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Theory of Operation
1780R-Series Service Manual
3–97
Diagrams 37 & 38 Timing Cursors & Counters
The timing cursors for the 1780R-Series are narrow bright-up pulses applied to
the CRT cathode. They can be moved independently in 5-nanosecond intervals.
The Microprocessor keeps track of the time difference between the two timing
cursors and displays the difference on the screen. The timing cursors are
implemented by starting an accurate oscillator at sync pulse time and digitally
counting cycles until the selected time delay is reached. The oscillator frequency
(10 MHz) gives 100-nanosecond time intervals when cycles are counted. A fine
Analog Delay is added after the final digital count providing delay increments of
5 nanoseconds. The digital delay, however, must remain accurate over a two-line
interval of 128 microseconds. To produce an accurate delay over a two-line
interval requires that the oscillator be accurate in frequency. The frequency
accuracy needed dictates that a crystal oscillator be used, but a crystal oscillator
Overview