Tektronix 516 series, Instruction Manual

Page: 23 / 132

Circuit Description — Type 5 1 6
FROM
TIME-BASE
GENERATOR
TO
B CHANNEL
SWITCHED AMPLIFIER
Fig. 4 -3 . Switching Circuit block diagram .
the beam, and then the signal in the other channel for the
next sweep of the beam. In this mode of operation, V585
becomes a bistable Switching Multivibrator which is switched
from one state to the other at the end of each sweep. When
the Switching Multivibrator is in one state— say, with V585A
conducting and V585B cut off— the low plate voltage on
V585A is coupled to the grid of V594B. This cuts off V594B
whose plate voltage then rises to about +63 volts, which
cuts off the B Channel Switched Amplifiers. The high plate
voltage of V585B is coupled to the grid of V594. This allows
V594A to conduct which lowers its plate voltage and the
voltage at the cathodes of the A Channel Switched Ampli­
fiers to about +57 volts. Therefore, the A Channel Switched
Amplifiers conduct. When the Switching Multivibrator is in
the other state— V585B conducting and V585A cut off— the
opposite set of conditions exists; the B Channel Switched
Amplifiers conduct and the A Channel Switched Amplifiers
are cut off.
The switching of the Switching Multivibrator in the AL­
TERNATE mode takes place as follows. At the end of each
sweep, a positive spike is formed at the screen of V145A
in the Time-Base Generator. This positive spike is applied to
the grid of the Alternate Trace Sync Pulse Amplifier, V574B,
and, since V574B is normally cut off, allows the tube to con­
duct. This allows current to flow also through the Sync Pulse
Coupling Diodes, V572, and applies a negative spike to both
grids of the Switching Multivibrator. Since one side of the
multivibrator is already cut off, the negative spike will have
no effect on that side. However, the negative spike will cut
off the side which is conducting and cause the multivibrator
to switch states.
When the MODE switch is in the CHOPPED position, V585
becomes an astable multivibrator with a frequency of about
150 kc. The outputs from the two sides of the Switching
Multivibrator are applied to the two sides of the Switching
Amplifier which turns the Switched Amplifiers off and on
alternately at a 150-kc rate. This causes the signals in the
two channels to be displayed on the crt alternately for a
little over three microseconds at a time.
Since the POSITION controls of the two channels are
normally set at different levels, it is necessary, in the CHOP­
PED mode, to turn off the crt beam as the switching between
channels is taking place. This action takes place in the Chop­
ped Blanking Amplifier, V574A. The negative-going pulses
formed as the two sides of the Switching Multivibrator al­
ternately come into conduction are differentiated and ap­
plied as negative spikes to the grid of V574A. V574A am­
plifies and inverts the spikes and applies them to the crt
cathode via SW848. These positive spikes on the crt cathode,
then, cut off the beam as switching is taking place.
TIME-BASE TRIGGER
The Time-Base Trigger consists basically of the Trigger
Selector switches, SW10A and SW10B, the Trigger Input
Amplifier, V24, and the Trigger Multivibrator, V45. The
TRIGGER SELECTOR switches select the triggering source, the
triggering slope, and the triggering mode. The Trigger In­
put Amplifier amplifies (and, when desired, inverts) the in­
coming triggering signal and applies it to the input of the
Trigger Multivibrator. The Trigger Multivibrator is a Schmitt
circuit which is switched from one state to the other by the
signal at its input. Its square-wave output is differentiated
to form negative and positive spikes which are applied to
the Time-Base Generator where the negative spikes are used
to start the horizontal sweep. The positive spikes are not
used.
A block diagram of the Time-Base Trigger is shown in
Fig. 4-4.
4-4