Philips PM 5390 Service Manual page 9

2-2
Output
attenuation
is
provided
in
two
stages:
fine
adjustment
is
carried
out
by
the pin-diode atteniito
r
in
the
RF
oscillator unit,
providing
a
continuous range
from 0
to
20 dB
in
1
dB
steps.
Coarse
attejH
^-
tion
is
achieved outside the
RF box by
a
100
dB
attenuator
in
10
dB
steps.
For the
PM
5390 S vefsion
a
20
dB power
amplifier
is
added.
The
RF
signal
is
then fed to the
RF
OUTPUT
socket.
Frequency modulation
signals
are fed to
the mixer
oscillator
VCO
3
in
order to have
a
constant moclLi-
lation coefficient.
FM
is
only possible
in
the
two
lower frequency ranges
up
to
340
MHz,
as
VCO^
is
active
in
those ranges only.
The
modulation
interface unit 3
processes
and matches
the
modulation
signals
for the
RF
units, i«e.
the
Internal
and external
FM,
AM
or
VIDEO. The
different
modes
are controlled
via
port
1
on
unit
1
.
The
sound
part consists
of
a
sound
carrier
oscillator/modulator (switched
in
during
VIDEO),
ajjilz
oscillator
and an input
circuitry
for external/internal
sound
signals
for generation of
AM
and
FM
moclu-
lation.