Digital Asic - Fluke 93 Service Manual

ClflCUIT
DESCRIPTIONS
3-9
3.3.4
DIGITAL ASIC
(D-ASIC)
cicultry
-
Introduction
The
Digital
Application Specific Integrated
Circuit (or
D-ASIC) D1203 forms
the core
of
the
digital
circuitry
of
the
ScopeMeter
ail
located
on
the
digital
A1
PCB.
Many
functions
are incorporated
in
this
complex
CMOS
Integrated
circuit
(see figure 3.^
on
the next
page);
-
Timebase
•
Trigger
•
Acquisition Corttrol
Logic
•
Acquisition
RAM
-
Min/max
- Display control
-
Decodmg
and
synchronization
D
Ig
Ital-to-analog
con
verters
(
DAOs
-
Detailed
circuit
description:
See
figure
3.4
and
circuit
diagram At
(figure 10.2).
The
following gives
a
short description of the separate
parts
of
the
D-ASIC, which
perform the
functions
mentioned above:
Tlmebase
The D-ASIC
contains
a
crystal oscillator,
which uses
the
25
MHz
crystal
G
1201.
An
internal
programmable
divider
generates timebase sign^
TRACK
with a
frequency from 0.8333
Hz
up
to
25
MHz
(see section
3.4.5).
This
TRACK
signal
is
used
to
sample
the
ScopeMeter
input signals.
Trigg&r
The
trigger
module
in
the
D-ASIC takes care
of
all
trigger related functions:
•
pre
triggering
•
post
triggering
•
event counting:
the
time
interval
corresponding to the trigger delay
is
Increased
by a
programmed number
of "evenlB" (trigger level
crossings ofihe external trigger
signal),
which
must
occur
before
triggering.
•
n-cycle mcxje:
trigger level
crossings of the input signal
are counted, and triggering occurs
every
n'*'
crossing
(2
<
n
< 255).
The
n-cyde
mode
can
be used
as a
digilal
trigger hold-off.
In
the real-time
sampling
mode
(<
1
ps^dlv),
the
D-ASIC
determines
the trigger
moment
with
digital
comparators.
In
the quasi
-random sampling
mode,
the A-
ASIC determines
the trigger
moment
with
analog comparators.