Tektronix 1730 Series, Instruction Manual

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1730–Series Theory of Operation
4–13
The DC Restorer back porch pulse is generated by a one-shot, U844A. When
SYNC occurs, U844A is cleared. The next line rate pulse output from U844B
will start the one-shot, whose pulse width is determined by C848 and R849. The
output pulse from Q99, an emitter follower, is time coincident with the back
porch of the input sync signal. The Q output of U844B sets the position of the
pulse, and C848 and R849 determine the pulse width.
U947A is an integrator whose output is normally low. The broad pulse in the
vertical interval will cause its output to ramp up. When the broad pulse ends,
and the serrated pulses begin, the output starts ramping back down. This
negative-going signal is coupled through C853 to comparator U753B, to output
the vertical rate sync pulses (V SYNC).
Under normal operation the base of Q856 is pulled down by CR955, which
causes Q855 to saturate and ground the amplifier output. When exerted the RGB
ENABLE, from the REMOTE connector, is inverted to provide a discharge path
for the emitter of CR955, which enables the RGB Staircase Amplifier.
The staircase signal from the REMOTE connector drives an operational
amplifier composed of Q856 and Q855, whose gain is approximately 0.5. The
amplifier is compensated, for optimum step definition (transient response), by
C953. R856, the RGB Offset adjustment, compensates for input dc level
variation.
U535 is a 16-input, registered gate array containing a D-type flip-flop. Line rate
sync (from U644) drives its Data input, which is clocked through by vertical
sync from U753B. Because of the half-line offset between fields, in the vertical
sync, alternately a high or a low line rate sync level is clocked out to enable the
flip-flop outputs. FIELD is a frame rate square wave that is high for one field
and low for the other, that provides field rate timing information to the Micro-
processor. The V TRIG output triggers the Field Rate Sweep Generator. There
is no field identification for RGB and Remote Sync operating modes.
Non-standard sync inputs may cause the field identifying circuit to stop
producing a field rate trigger signal for the Sweep Generator. When this
happens, V SYNC is automatically used for triggering (without any field
identification taking place). Q806 detects the absence of field identified vertical
sync. The field rate square wave output as FIELD, keeps C906 charged, which
holds pin 19 of U535 high when there is field identification.
Vertical Sync Generator
RGB Amplifier
Field ID and Trigger Select