Tnnebase
Generator
2212 Service Manual
Horizontal System and Z-Axis Circuit
PAL U501 contains
the
horizontal logic. It is a state
machine
that
controls
the
horizontal magnifier (diagram
6)
to
switch
between
lx
gain,
10x
gain
and
50x
gain.
The
hold
off
pulse
is the clock signal for
this
state machine. Some gates
inside
U501 are used to decode TIME6
and
TIME?. These bits are
part
of the
control byte
that
sets
the
sweep speed.
The
decode logic selects
the
proper
capacitor for the hold off generator to obtain
the
correct
hold
off time for
each
sweep
speed.
Auto
Cal
and AUTO Setup
In
calibration mode,
the
sweep length is set to exactly
10
divisions
and
appro-
priate time markers are applied to
the
scope.
The
first
time
marker triggers a
sweep. A second time marker
is
detected
by
U503B. After EOS, U503A samples
the
status of U503B
and
generates
the
AUTOTIMOK signal.
The
system proces-
sor reads AUTOTIMOK
and
controls
the
sweep
speed
(VARTIM
in
diagram
5B)
The
sweep
speed
is adjusted so a 10 division
sweep
fits exactly
within
two
time
markers. In auto set
up
the
same circuit is
used
with
the
normal sweep length
>10 divisions to make
an
estimation
of
the
input
signal frequency.
In
this
way
the
proper sweep
speed
can
be selected for
that
signal.
The
timebase generator consists basically of a capacitor ( C554, C555, C556,
C557)
and
a current source (Q551
and
U552A)
that
charges
that
capacitor.
Q552 and Q553 switch
on
the desired capacitor
in
a 1 to 100 sequence. Multi-
plexer U550 controls
the
current source
in
a 1 to
10
sequence, U551 controls the
current source
in
a 1, 2, 5 sequence.
So
there is a 3 dimensional switching
matrix to obtain all the possible
sweep
speeds.
The
matrix is controlled
by
the
code
byte from the serial data
chain
TIMCTR[0
..
5). TIMCTR[6
..
7)
are
used
as
TIME6
and
TIME? for-the hold off time setting. (See
Sweep
Logic diagram 5A).
There is
some
redundancy
in
the matrix, some
sweep
speeds
can
be
obtained
in
more
then
one way.
The
system processor is programmed to select the
most
accurate ones. With
the
VARTIM signal,
the
current
source
and
as a result the
sweep speed can be controlled continuously.
This
signal is used for
sweep
calibration
and
for variable sweep.
When
SWPGT
goes low, U554A,
-Band-Dare
on.
The
timing capacitor is discharged to a level
of
about -2.5
V.
When SWPGT goes high, U554C
turns
on
and
U554A,
-Band
-D
turn
off.
The
current from Q551 charges the
timing
capacitor
and
the
sweep
runs.
SWPSTRT
is
a signal from the daculator
and
is programmed according to
the
diode curve of Q554B
and
the
selected current.
This
compensates for
the
voltage
across U554B to achieve a sweep
that
always starts at
the
same
level.
The
buffered
sweep
is passed
on
to the
end
of
sweep
comparator
and
the horizontal
magnifier.
In
X-Y
mode
the
sweep does
not
run. Q561 switches
XAXIS
from the
CH1
storage pick off to the horizontal magnifier.
3 · 15
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