Tektronix 511 Instruction Manual page 19

multivibrator pulse length.
Unblanking Circuit
During the waiting period the bias on the
cathode-ray tube is such that it is completely
cut off. As soon as a trigger appears and the
sweep starts , it is necessary to provide a
positive pulse on the grid of the cathode-ray
tube and thus turn on the electron beam. This
pulse should have an extremely fast rise and a
very flat top so that the brightness of the image
is uniform.
To secure a pulse of this nature, in instru­
ments S/N2869 and up, the negative pulse from
the grid of V5 is limited by plate current cutoff
in V7.1A before it reaches full amplitude.
The voltage divider R37, R38, sets the d.c.
bias on the grid of V7.1A at approximately 0
volts and supplies a negative pulse of 45 volts
amplitude. C l2 insures that the leading edge will
be coupled through without loss of risetime due
to grid and wiring capacities to ground. The
45 volt negative pulse is more than adequate
to drive V7.1A into cutoff, thus producing at
the plate a square, flat-topped positive pulse
having an amplitude of 80 volts. To provide
increased risetime, L2.1 is employed in the
plate circuit of V7.1A. The cathode follower,
V7. IB , serves to isolate the plate circuit of V7.1A
from the capacity-to-ground of the cathode-ray
tube grid and associated wiring, thus preventing
i
any appreciable reduction in risetime due to
their effect. This circuit is capable of un-
blanking the tube in less than .1 microsecond
after a sharp trigger pulse reaches the TRIGGER
I
INPUT binding post.
To secure a pulse of this nature, in instruments
S/N101 to 2868, the positive pulse from the
multivibrator is clipped before it reaches its
normal amplitude by the diode V6. The cathode
of V6 is held at approximately + 55 v and thus
the plate cannot rise more than a few volts
above this value. In order that the capacity - to-
ground of the cathode-ray tube grid and its
associated components and wiring should not r e ­
duce the rate of rise appreciably, the cathode
follower V7 is used. This circuit is capable
of unblanking the tube within .1 microsecond
after a sharp trigger impulse reaches the TRIG­
GER INPUT binding post.
Sweep Generator
basic wave necessary to sweep the beam linearly
across the tube and return it to its starting
point. During the waiting period the 6AG7 tube
V9 is held fully conducting since it has no bias
and therefore its plate is about at ground poten­
tial. When a trigger occurs the negative pulse
from the multivibrator cuts the grid of V9 off
very rapidly.
in exponential fashion at a rate depending upon
R20D, R20D1-9 ( as determined by setting of
S10B), R39, the voltage applied across the c ir ­
cuit, and C15. When the multivibrator pulse
ends V9 conducts again, thus discharging C15.
Since C15 is charged to only about 5% of the
supply voltage the rate of rise is very nearly
linear. Five values of C15 as determined by
the SWEEP RANGE switch in combination with
the variable resistors R20D and R20D1-9 pro­
vide a continuous variation of sweep speed of
from .1 sec to 1 microsecond for a 10 cm
sweep. In order to maintain the correct multi­
vibrator pulse length as the sweep speed is
increased, the SWEEP RANGE switch changes
C7 in the multivibrator and C15 simultaneously.
In a like manner the SWEEP SPEED MULT,
controls are dual controls changing R20C, R20D
and R20C1-9, R20D1-9 simultaneously. In order
to maintain the calibration of the SWEEP SPEED
MULT, dials when tubes are changed, etc.,
the voltage applied to R20D may be varied by
R42 which sets the grid potential of the cathode
follower V15. The use of the cathode follower
instead of a potentiometer maintains a con­
stant charging potential as R20D, R20D1-9 is
T Y P E 5 1 i - C I R C U I T
Gate Output Generator
V8 serves to isolate the gate output binding
posts from the multivibrator. It receives a p osi­
tive pulse on its grid of the same duration as
the sweep from the plate of V5 via the voltage
divider R32 and R33. CIO maintains the steep
sides of the pulse. Since the plate and cathode
load resistors are equal, pulses of equal size
but opposite polarity appear at the plate and
cathode. The + gate from the cathode has no
blocking capacitor and therefore preserves the
square pulse for the longest sweeps. It was
not felt desirable to have the -Gate binding
post carry the +225 v potential of the plate;
therefore a blocking capacitor is used.
The sweep generator circuit produces the
In instruments S/N 1406 and up the plate rises
D E S C R I P T I O N
P A G E 3 - 3