Chapter 4 Technical Description; Introduction; Function And Range Switching; Control Codes - Racal Instruments 9901 Maintenance Manual

CHAPTER
4
TECHNICAL DESCRIPTION
INTRODUCTION
4.1 Apart
from
some
switches
and
certain items of the power
supply, the
circuit for
the
instrument
is
mounted on one main p.c.b.
assembly.
The
optional
fast
warm-up
oscillator
is
available
as
a separate
unit.
LOGIC
CIRCUIT
SYMBOLS
4.2
Extensive use
is
made
of integrated
circuits (IC's)
and
these are identified by a
number and
suffix letter.
In
the circuit description a particular 1C pin
will
be
identified by a reference such
as 'IClOa/3',
which
indicates pin 3
on that particular gate.
The
logic
symbols used
in
the
circuits
are those found
in
most manufacturers 1C data
sheets
to
which
reference should be
made
if
detailed information
is
required.
The
CDI
Chip, IC2
is,
however,
available only through
the Service
Department
of Racal Instruments Ltd.
OPERATING CONTROLS
4.3
It is
assumed
that the reader understands the operational function of each
control,
as described
in
Chapter
2.
FUNCTION
AND
RANGE
SWITCHING
Switch Selection
4.4
In
the circuit diagram.
Fig.
4
at the
back
of the
book,
the Function
and Range
push-button banks
SI
and
S2 are
at the lower
left
corner.
It
should be noted
that
the
PERIOD
and
'n'
(1)
switches are
shown
as
selected
and
all
other push-buttons are
at 'off.
Switching
functions are directed by
logic signals, the
+5V
supply
and
'pull-up'
resistors
being
provided
in
the d.i.l.
resistor
array
R63 and
discrete
resistors
R53-R55.
Control
Codes
4.5
Operational modes
within the
CDI
Chip
(other
than
AUTO)
are selected
by
the
switch bank
SI
and
the
a,b and
c lines,
which
are
connected
via inverters ICla,
c
and e
to
IC2. Table 4, below, gives
the logic coding
at the switch connections. Time base
(range) selection
is
made
via
switch bank
S2
and
lines
x,y and
z,
which
control the
gate
times
in
IC2
via the inverters IC1b,d and
f.
Table
5
gives the logic coding
at the switch
connections.
Frequency Auto
Mode
4.6 The
autoranging
facility operates
on Frequency mode
and
automatically
selects the
4-1
9901 Vo. 2