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10.1
DESCRIPTION
The Receiver board recovers baseband signals and
determines
in put-carrier
levels. T he
10.7-MHz
IF
input signal
can
be amplitude (AM), frequency (FM),
or
si ngle
sideband
(SSB)
modulated
with
t he base-
band
information.
Separate detectors are
provided for
AM,
F M,
and
SSB;
the demodulated outputs are
routed
to t he audio
fi
lters.
Selectable
high- pass
and
low-pass
filters
then
fur-
t
her
process the
baseband signal
before
it
leaves
t
he
Receiver
board. Separate
board
out puts
provide
base-
band
signals
to t he front
panel,
to the
circuitry
for
dete rmining mod ulation level,
and
to
the
speaker
amplifier.
In put-carri er
level is
determined
by
a
logarithmi-
cally
respond ing amplifier. The amplitude of the out-
put signal
from t he log
amplifier
is
proportional
to the
log of
the
input-carrier level.
This output,
which
is
used
for t he
spect rum-analyzer
mode, also provides a sig-
nal-strength level
for t he
squelch circuitry.
T he system
processor
provides primary
control
of
the
Receiver
board's
functions via
a
serial
data
bus.
The data
bus uses
a single data line, a
clock
line,
and
a
latc h-enable line to serially shift
control information
from
the
Processor
board to t he
Receiver
board.
Block d iagrams of the Receiver
board and
its func-
tions are
shown
at the
end of
the
section in
Figure
10-1a-e, a schematic
in
Figure 10-2,
and
the
printed
wiring hoard assembly
and
parts list
in
Figure 10-3.
10.2
THEORY OF
OPERATION
1
0.2.1 LINEAR
IF SECTION
1
0.2.1.1 General
The linear
IF
section
consists
of
1)
a tuned
,
gain-
controlled
fi lter
amp
lifie
r
with switchable band-
widths.
:2)
an AGC
amplifier, 3) an
overload detector
driven
by the
AM-detected output, and 4) an
AM
detector.
RF
outputs
are
provided to the
scope,
the
FM
limiter.
the SSB-product
detector
and t
he logarithmic
amplifier.
The AM
demodulated
audio
s
ignal
is
pro-
vided
to
the
audio-filter circuits.
A
block
diagram of
the linear
IF
amplifier
is
shown
in Figu
re
I
0-1 b at
the
end
of
t.he section.
10-1
SECTION 10.
RECEIVER BOARD (AS)
1
0.2.1.2
Pre-Selector
The linear IF
is preceeded
by
a
pre-selector. This
limits
the number of
out-of-band
signa
ls
reaching the
pre-amp lifie
r
and provides
a good
termination
for the
first
mixer/
low-pass
filter
located
on Al
7
A2. T
he
pre-
selector
is
fixed -t uned
wit
hout adjustment and has
a
bandwidth
of 2
MHz (Q
=
5).
1
0.2.1.3
Pre-Amplifier/Filter
T he pre-selector
is followed
by
a
low-noise pre-
amplifier
(a
grounded gate with
a
source
follower)
with
a gain of 28
dB. This
pre-amplifier
provides
good
impedance-matching
for input/output of
the mixer and
FL301
(a 6-pole
ceramic
fi lter with a
bandwidth
of
280
kHz),
as
well
as t he ability to
handle
high-level
sig-
nals.
The
high
gain
of
this pre-amplifier
negates
both
t he 9-d
B
loss
associated
with FL301
and
the
3-dB
power-split
following
the filte r.
FL301 det
ermines
the
wideband
frequency
response
and
rejection
of
the
Receiver and logarithmic
amplifier (log
amp).
1
0.2.1.4
Active
Power Split
A matched pair
of
parallel,
grounded-gate
J Fet
ampl
ifiers
equally split the
received signal
between
the
linea
r
IF and log
IF.
The grounded-gate
amplifier
pro-
vides
isolation
between
the
two
IF
strips
as
well as a
good
im
pedance
match
for the
output
of FL301.
No
au to
matic
gain
control
(AGC)
is appl ied
before
this
point
to
maintain calibration
of the
log amp.
This
requi
res
a high
dynamic-range
ampl
ifier inherent
in
the
grounded-gate
approach.
1
0.2.1.5
Switched
IF Filters and 10.7
-MHz Amplifier
The
IF
bandwidth is
determined
by
a
wideband/
narrowband
filter-set, selectable
under software
con-
trol. Switching occurs
downstream of
the
log-amp
pick-
oiL
in
order
to
main tain log-amp calibration.
FL303,
(a
~-pole,
wideband
ceramic
filter)
reduces image noise
to the mixer in
wideband
mode. FL302
sets
t
he nar-
l'owband
pre-detection
bandwidt
h
and adjacenL-
r
hannel
rejection
of the
Receiver. Q305 serves
as an
AG(''d
amplifier to terminate
the filte rs
and
isolate
them
from
the active
mixer (U302).
Diodes
driven by
LJ:lO
I
(a
n
RS232
d river)
do the filter-switching.
This
provides
level translation
and drive
capability
from
the
r
ont rot
ci
rcuit.
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