Tektronix 7A19 Instruction Manual page 12

Circuit Description--7A19
Signal Line Terminator
The Signal Line Terminator consists of a current sensing
operational amplifier connected to the Signal Line . This
operational amplifier senses changes in currents at the
Signal Line and compensates by adding or subtracting
currents. The Input Current control sets the quiescent
current at the Signal Line . This is the major controlling
factor for input impedance . Changes of input current due
to positioning or polarity switching are also compensated
by this circuit. R121 and C121 compensate for the addition
of this circuit to the Signal Line.
Limiter
CR124, CR125 provide overload protection . An over-
load condition at the signal INPUT connector will cause
one or both diodes to forward bias . If this condition
continues, the current flowing through the diodes will soon
cause the fuses F6, F7 to burn out . Diodes CR124,CR125
are connected to the Signal Line through an impedance
matching "T" network, L124, L125, and C123 .
Signal Amplifier and "Trigger Amplifier
The Signal Amplifier and Trigger Amplifier circuitry are
identical . The description for the Signal Amplifier applies
equally to the Trigger Amplifier.
The Signal Amplifier consists mainly of a TEKTRONIX-
made hybrid integrated circuit (U40) and its related
circuitry . A representation of U40 is shown in the
schematic diagrams section . The Signal Line is connected to
the Signal Amplifier input through a "T" network con-
sisting of C20, L21, and L22. The "T" coils, L21 and L22
are adjusted for minimum aberrations at high frequencies .
-The paraphase input of U40 converts the single ended input
to a differential signal . Frequency compensation, signal
balancing, gain, and temperature compensation are all
accomplished in the emitters of the paraphase amplifier .
Frequency compensation is achieved through the use of the
series of resistors, capacitors, and an inductor connected to
pins 5 and 11 . R38 signal balance control balances our the
resultant currents in the amplifier .
Gain Current Source
Temp Comp control, R76, sets the voltage at the emitter
of Q76. The current through R74 is set by the current
generator Q73 and Gain control R73. As the thermal
resistor R74 changes with temperature so will the voltage at
the collector of Q73; therefore, compensating for temper-
ature changes within the plug-in .
Positioning is achieved by varying the DC level at the
input of the paraphase amplifier (pin 4, U40) . Q56A and
Q56B is a DC amplifier having an inverting input and
non-inverting input. Gain of this amplifier is selected by the
VOLTS/DIV switch . With the VOLTS/DIV switch in the
10 mV position, the gain of Q56 is set by R66. In the
20 mV position the gain of 056 is doubled . The POSITION
control (R68) applies a DC signal to either the inverting or
non-inverting input. This is selected so that a clockwise
rotation of the POSITION control will always produce an
upward deflection of the displayed trace . The output of the
position amplifier is applied to the Signal and Trigger
Amplifier through an inverting buffer Q50. This signal is
also applied to the Signal Line Terminator which com-
pensates for the current changes that occur as a result of
the positioning circuitry .
Polarity Circuit
The output from the Signal and Trigger Amplifiers is
connected through the Polarity switch to the interface
connector. 'The Polarity switch, in the INVERT position,
not only inverts the Signal and Trigger outputs but it
inverts the polarity signal so that a clockwise rotation of
the POLAR ITY control always produces an upward deflec-
tion . See the description for the Position Circuit for details.
Connectors
All connections made to the mainframe by the 7A19 are
shown on the schematic located in the Diagrams section of
this manual . Also shown are the power supply decoupling
components .
Readout
The 7A19 incorporates standard vertical readout en-
coding circuitry . Refer to any 7000-Series Oscilloscope
with readout for more information .