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Fast Sweep Intensity Control @
To provide uniform intensity between the fast sweep trace and the other information displayed
on the CRT, the Fast Sweep Intensity Control circuit generates a voltage to control the CRT
beam intensity (Z-axis) when the fast sweep ramp is multiplexed in.
The FSZ voltage is dependent on the duty cycle of the fast sweep ramp; that is, the ratio
of time during which the ramp is in progress to the sum of dead time and sweep time. FSZ
varies from about 150 mV (low beam intensity) for high duty cycle operation to about 2 V
(high beam intensity) for very low duty cycle operation.
The LRTRC line, high during the fast sweep ramp, provides a digital waveform which is
averaged and inverted by Q2, C15, and associated circuitry, providing an output voltage to Ql
that is dependent on the average value of the LRTRC signal. Ql is an emitter follower which
provides drive when the FSZ signal is above - - 150 mV for low duty cycle sweep operation
(less than 20 percent). when the voltage at the collector of Q2 is below +0.7 V (high duty
cycle operation), the base-emitter junction of Ql is off, and the FSZ voltage is held at about
150 mV by voltage divider R71 and R72.
Much of the circuitry on the A3Al Trigger can be tested without removing the assembly from
the instrument. First key in Instrument Preset,
10 ms. Noise should be displayed at approximately 2.5 divisions up from the bottom graticule.
If the display is blanked, try ILINE) and then (vIDEOJ triggering.
The waveforms shown on the service sheet for TP2 and TP5 are valid only in the (FREE]
and Normal mode ([SHIFT) (VIDEO], [SHIFT) (EXT)). The Fast Sweep time should be equal to
the time it takes the ramp at TP5 to sweep from 0 V to +2 V. The oversweep above 2 V is
determined by the Sweep-Comparator and is not critical for instrument operation. The dead
time when the ramp and LRTRC are low is approximately 15 ps for sweep times less than
200 ps, and 120 ps for sweep times from 200 ps to 10 ms. During the dead time the Free Run
Oscillator is on. See Figure l-l for the relationships between the waveforms.
6 A3A1
0 Hz and c-1
(
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FREQUENCY SPAN
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