Tektronix 2213, Инструкция по эксплуатации

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Theory of Operation—2213 Service
capacitor at a constant value. When a capacitor is charged
in this manner, the rise o f voltage across the capacitor is
linear rather than exponential.
Field-effect transistors Q630A and Q630B are matched
devices. As such, the l DSS (drain current w ith gate-to-
source shorted) characteristics o f each are nearly identical.
FET Q630B acts as a source-current supply fo r Q630A
and holds the gate-to-source voltage o f Q630B at zero volts.
Figure 3-5. Sweep timing diagram.
Before a sweep starts, pin 6 o f U620 (the Sweep Logic
Gate) is HI, and both disconnect diodes (CR626 and
CR630) are forward biased. The charge on the selected
tim ing capacitor w ill be zero volts. When U620 pin 6 goes
LO, the disconnect diodes become reverse biased and the
tim ing capacitor begins charging through the tim ing resistor
to start the sweep.
The overall gain o f the amplifier composed o f source-
follow er Q630A and common-emitter amplifier Q631 is
very high. As the tim ing capacitor charges, Q631 supplies
feedback to the gate o f Q630A to hold the gate voltage
nearly constant. Voltage across the tim ing resistor is there­
fore constant, and the charging current to the tim ing
capacitor is constant. The resulting voltage waveform
produced at the collector o f Q631 is a linear ramp.
When the sweep waveform amplitude reaches about
+13 V, the End-of-Sweep Comparator (Q640) is biased
on and the Sweep Logic circuit resets. Pin 6 o f U620 goes
HI to forward bias disconnect diode CR626, and the
current through the tim ing capacitor reverses direction.
The sweep output waveform drops rapidly until disconnect
diode CR630 also becomes forward biased. A t this point,
the Sweep Generator is ready to start another sweep.
Sweep Logic
Following the sweep completion, a fin ite tim e is
required to discharge the tim ing capacitor. The Sweep
Logic circuit is prevented from responding to a trigger
signal during this time by the H oldoff circuit. The end o f
sweep (and start o f the h o ld o ff period) is determined by
the End-of-Sweep Comparator (Q640).
The Sweep ramp waveform is applied to the base o f
Q640 through both a voltage divider and a biasing network
composed o f R637, R638, and C637. When the ramp
amplitude reaches the threshold level o f Q640, the collector
o f Q640 goes LO, and a LO is placed on both U640B
pin 11 and U607C pin 10. The output o f U607C goes HI,
and the positive feedback supplied to the base o f Q640
through R639 speeds up the change o f state o f Q640. By
reinforcing the switching action o f Q640 in this manner,
noise occurring at the threshold level o f Q640 is overridden.
The sweep h o ld o ff period commences when the LO
from Q640 is applied to pin 11 o f monostable m ultivibrator
U640B. The Q output on pin 9 goes LO and remains LO
fo r a length o f time determined by the RC tim ing
components connected between pins 14 and 15 o f U640B.
Holdoff time can be varied from the normal period by
using VAR HOLDOFF control R557. Potentiometer R557
3 -1 6
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