Tektronix 2213 Instruction Manual

Theory of Operation—2213 Service
The collector current changes from Q474 are filtered
by a network composed of C476, C477, R477, and R478.
The filter network rejects TV video information and
averages the TV horizontal-sync pulses. Setting the trigger-
level threshold at near the center of the horizontal-sync-
pulse swing establishes the untriggered level. When the TV
vertical-sync block occurs, the output of the filter rises to
a level that will cause the Schmitt Trigger circuit to switch.
Precise TV field synchronization is obtained as a result of
the filtering action.
The output signal from the filter is applied to U480B
pin 6. The Schmitt Trigger circuit responds only to the TV
sync signal because pin 7 is held LO by the output of
U480C.
Schmitt Trigger Circuit
With a LO on U480B pin 7, the output at pin 3 goes
LO as soon as the signal on U480B pin 6 reaches the
switching threshold. The LO is applied to U480A pin 4 and,
together with the fixed LO on pin 5, causes the output of
U480B pin 6 via R480 to reinforce the switching action.
As a result, the output signal at U480A pin 2 switches
rapidly.
When the level from the filter network falls to the LO
threshold level, the feedback supplied by R480 holds the
Schmitt Trigger switched HI for a short time. The amount
of time involved prevents noise occurring exactly at the
threshold level from causing false triggering.
When either AUTO or NORM triggering is selected,
input pin 6 of U480B is held LO, and the Comparator
output signal on U480B pin 7 supplies the input to the
Schmitt Trigger circuit.
The output of the Schmitt Trigger circuit is obtained
from U480D pins 9 and 15. The differential output signal
derived from U480D is applied to a two-transistor level-
shifting circuit composed of Q492 and Q493. The level-
shifting circuit converts the ECL logic levels to TTL logic
levels required for the Sweep Generator. A signal obtained
from the collector of Q493 is used to drive the Auto
Baseline circuit.
Auto Baseline Circuit
The Auto Baseline circuit (composed of U640A, Q605,
and associated components) is enabled in both AUTO and
TV FIELD triggering modes. This circuit provides a signal
to the Sweep Generator circuit (Diagram 5) that initiates
a sweep if a triggering signal is not received by the Schmitt
Trigger circuit within a period o f about 100 ms. A second
output from the circuit illuminates the TRIG'D LED on
the instrument front panel when the sweep is triggered.
When adequate triggering signals are being received, the
output of Q493 is applied to pin 5 of monostable multi
vibrator U640A. The negative-going edge of the signal
causes pin 6 of U640Ato switch HI. The HI forward biases
CR615, and Q605 is then biased into conduction. With
Q605 conducting, the Auto Baseline signal line is held LO
to prevent the Sweep Generator circuit from free running.
The amount of time that pin 6 of U640A stays HI
without receiving an input signal is determined by timing
components R614 and C614. If a trigger signal is not
received in about 100 ms, pin 6 of U640A will go LO and
Q605 will be biased off. The Auto Baseline signal line then
goes HI through pull-up resistor R610, and the Sweep
Generator free runs to produce the baseline trace.
In NORM triggering mode, the Auto Disable signal
(+8.6 V) is applied to the base circuit of Q605 via CR611
and R611. The signal holds Q605 forward biased and
prevents the Sweep Generator from free running.
The other function of the Auto Baseline circuit is to
illuminate the TRIG'D LED when the sweep is properly
triggered. As long as U640A pin 6 remains HI (triggering
signals occurring with the proper time), TRIG'D LED
DS618 will be illuminated. The trigger mode in use does
not affect the operation of the TRIG'D LED.
SWEEP GENERATOR AND LOGIC
The Sweep Generator and Logic circuitry, shown on
Diagram 5, produces a sawtooth voltage that is amplified
by the Horizontal Amplifier to provide horizontal deflec
tion on the crt. This sawtooth voltage (sweep) is produced
on command from the Sweep Logic circuits. The Sweep
Generator circuits also produce gate waveforms that are
used by the Auto Intensity and Z-Axis circuits to establish
the correct timing of the crt unblanking and intensity
levels used for viewing the display. See Figure 3-5 for the
Sweep timing diagram.
The Sweep Logic circuitry controls the holdoff time,
starts the sweep upon reception of a trigger signal, and
terminates the sweep at the proper sweep level. When using
AUTO or TV FIELD triggering, the Sweep Logic circuitry
will cause the Sweep Generator to free run, producing a
baseline trace if a trigger signal is not received within the
predetermined time period.
Miller Sweep Generator
The Miller Sweep circuit is composed of Q630A, Q630B,
Q631, and associated timing components. The circuit
operates to hold the charging current to the timing
@ 3-15

Tektronix 2213 Specifications

General

Brand	Tektronix
Model	2213
Category	Test Equipment
Language	English