Control Unit - Philips PM 5390 Service Manual

3-3
3.2.
CONTROL
UNIT;
UNIT
2, fig.
38
Unit 2 contains
all
ports,
buffers
and
drivers
to control the different circuits.
The
ports are fed with
information
from
the microprocessor
on
unit
1
via
the
multiplexed address/data bus; this
information
is
stored and
sent via
buffers
an
drivers
to the reed
relais
and attenuators
by means
of
which
the required
parameters
are
converted.
The
ports 2
and 3 contain
memory
registers
to store
the data
bus information
from the processor
and
output
buffers to drive the
output
lines.
These
lines
are
divided into
three
groups
A, B and
C,
each
of
them
with 8
input/output
lines; in
this
instrument they are
used only
as
outputs.
The
output
lines
A0-A4
of port
305 are directly fed via
connector
D
and buffers
on
the
motherboard,
unit
10, to the
RF
unit
2 for control of
the
programmable divider
2,
ICs
308-311
.
The
outputs
A6
and
A7
as well as
B0-B3 send their
information
via
inverting
buffers
316 and connector
D
to the mother-
board and control the
relais
801
-
806,
the contacts of
which switch the supply
voltages for the
6
VCOs
of the main
oscillator in
the
RF
unit.
The
output
lines
B4
and
B5 control the
relais
808
and
807
via
the transistors
337 and 338: they switch
the output
signal
of the pin-diode attenuator to
mixer
2
and actuate
VC03
and the low-pass filter
U10/U1
1
on
the
RF
unit
1
for the
2 lower frequency ranges, 0.1
-
340
MHz.
All
these points in
the
RF
unit
require control voltages being quite accurate
and without saturating voltage of transistors
or drivers;
therefore
reed contacts are used.
The
transistor
335 on
unit
2
is
not
fitted
in
the standard version,
but for the option
PM
5390
S.
The
input of
the
transistor
is
driven
from
the port
output B6; the
output drives
a
relay
which
activates
the
20 dB power
amplifier in
order to
get a higher
output amplitude of
1
Vrms. Actuation
is
done by
switching the
amplifier supply
voltage
from
-12
V
to
+24
V.
The
remaining
output
lines
of port 2
(B7 and C0-C7) are used to control
the coarse
and
fine
attenuator
in
order
to set
the
output amplitude
from -127
dBm
to -7
dBm. The
lines
C5-C7
are
converted
from
TTL
level
at
the port
output to
+12
V;
this level
is
necessary to
control the
programmable attenuator
which
sets
the
output
level in
steps of
1
0
dBm.
Fine
attenuation of the
RF
signal
is
performed by pin-diodes
in
the
RF
unit controlled
by
the
D/A
converter
on
unit 2.
The
port outputs
C0-C4 feed their
currents
through
resistors
605
-
609
into
the
summing
point of the operational amplifier 308.
The
values of these resistors are
weighed. 3
more
lines
control the pin-diode
attenuator: the
outputs BO'
-
B2',
port
3,
1C 309, are
converted
from
TTL-level
at the port
output to
12
V
level
for
CMOS;
they control the solid state
switches 1C 310,
by means
of
which the
resistors
639,
652 and 653
are
switched to
+12
V
feeding
a
current
into
the
summing
point of
1C 308.
Thus
with port
2,
C0-C4,
a total
number
of 8
lines
control the
D/A
converter for
the pin-diode attenu-
ator,
which corresponds to
a resolution of
256
points;
the
sum
of the currents
will
produce
a volt39s
drop
at resistor
683.
Thus
the
output
of 1C
308 shows
a
DC-level
depending
on
the current
through
resistor
683.
The
working point
is
set
by resistor/potmeter
638/681, the gain
by
resistor
662, depen-
ding
on
the characteristic
of the assigned pin-diode.
For further details see also
chapter 3.5.4.
The programmable divider
1,
1C 301,
HEF
4751,
together with
HEF
4750,
VCO
1
and
two
10/1
1:1
prescaler
on
RF
unit
2
build
the phase-locked loop
frequency synthesizer
system
of
the instrument,
see
figs.
3 and
30.