlling the electron beam of a display in an ultrasonic scanning system includes circuitry for receiving a position voltage from a scanner in the system, and a voltage from a variable voltage source. When the variable voltage exceeds the position voltage the variable voltage is locked whereby a new position voltage for the display is derived from the sum of the locked voltages and the scanner position voltage. The variable voltage source includes a digital counter and a digital to analog converter. A comparator compares the sum of the variable voltage and the scanner position voltage, with a reference potential and a flip-flop responds to the comparator to disable the counter when the summed voltages exceed the reference potential.
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