Tektronix 2445, Instruction Manual

Page: 68 / 351

Theory o f Operation—2445 Service
Switching between unmagnified (X I) gain and magnified
( X I 0 gain) is also controlled by signals from the Display
Sequencer. For normal horizontal deflection, the MAG
signal on pin 14 o f U800 is HI, and the gain of the output
amplifier produces normal sweep deflection. Precise X I
deflection gain is set by adjusting X I Gain pot R860.
When the X10 MAG feature is selected, amplifier gain fo r
the magnified sweeps is increased by a factor of 10. The
MAG signal from the Display Sequencer goes LO when
magnified sweep is to be displayed. This switches the
amplifier gain and switches analog switch U860C from the
X I position to the X10 position. A m plifier gain in the
magnified mode is adjusted by adding or subtracting a
small bias current using X10 Gain control R850. Dc offsets
in the amplifier and crt are compensated for, using Horiz
Centering pot R801 to precisely center the display. An
intensity-dependent position correction signal, used to hold
the horizontal centering stable over a wide range o f varying
display intensities, is added at this point by the Dynamic
Centering circuitry.
As w ith the Vertical Output Am plifier, the Beam Find
feature reduces the dynamic range of the Horizontal
Output Am plifier. While the front-panel BEAM FIND
button is pressed in, a HI is placed on U800 pin 15 via
pull-up resistor R615, and the horizontal deflection is
reduced, moving horizontally off-screen displays to w ithin
the graticule viewing area.
Timing and linearity of the sweep is set by voltage
divider R802-R803.
Z-Axis Amplifier
Z-Axis A m plifier U950 turns the crt beam o ff and on at
the desired intensity levels as the oscilloscope goes through
its display sequence. The BRIGHT (brightness) signal
applied to U950 pin 44 from the Display Sequencer U650
(diagram 5) is amplified to the level required to drive the
crt control grid (via the DC Restorer circuitry) and sets
the crt beam intensity. The BLANK input signal applied to
U950 pin 5, also from the Display Sequencer, blanks the
trace during sweep retrace, chop switching, and readout
blanking by reducing the V ZOUT signal to a blanked level.
Sweep gate z-axis signals (SGAZ and SGBZ) from the A
Sweep and B Sweep hybrids (U700 and U900 respectively,
diagram 5) are applied to the Z-Axis A m plifier on pins 4
and 5. These signals turn the beam current on and o ff for
the related displays and, when used in conjunction w ith the
BLANK signal on pin 5, enable the sweeps to be blanked
while still allowing the Readout circuitry to blank and
unblank the crt fo r the readout displays.
Control signals applied to U950 pin 48, pin 2, and
pin 1 (HSA, HSB, and T X Y respectively) switch some
internal logic circuitry to enable or disable different input
signals fo r the various types o f displays. Table 3-5 illustrates
the effects of the various input signals on the output signal
fo r different combinations o f HSA, HSB, and TXY.
The Z-Axis hybrid has an internal lim iter circuit that
prevents the crt from being damaged during high-intensity,
high-repetition-rate displays. For high-rep-rate displays,
capacitor C956 is shunted to ground via U850A. A signal
representative of the intensity setting and the sweep
repetition rate is integrated on C957 and results in a control
level at pin 7 of U950 used to lim it intensity o f the crt
beam. For the slower repetition rate displays, the SIL bit
(slow intensity lim it) from A uxiliary Control Register U140
(diagram 4) opens CMOS switch U850A, gently reducing
the effective capacitance at pin 7. In this slow-sweep mode,
lim iting depends prim arily on the intensity setting.
Table 3-5
Blanking and Intensity Control Selection
Control Inputs
|
Intensity
Affected
By
Blanking
Affected
By
Typical
T X Y
HSA HSB
Display
Xa H H
BRIGHT (RO level) BLANK Readout
X H L
BRIGHT, Z EXT
BLANK, SGAZ, SGBZ
Delayed Sweep
X
L H BRIGHT, SGBZ, Z EXT BLANK, SGAZ Main Sweep
L
L L BRIGHT, SGBZ, Z EXT BLANK X-Y
H
L
L
BRIGHT, SGBZ, Z EXT BLANK, SGAZ
X-Y
aX = State doesn't matter.