Tektronix 7A26 Instruction Manual page 7

In the AC coupling position, the DC component of the
signal is blocked by a capacitor in the input circuit . The AC
coupling position provides the best display of signals with a
DC component much larger than the AC components . The
precharge feature should be used with large DC inputs . To
use this feature, first set the coupling to GND . Connect the
probe to the circuit and wait about two seconds for the
coupling capacitor to charge . Then set the coupling to AC .
The GND position provides a ground reference at the
input of the amplifier without externally grounding the
input connectors . However, the signals connected to the
inputs are not grounded, and the same DC load is presented
to the signal source .
VOLTS/DIV and VARIABLE Controls
The amount of vertical deflection produced by a signal is
determined by the signal amplitude, the attenuation factor
of the probe, the setting of the VOLTS/DIV switch, and
the setting of the VARIABLE control . Calibration deflec-
tion factors indicated by the settings of the VOLTS/DIV
switch apply only when the VARIABLE control is in the
calibrated (CAL IN) position .
The VARIABLE control provides variable, uncalibrated
settings between the calibrated steps of the VOLTS/DIV
switch . With the VARIABLE control fully counter-
clockwise and the VOLTS/DIV set to 5 volts/division the
uncalibrated vertical deflection factor is extended to at
least 12 .5 volts/division . By applying a calibrated voltage
source to the input connector, any specific deflection
factor can be set within the range of the VARIABLE
control .
CH 2 POLARITY Switch
The CH 2 POLARITY switch may be used to invert the
displayed waveform of the signal applied to the CH 2 input .
This is particularly useful in added operation of the 7A26
when differential measurements are to be made . The CH 2
POLARITY switch has two positions, +UP and INVERT . In
the +UP position, the displayed waveform will have the
same polarity as the applied signal and a positive DC voltage
will move the CRT trace up . In the INVERT position, a
positive-going waveform at the CH 2 input will be displayed
on the CRT in inverted form and a positive DC voltage will
move the trace down .
DISPLAY MODE Switch
For single-trace operation, apply the signal either to the
CH 1 input or the CH 2 input and set the DISPLAY MODE
switch to the corresponding position : CH 1 or CH 2 .
REV, B, MAR . 1975
Operating Instructions-7A26
To display a signal in one channel independently when a
signal is also applied to the other channel, simply select the
desired channel by setting the DISPLAY MODE switch to
the appropriate CH 1 or CH 2 position .
Alternate Mode . The ALT position of the DISPLAY
MODE switch produces a display which alternates between
channel 1 and channel 2 with each sweep on the CRT .
Although the ALT mode can be used at all sweep rates, the
CHOP mode provides a more satisfactory display at sweep
rates below about 0 .5 millisecond/division . At slow sweep
rates alternate mode switching becomes visually percep-
tible .
Add Mode . The ADD position of the DISPLAY MODE
switch can be used to display the sum or difference of two
signals, for common-mode rejection to remove an undesired
signal . The overall deflection factor in the ADD mode with
both VOLTS/DIV switches set to the same position is the
deflection factor indicated by either VOLTS/DIV switch .
However, if the CH 1 and CH 2 VOLTS/DIV switches are
set to different deflection factors, the resultant amplitude
is difficult to determine from the CRT display . In this case,
the voltage amplitude of the resultant display can be
determined accurately only if the amplitude of the signal
applied to one channel is known . In the ADD mode,
positioning of the trace is controlled by the channel 1
POSITION control only .
Chop Mode . The CHOP position of the DISPLAY
MODE switch produces a display which is electronically
switched between channels at approximately a 500 kilo-
hertz rate (controlled by mainframe) . In general the CHOP
mode provides the best display at sweep rates slower than
about
0 .5 millisecond /division or whenever dual-trace,
non-repetitive phenomena is to be displayed .
TRIGGER SOURCE Switch
CH 1 . The CH 1 position of the TRIGGER SOURCE
switch provides a trigger signal obtained from the signal
applied to the CH 1 input connector . This provides a stable
display of the signal applied to the CH 1 input connector .
CH 2 . The CH 2 position of the TRIGGER SOURCE
switch provides a trigger signal obtained from the signal
applied to the CH 2 input connector . This provides a stable
display of the signal applied to the CH 2 input connector .
MODE . In this position of the TRIGGER SOURCE
switch, the trigger signal for the time-base unit is dependent
on the setting of the DISPLAY MODE switch . The trigger