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12.2.3
1-KHz
TONE
A filtered
,
1-kHz square wave provides the fixed,
1-kHz modulation
source.
The SYNTH 1KHz
signal
from the Processor
Interface
board (All)
is
filtered to
less than 1 percent
distortion by
bandpass filter U46.
12.2.4
INTERNAL MODULATION
The
internally
generated modulation consists of the
output
of either
the
audio
synthesizer
or the DPL
gen-
erator.
12.2.5
EXTERNAL
MODULATION
External modulation can come from either
the
microphone input
or the
external modulation input
port (Ext Mod In)
on the
front panel. An instanta-
neous deviation-control (IDC) circuit (U45) amplifies
and
limits
the
microphone
signal (MIC
IN)
before
summing
it with the
signal
EXT MOD
IN
from the
front panel's port. The resulting signal is the EXT
MOD
source.
12.2.6
MODULATION
CONTROL
The three modulation
sources, internal,
exterrral,
and 1-kHz, are
level-controlled
by either the front
panel controls or the
IEEE
Interface board (A13).
The
level-adjusted sources
are then returned to the Audio
Synthesizer board, where they are summed together
12-2
and de-adjusted by
U38 for
0
Vdc,
forming
the
com-
posite modulation audio. The
composite
signal
is
routed to four places: the modulation-select
switch,
the
MOD
CAL
AUDIO
line to
the
Scope/DVM
Control
board
(A 7),
the F.P. MOD OUTPUT line to the front
panel, and VOL CONTROL AUDIO
line
to the
speaker
(for generate
modes
only).
The modulation-select
switch (U24)
routes
the
composite
modulation
signal
to any of the four mod-
ulators:
to
the
double
sideband-suppressed
carrier
modulator
(DSBSC
MOD) for
sideband
modulation,
to the
RF Synthesizer for
freq uency
modulation (FM
MOD), to the Duplex Generator for frequency
modu-
lation
(OFFSET
MOD)
,
and to the
RF output-level-
ing
loop for
amplitude
modulation.
U47 sums
the
signal
for
amplitude
modulation with a 5-Vdc
level
and then
routes
it to the
variable
RF Level control on the front
panel
(5
VDC
+
AM MOD). At
the
RF Level control,
the signal
is attenuated according
to
the level
setting
to give
the de-plus-AM reference
signal
for the
out-
out-leveling loop.
12.2.7
BOARD
CONTROL
The AF bus provides
all the control
inputs for the
Audio
Synthesizer
board. The
4-bit
address bus
(AF
ADD BUS 0-3) is decoded by the address decoder
(U35)
to
determine which
control
latch is to be
accessed.
Control
data is
transferred
to
the
accessed
latch
on
the 4-bit data bus (AF DATA BUS
0-3).
Syn-
chronization
of
the
data
transfer is the function of
the
AF BUS EN 1
signal
line.
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