|Category||Analog & Mixed-Signal Processing|
|Description||Private Squelch Circuit|
|Company||Consumer Microcircuits Limited|
|Datasheet||Download FX375 datasheet|
Tone Operated Private/Clear Switching CTCSS Tone Encode/Decode Separate Rx/Tx Speech Paths Fixed Frequency Speech Inversion µP Compatible Interface with Serial or Parallel Control Loading
On-Chip Pre- and De-Emphasis Filtering in the Tx Path 38 Programmable Tones + 'NoTone' Facility Audio Path Filtering 3033Hz) Low Power 5V CMOSRx TONE DECODER OUTPUT DECODE COMPARATOR INPUT Rx TONE DETECT OUTPUT
XTAL Rx TONE INPUT LOAD/LATCH D5 or SERIAL ENABLE D4 or SERIAL ENABLE 3 or SERIAL DATA INPUT 2 or SERIAL CLOCK INPUT D 1 INPUT D0 Rx/Tx CONTROL PRIVATE ENABLE PRESS TO LISTEN PTL 8-BIT SHIFT REGISTER AND LATCHES CONTROL
The 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 to 250.3Hz, these Xtal derived tones are selected 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 subaudio 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.
Xtal/Clock: The input to the clock oscillator inverter. An external 4MHz Xtal or clock input to be applied at this pin. See Figure 2.Xtal: The 4MHz output of the clock oscillator inverter. See Figure 2.
Load/Latch: This input regulates the operation of the eight input latches D4, D5, Rx/Tx and Private Enable for both parallel and serial input load modes. Rx/Tx and Private Enable inputs can be used independently in either mode by the use of Load/Latch and Control inputs configured as shown in Table 3, the data format D5), remains as set. This input has an internal 1M pullup resistor.D5 (Serial Enable D4 (Serial Enable D3 (Serial Data Input) D2 (Serial Clock Input) D1 D0
The Rx/Tx tone programming and function inputs. Programmed as shown in Table 2 these inputs will select the CTCSS tone frequency and parallel or serial loading function. Notone, when set in receive, enables the Rx Audio Output and forces the Rx Tone Decode Output to a logic '0,' in transmit the Tx Tone Output is held at VBIAS (Notone). These inputs each have an internal 1M pullup resistor. or LS package styles are used Pin 5 (Serial Enable 2) should be externally connected to VSS.
Rx Tone Decode Output : The output of the decode comparator. Rx a logic '0' indicates 'CTCSS tone decoded' above the internal reference level, or Notone programmed. This action internally enables the Rx audio path and Frequency Inversion function (when applicable) as shown in Table In Tx this output is a logic '1'.
Decode Comparator Input : A logic '1' at this pin, in Rx, is compared internally with a fixed reference level, a more positive input value will produce a logic '0' at the Rx Tone Decode Output. This input should be externally connected to the Rx Tone Detect Output via external integrator components R3, D1 (see Figure 2).
Rx Tone Detect Output : This output, in Rx, goes to a logic '1' when a valid, programmed CTCSS tone is received at the Rx Tone Input. This input should be externally connected to the Decode Comparator Input via external integrator components R3, D1 (see Figure 2).
Notone Output : Outputs a logic '0' when a " Notone" CTCSS code has been programmed. It can be used to operate squelch circuitry under receive "Notone" conditions.
VSS : Negative supply rail (GND). The FX375LG and LS package styles are configured as a serial-data loading device, Parallel Programing Inputs D0, D1 and D5, and the NOTONE Output pin functions are not available. 2
Tx Tone Output : This is the buffered, programmed CTCSS tone sinewave output in Tx. During Rx and Notone operation this output is held at VBIAS. See note "g," page 7 with reference to capacitive load limits of this output. VBIAS : This bias pin is set internally VDD/2. It must be externally decoupled using a capacitor, of 1.0µF (minimum) to VSS, see Figure 2. Filter Output : The Input Audio Bandpass Filter output, this pin must be connected to the Balanced Modulator Input via a capacitor, C6, and decoupled to VSS by C10, see Figure 2. Balanced Modulator Input : The input to the Balanced Modulator, this pin must be connected to the Filter Output via a capacitor, C6, see Figure 2. Rx Audio Output : Outputs the received audio from a buffered output stage and is held at VBIAS when in Tx. Tx Audio Output : The output of the audio path in the Tx mode and is held at VBIAS when in Rx. Rx Audio Input : The Audio input pin for the Rx mode. Input signals should be a.c. coupled via an external capacitor, C4, see Figure 2. Tx Audio Input : This is the voice input pin for the Tx mode. Signals should be a.c. coupled via an external capacitor, C3, see Figure 2. PTL : The "Press To Listen" function input, in the receive mode a logic '0' enables the Rx Audio Output directly, overriding tone squelch but not intercepting a private conversation. In the transmit mode a logic '0' reverses the phase of the Tx Tone Output for "squelch tail" reduction (see Table 1), this function, in Tx, should be accurately applied by a timing circuit to ensure correct system operation. Control : This input, with Load/Latch, selects the operational mode of Rx/Tx and Private Enable functions, see Table 3. Rx/Tx : Selects the receive or transmit mode (Rx = '0') and can be loaded by serial or parallel means, as described in Table 3. Private Enable : This input selects either Private or Clear modes (Clear = '1', Private ='0'), and can be loaded by serial or parallel means, as described in Table In Rx this input could be taken from the Rx Tone Decode Output. This input has an internal 1M pullup resistor. Rx Tone Input : The received tone input to the on-chip CTCSS decoder and should be a.c. coupled via capacitor C5, see Figure 2. VDD : Positive supply rail. A single +5V power supply is required.
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