TM 11-5820-922-40-1
the base of Q2 and turns it on, which
closes the dc loop, making it possible
receive mode of operation, Rcv Audio is
for the teletypewriter to apply its
routed from the receiver/exciter into
binary-coded output to the FSK tone gen-
the TTY-FSK converter via P1-9. Rcv
erator. The heart of the FSK tone gener-
Audio is amplified in amplifier AR1, and
ator is the crystal oscillator (consist-
routed through clipper CR1 and CR2 to
ing primarily of Y1, U1-12 and -13, R29,
clip and square the amplified signal.
C14, and C15). When P1-25 is at ground
The output of the clipper is approxi-
potential, the crystal oscillator will
mately 1.4v p-p, with Marks represented
operate at 4.88880 MHz. If the operat-
by approximately 1575 Hz, and Spaces
ing frequency is changed on the receiver/
represented by approximately 2425 Hz.
exciter control panel, a Tune In Prog-
The Spaces are routed to narrow bandpass
ress signal is applied to P1-22, which
filter (2425 Hz) AR1, and the Marks are
cuts off the crystal oscillator until
routed to narrow bandpass filter (1575
tuning is completed. The output of the
Hz) AR1. The Marks and Spaces are
crystal oscillator is a sine wave and is
routed to CR3-C5 and CR5-C9 detector
routed to the buffer U1. The sine wave
circuits, respectively. The dc level
gates the buffer on and off to produce a
outputs of the detector circuits are
applied to comparator AR1, where the dc
square-wave output. The square-wave out-
put is applied to the programmable
levels either gate-on or gate-off the
divider circuit U2 and U3.
comparator to produce a binary output.
The binary output is applied to the base
of Q2. If the base of Q2 is positive,
depending on whether a Mark or a Space
it will turn on and close the dc loop.
has been received from the teletype-
(The dc loop performs the electrical
writer. At U1 inverting amplifier, a
interface between the TTY-FSK converter
low potential indicates a closed dc
and the teletypewriter, and is used in
loop, or a Mark, and a high potential
both receive and transmit modes of oper-
indicates an open dc loop, or a Space.
ation.) When the dc loop is closed, it
A Space is routed in on U1 inverting am-
produces a Mark in the teletypewriter.
plifier and out as a low potential.
When the dc loop is open (negative pulse
This low potential, representative of a
at the base of Q2, shutting Q2 off), a
Space, and the high potential at U1
Space is produced. The dc loop can be
input are applied to programmable
switched from 60 milliamperes operation
divider U2 and U3 and turn on the divide-
to 20 milliamperes operation depending
by-63 network. The frequency out of the
on the type of teletypewriter used.
divide-by-63 network (77.6 kHz) is
routed through the symmetrical divide-
by-32 flip-flop U4, and the resulting
transmit mode of operation, the TTY KEY
switch on the teletypewriter is placed
2425-Hz output is applied to the output
buffer circuit and output to the
in the transmit position, which puts
receiver/exciter as an AFSK signal. A
P1-25 at ground potential forward-
Mark is routed in on U1 inverting
biasing CR4. When CR4 is forward-
amplifier and out as a high potential.
biased, a ground potential is applied
to P1-17, the receiver/exciter PTT line,
This high potential, representative of a
Mark, and the low potential at U1 input
which places the receiver/exciter in the
are applied to programmable divider U2
transmit mode. Placing the TTY KEY switch
and U3 and turn on the divide-by-97
in the transmit position also applies a
network. The frequency out of the
low potential to the noninverting input
divide-by-97 network (50.4 kHz) is
of AR1 (2425-Hz narrow bandpass filter).
routed through the symmetrical divide-
The low potential output of this filter
by-32 flip-flop, and the resulting
is applied to the inverting input of the
1575-Hz output is applied to the output
AR1 comparator, the output of which is a
buffer circuit and outputted to the
high potential. The high potential out-
receiver/exciter as an AFSK signal.
put of the AR1 comparator is applied to
