Tektronix 7a15a Instruction Manual page 14

Circuit Description -7A15A/7A15AN
For switch positions above five millivolts, the attenua-
tors are switched into the circuit singly or in pairs to pro-
duce the deflection factor indicated on the front panel .
These hybrid attenuators are frequency-compensated
voltage dividers . For DC and low-frequency signals, the
attenuators are primarily resistance dividers and the voltage
attenuation is determined by the resistance ratio in the
circuit. The reactance of the capacitors in the circuit is so
high at low frequencies that their effect is negligible . How-
ever, at higher frequencies, the reactance of the capacitors
decreases and the attenuator becomes primarily a capaci-
tance divider.
In addition to providing constant attenuation at all
frequencies within the bandwidth of the instrument, the
Input attenuators are designed to maintain the same input
RC characteristics (one megohm X 20 pF) for each setting
of the VOLTS/DIV switch . Each attenuator contains an
adjustable series capacitor to provide correct attenuation at
high frequencies, and an adjustable shunt capacitor to pro-
vide correct input capacitance .
General
Input Amplifier
The Input Amplifier converts the single-ended signal
applied to the Input connector to a differential (push-pull)
output . A schematic of this circuit is shown on Diagram 2
in the Diagrams section .
Input Source Follower
The Input Source Follower 0210A provides a high input
impedance with a low impedance drive for the following
stage. R210 limits the current drive to the gate of Q210A.
Dual-diode CR210 provides circuit protection by limiting
the voltage swing at the gate of Q210A to about ± (positive
or negative) 15 volts. Q210B provides a constant current
source for 0210A. Q210A and Q210B are encapsulated in
the same case so that Q210B temperature compensates the
circuit.
Amplifier
The signal from the Input Source Follower is applied to
paraphase amplifier Q220-Q320. The paraphase amplifier
converts the single-ended input to a differential (push-pull)
output . It also provides a means of compensating for stray
currents throughout the entire amplifier by varying the DC
Level at the base of Q320 via the DC Bal control R322 . The
differential signal from the paraphase amplifier is cascoded
to the Inverting Amplifier, a set of common base differen-
tial amplifiers Q230-Q330 and Q235-Q335. With the
POLARITY switch set to +UP, Q230 and Q330 are forward
biased while Q235 and Q335 are reverse biased . The signal
is therefore allowed to pass un-inverted through
Q230-Q330. By setting the POLARITY switch to INVERT,
3-2
0230 and Q330 are reverse biased and Q235 and Q335 are
forward biased . The signal is inverted through Q235-Q335.
Current gain for amplifiers Q230-Q330 and Q235-Q335 is
controlled
by the GAIN potentiometer R238 and
VARIABLE control R239 . The output from Q230-Q330 or
Q235-Q335 (depending on the POLARITY switch) is
connected to the X1 amplifier, 0240-Q340, and the X10
amplifier, Q245-Q345. The MAG switch determines which
amplifier (X1 or X10) is on, by switching their emitter
supply voltages . Current gain for the X10 amplifier is
adjusted by R245 . R341 and C341 provide frequency com-
pensation for the X1 amplifier. The signal from the X1 or
X10 amplifier (depending on the MAG switch) is cascoded
through the common base amplifier, Q250-Q350, to the
Signal Amplifier, Q260-Q360, and Trigger Amplifier,
Q270-Q370.
General
The Connectors and Readout circuit consists of the
power supply and signal distribution from the Interface
Connector and the Readout Encoding circuit. A schematic
of this circuit is shown on Diagram 3 in the Diagrams
section .
Connectors
Connectors and Readout
All the connections made to the mainframe by the
7A15A/7A15AN are shown on the Connectors portion of
Diagram 1 . Also shown are the power supply decoupling
components .
Readout Encoding (7A15A only)
The Readout Encoding circuit consists of switching
resistors and probe sensing stage Q620 . This circuit encodes
the Row and Column output lines for readout of deflection
factor, uncalibrated deflection factor (VARIABLE) infor-
mation, and signal inversion. Data is encoded on these
output lines by switching resistors between them and the
time-slot input lines or by current added through Q620 .
R647-C647 are switched between time-slot three (TS-3)
and the Column output line when the CAL switch is in the
uncal position . This results in the symbol > (greater than)
being displayed preceding the deflection factor readout.
R648 is switched between TS-2 and the Column output line
when the POLARITY switch is in the INVERT position .
This results in the symbol
f
(inverted) being displayed
preceding the deflection factor readout.
Switching resistors are used to indicate the setting of the
VOLTS/DIV switch to the mainframe readout system . The
dots on the contact-closure chart (see Diagram 3) indicate
when the associated contacts on the VOLTS/DIV cam
O