CML Semiconductor Products
PRODUCT INFORMATION
FX375 Private Squelch Circuit
Publication D/375/4 July 1994
Features
Tone Operated Private/Clear
On-Chip Pre- and De-Emphasis
Filtering in the Tx Path
Switching
CTCSS Tone Encode/Decode
Separate Rx/Tx Speech Paths
38 Programmable Tones
+ 'NoTone' Facility
Fixed Frequency Speech Inversion
Audio Path Filtering
(300Hz – 3033Hz)
µP Compatible Interface with Serial
or Parallel Control Loading
Low Power 5V CMOS
CLEAR
Tx
BALANCED
MODULATOR
INPUT
Tx
FILTER
OUTPUT
Tx AUDIO OUTPUT
6dB/OCTAVE
Tx AUDIO INPUT
– 6dB/OCTAVE
Rx
fEMPH
fEMPH
BIAS
BIAS
Rx
fCLK1
fCLK2
fCARRIER
Tx
PRIVATE
Rx AUDIO INPUT
Rx AUDIO OUTPUT
REFERENCE
HYSTERESIS
+
–
3333Hz
FILTERS
XTAL
Rx TONE DECODER OUTPUT
CLOCKS
DECODE COMPARATOR INPUT
Rx TONE DETECT OUTPUT
XTAL
Rx TONE INPUT
CTCSS
TONE
FX375
DETECT
LOAD/LATCH
3333Hz
D5 or SERIAL ENABLE 1
TX
TONE
D4 or SERIAL ENABLE 2
D3 or SERIAL DATA INPUT
OUTPUT
LD
5
LD
0
8-BIT
SHIFT
D2 or SERIAL CLOCK INPUT
D1INPUT
NOTONE OUTPUT
fCARRIER
REGISTER
AND
LOGIC
fCLK1
D0 I N P U T
VDD
CONTROL
fCLK2
LATCHES
Rx/Tx
fEMPH
VBIAS
Rx PATH
CONTROL
PRIVATE ENABLE
Tx PATH
PRIVATE
VSS
PTL
PRESS TO LISTEN
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.
The Privacy function is exclusive only to units using the
same tone set, other intercepted signals remain "as
transmitted."
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.
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 µProcessor.
External requirements are a single 5-volt supply, an
external 4.0MHz Xtal or clock input and signal coupling
components.
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