Circuit Description; Switching Circuit - Tektronix 53C Instruction Manual

AMPLIFIERS
The Type 53/54C Plug-In Unit consists of
two identical amplifier channels and a channel ­
switching multivibrator. The following descrip ­
tion of the amplifiers applies equally well to
either channel.
Input Coupling and Attenuation
The signal to be displayed is applied to the
input cathode follower V3 0 53 (V4053) by way
of the AC-DC switch and the VOLTS/CM
switch. The AC-DC switch is a two-position
slide switch that shorts out C3003 (C4003) in
the DC position so the input is d c coupled. In
the AC position of this switch the signal must
pass thru C3003 (C4003) so the d c component
of the signal is blocked.
The VOLTS/CM switch is a 9-position rotary
switch that selects the various frequency-com ­
pensated rc attenuator sections. The basic sen ­
sitivity of the unit is .05 volts/cm. This sensi ­
tivity is reduced by the eight individually se ­
lected attenuator sections to give nine fixed cali ­
brated sensitivities.
Input Stage
The input stage consists of the cathode fol ­
lower V3053 (V4053) and the cathode-coupled
phase inverters V3083 and V3263 (V4083 and
V4263). The control-grid d c level of V3083
(V4083) is established by the d c connection to
the cathode of V3053 (V4053). The control ­
grid dc level of V3263 (V4263) is adjustable by
means of the DC BAL control so that the d c
level of the cathodes of V3083 and V3263 (V4083
and V4263) can be made equal. Any d c level
difference between these two cathodes would
act as a signal and cause the trace to shift posi ­
tion when the VARIABLE control is rotated.
The VARIABLE gain control establishes the
amount of cathode coupling and thus allows the
stage gain to be varied over about a 2½ to 1
range.
The GAIN ADJ control permits the basic
gain of the unit to be accurately set to agree
with the front-panel calibration.
Polarity and Positioning
With the POLARITY switch in the NOR ­
MAL position the displayed waveform will have
the same polarity as the input signal. Placing
the POLARITY switch in the INVERTED
position reverses the signal-grid connection of
V3303 and V3343 (V4303 and V4343) and turns
the displayed waveform upside down. Rotation

CIRCUIT DESCRIPTION

TYPE 5 3/54C — CIRCUIT DESCRIPTION — PAGE 1
of the VERTICAL POSITION control forces
one plate of the input stage toward a higher
potential and the opposite plate toward a lower
potential. The resulting dc level shift moves the
trace vertically.
Amplifier Stage and Output CF
The signal is further amplified by V3303 and
V3343 or V4303 and V4343, depending on which
channel is conducting. V3303 and V4303 have a
common plate load and likewise V3343 and
V4343. Since one amplifier is always cut off
while the other is conducting, the shunt loading
effect is negligible.
V3503 is the output cathode follower that pro ­
vides a low-impedance source for driving the
oscilloscope ' s vertical amplifier.
TIONING ADJ control located in the grid
circuit of the output cf permits the trace to be
centered vertically under no-signal conditions.

SWITCHING CIRCUIT

A Only, B Only
V37 0 3 is a multivibrator that is controlled by
the MODE switch. With the MODE switch
in the A ONLY or B ONLY position the multi ­
vibrator is held in one of its two possible states
by returning one grid to a positive voltage and
the other grid to a negative voltage. For ex ­
ample, in the A ONLY position the grid of
V3703A is held positive and this half of the
multivibrator conducts while the grid of V3703B
is held negative and this half is cut off. While
V3703A is conducting the cathode is above
ground which causes V3903A to conduct and in
turn pulls the grid of V3953B toward ground
lowering the plate voltage of V4083 and V4263.
This reduced plate voltage cuts off the following
stage (V4303 and V4343) and the B-channel
amplifier is held in a non-conducting state. The
converse is true of the A-channel amplifier. The
grid of V3903B is near ground potential and
with reduced plate current the plate of V3903B
and consequently the grid of V3953A are per ­
mitted to become more positive providing plate
voltage for V3083 and V3263 and the A-channel
amplifier then conducts.
Chopped
Turning the MODE switch to the CHOP ­
PED position returns both grids of the multivi ­
brator to a positive voltage and the multivibra ­
tor free runs at a rate determined by the time
constant of the grid circuits. The two ampli ­
fiers are alternately cut off and allowed to con ­
duct at the free-running rate of the multivi ­
brator.
The POSI ­