1 kW TTS1000B TRANSMITTER IPA ASSEMBLY
PUB96-30 rev 1: Jul 1, 2010
30-2
1 kW IPA Assembly VHF
In
the
High
Band
unit,
the
output
of
the
hybrid
is
also
fed
via
an
attenuator
R5
(GAIN)
but
this
time
to
an
additional
preamplifier
stage
U4,
whose
output
appears
at
the
input
of
U2,
which
feeds
terminal
J2.
The
spec'd
gain
of
type
MWA330
in
the
U4
position
is
6
dB,
and
type
MHW6185
or
CA2885
(U2)
is
18
dB.
A
few
dB
of
losses
exist
on
the
board,
so
the
effective
gain
of
the
High
Band
preamp
board
10A1453G3
with
R5
at
maximum
is
about
18
to
20
dB.
(High
Band
preamps
10A1453G1
in
higher
powered
externally
diplexed
transmitter
aural
service,
use
a
type
MWA130
as
U4.
Specified
gain
is
about
12
dB,
but
substitution
of
U4
in
the
internally
diplexed
systems
is
not
recommended
due
to
overall
linearity
considerations).
At
the
output
of
U2,
a
match
to
50
Ω
is
provided
by
C12
and
the
device
lead
inductance,
which
together
create
a
matching
network
in
boards
where
a
type
CA2885
device
is
used;
conversely
a
type
MHW6185
device
will
drive
50
Ω
directly
therefore
no
special
output
matching
is
necessary,
and
C12
is
not
present.
Most
LARCAN
exciters
produce
their
best
linearity
at
or
near
their
maximum
rated
output
levels,
and
often
the
overall
system
gain
is
sufficient
to
result
in
overdrive
of
later
stages
of
the
transmitter.
The
transmitter
or
translator
lineup
may
therefore
include
an
in
‐
line
attenuator
in
the
RF
chain
ahead
of
the
IPA
module,
in
order
to
prevent
overdrive
from
certain
models
of
exciter
‐
modulator.
U3
is
a
voltage
regulator
providing
B+
to
the
amplifier,
and
biasing
for
the
varactor
diodes.
RF
isolation
is
provided
by
inductors
L1
and
L2
while
capacitors
C1
and
C2
act
as
DC
blockers.
SRF
3943
‐
2
Intermediate
Power
Amplifier:
Figures
4,
5,
6,
9,
10,
and
11.
The
Intermediate
Power
Amplifier
(IPA)
in
both
the
Low
Band
and
High
Band
versions,
is
configured
in
push
‐
pull,
using
dual
N
‐
channel
enhancement
mode
Field
Effect
RF
power
transistors
which
are
operated
in
class
AB.
The
IPA
circuit
is
very
similar
to
the
circuit
of
a
single
amplifier
of
the
1.5
kW
PA
module
described
in
another
Section
of
this
manual.
The
Low
Band
and
High
Band
versions
of
the
IPA
differ
slightly
due
to
the
frequency
ranges
to
be
covered.
Low
Band
IPA
Circuit
Description
The
IPA
consists
of
two,
source
grounded
N
‐
channel,
insulated
gate
Field
Effect
Transistors
(FETs)
packaged
in
a
single
case,
operating
class
AB
in
a
push
‐
pull
configuration.
The
original
schematic
indicates
a
type
MRF
‐
151
‐
G
as
the
dual
FET
used;
actually
we
now
use
a
"selected
MRF
‐
151
‐
G
to
tightly
controlled
specifications"
which
is
proprietary
to
LARCAN
and
designated
type
SRF
3943
‐
2.
The
