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HYSTERESIS
Rx TONE DECODER OUTPUT
DECODE COMPARATOR INPUT
Rx TONE DETECT OUTPUT
3333Hz
XTAL
Rx TONE INPUT
LOAD/LATCH
D
5
or SERIAL ENABLE 1
D
4
or SERIAL ENABLE 2
D
3
or SERIAL DATA INPUT
D
2
or SERIAL CLOCK INPUT
D
1
INPUT
D
0
I N P U T
Rx/Tx
CONTROL
PRIVATE ENABLE
PRESS TO LISTEN
PTL
8-BIT
SHIFT
REGISTER
AND
LATCHES
CONTROL
LD
5
LD
0
CTCSS
TONE
DETECT
FX375
TX
TONE
OUTPUT
NOTONE OUTPUT
LOGIC
f
CARRIER
f
CLK1
f
CLK2
f
EMPH
Rx PATH
Tx PATH
PRIVATE
V
DD
V
BIAS
V
SS
Fig.1 Internal Block Diagram
Brief Description
The FX375 is a Low-Power CMOS LSI microcircuit
designed for Tone Operated Voice Privacy in
communication systems.
This half-duplex device consists of a Fixed Frequency
Voice Band Inverter interfaced with a Continuous Tone
Controlled Squelch System (CTCSS) Encoder/
Decoder, whose allocated tone is used for voice
privacy and audio squelch operation.
Frequency Inversion is achieved by modulating the
input audio with a fixed carrier frequency to exchange
the high and low frequencies of the voice band, making
the resulting audio output unintelligible to receivers not
equipped with a compatible system.
The on-chip CTCSS Dencoder is capable of encoding
and decoding any one of 38 sub-audio tones in the
range 67.0Hz to 250.3Hz, these Xtal derived tones are
selected by a 6-bit binary word that can be loaded to
the device in either a serial or parallel format.
The Privacy function is exclusive only to units using the
same tone set, other intercepted signals remain "as
transmitted."
A 'Press to Listen' facility allows monitoring of the
channel prior to transmitting.
This device has separate, switched Rx and Tx voice,
and tone audio paths. Voice paths use switched
capacitor bandpass filters for the attenuation of sub-
audio tones and unwanted modulation products.
6dB/octave pre- and de-emphasis filtering in the Tx
path maintains natural sounding audio from this device
when embodied in communication transceivers.
The FX375, which is available in DIL and SMT
packages, can be simply controlled by switches, or
interfaced to a
碌Processor.
External requirements are a single 5-volt supply, an
external 4.0MHz Xtal or clock input and signal coupling
components.
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