Philips PM 3266 - Page 93

98
3.1.3.
Time
bases
3.
1.3. 1. Main
time-base
The M.T.B.
trigger
and
X-Deflection
amplifier
receives
its signal
from one
of the
vertical
channels
or both
(COM-
POSITE),
from
the
attenuator/impedance
converter
for
external
trigger
or X
deflection
signals,
or from
the
power
supply
(MAINS).
One
of
those signals
can be
selected
by operating
one
of the
controls
incorporate
in this
stage.
From
this
stage the
signal
is
fed
to either
the
X-Deflection
amplifier
for
horizontal
deflection,
or the sweep-
gating
logic
for
starting
the
time-base
generator.
The MTB
trigger
and X-Deflection
amplifier
is
a
differential
one,
containing
the
controls
for trigger
-level
adjustment,
slope
selection
and
coupling
(i.e.
DC/LF/HF)
selection.
The
slope
selector
allows the
polarity
of the trigger
signal
to be inverted,
enabling
triggering
on the
positive
as well
as on the
negative
slope
of the
input
signals.
The
sweep-gating
logic
starts
and
stops the
time-base
generator
which
delivers
the
sawtooth
signal
required
for
normal
time-base
operation.
The
generator
comprises
the charging
capacitors
and
resistors
selected
by the
TIME/DIV
switch
in order
to set
the
time
coefficients
in
calibrated
steps.
Continuous
control
of
the time
coefficients
is
obtained
by varying
the
charging
current
of
the time
determining
capacitors
by means of
the
TIME/DIV
continuous
potentiometer.
The
amplified
output
signal
of
the
time-base
generator is
fed
to
the
X
deflection
selector,
the
comparator
which
is part
of the
delayed
time-base
unit and
via
a
feedback
loop
to
the
hold-off
circuit.
The
hold-off
circuit
resets
circuit
resets
the
sweep-gating
flip-flop
(D901)
and
blocks
its input during
the
flyback
of
the
sawtooth
signal.
The
hold-off
circuit also
incorporates
the
single-sweep
circuit.
The three
modes
of
operation
of the
main
time-base
are
determined
by the
three-position
switch
AUTO/TRIGG/
SINGLE.
In the
AUTO
mode,
the
automatic
free-run
circuit
is
operative
when
triggering
pulses
are
absent. Thus
a trace,
though
not
necessarily
a stationary
one,
is
always
displayed
even
though
the trigger
controls
may
not be
correctly
adjusted.
In this
way, correct
adjustment
of
the
oscilloscope
trace is
greatly
facilitated.
However,
when
trigger
pulses
are
present
the
circuit reverts
to
the
normal
triggered mode.
If
trigger
pulses
disappear,
the
time-base
free-runs
after
a lapse
of
approx.
100
ms. In
the
TRIGG,
mode,
a display
is
present
only
when
suitable
trigger
pulses
are
available.
In the
SINGLE mode,
events that
occur
only once
can be
observed
and
photographed
if
necessary.
It is often
desirable
to
ensure
that only
one
sweep
is generated,
even though
other
trigger
pulses might
follow the
phenomenon
of
interest.
In
this mode,
after the
trigger
pulse has
initiated the main
time-base
to produce
one
sweep,
the circuit
is
unaffected
by further
trigger pulses
until
it is reset for the
next
event
by operating
the reset
push-button.
3.
1.3.2.
Delayed time-base
The delayed
trigger-circuit
and delayed
time-base
generator
comprise in principle
the
same circuitry
as the main
trigger-circuit
and main
time-base
generator.
The delayed
time-base works
always in the
single-shot
mode. It is
started by the main
time-base
generator which
also serves
as
hold-off
circuit for the
delayed
time-base.
The
DELAY
TIME multiplier
control, the
comparator and
the reset multivibrator
determine
the
delay
time
for the
delayed time-base
generator.
When push-button
MAIN
TB of the
horizontal deflection mode
controls has been
depressed, the
part of the
trace
coinciding
with the
delayed sweep
is intensified, except in
the
OFF
position of
the delayed
TIME/DIV
switch.
3.1.4. X-Deflection
selector
The
X-deflection
selector
couples
the external
X-deflection
signal, the output
signal
of the main time-base
generator,
the
output
signal of
the delayed
time-base
generator
or the
combined
output signals
of the main
and
delayed
time-base
generators via
the X-final
amplifier,
to the horizontal-deflection
plates.
The
X-final amplifier
comprises the
horizontal
trace
positioning
and lOx
magnification
controls.
The "alt-" and
"chop"-
mode stages
supply
blanking pulses to the Z amplifier.
"Alt"
pulses blank the trace at
the end of the sweep
of the main
time-base
and
provide
an extra
bright-up
pulse if the
oscilloscope
operates
with a
portion of the
trace intensified.
"Chop" pulses
suppress the
trace during the
switching from channel
to channel
Y
0
and/or the 3rd
channel
TRIG VIEW.