FX375 private squelch circuit cml semiconductor products product information publication d/375/4 july 1994 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 m processor. external requirements are a single 5-volt supply, an external 4.0mhz xtal or clock input and signal coupling components. FX375 on-chip pre- and de-emphasis filtering in the tx path 38 programmable tones + 'notone' facility audio path filtering (300hz C 3033hz) low power 5v cmos tone operated private/clear switching ctcss tone encode/decode separate rx/tx speech paths fixed frequency speech inversion m p compatible interface with serial or parallel control loading tx audio output rx audio output bias rx tx bias rx tx tx private clear 6db/octave C 6db/octave tx audio input rx audio input filter output balanced modulator input f clk2 f carrier f emph f emph f clk1 rx tone detect output rx tone decoder output control private enable logic rx path tx path private tx tone output C + ctcss tone detect hysteresis reference load/latch f carrier f clk1 f clk2 ld 5 ld 0 xtal rx tone input rx/tx control press to listen v ss v dd v bias ptl decode comparator input 3333hz d input 0 d input 1 d or serial clock input 2 d or serial data input 3 d or serial enable 2 4 d or serial enable 1 5 8-bit shift register and latches xtal 3333hz filters f emph clocks notone output features
2 pin number function FX375j FX375lh 1 2 3 4 5 6 7 8 9 10 11 12 13 14 xtal/clock: the input to the clock oscillator inverter. an external 4mhz xtal or clock input is 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 : d 0 , d 1 , d 2 , d 3 , d 4 , d 5 , 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 (d 0 C d 5 ), remains as set. this input has an internal 1m w pullup resistor. d 5 C (serial enable 1) : d 4 C (serial enable 2) : d 3 C (serial data input) : d 2 C (serial clock input) : d 1 d 0 rx tone decode output : the output of the decode comparator. in 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 1. 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 c 7 , r 2 , r 3 , d 1 (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 c 7 , r 2 , r 3 , d 1 (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. v ss : negative supply rail (gnd). the FX375lg and ls package styles are configured as a serial-data loading device, parallel programing inputs d 0 , d 1 and d 5 , and the n otone output pin functions are not available. FX375lg FX375ls 2 3 4 C 5 6 7 C C 8 9 10 C 11 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 v bias (notone). these inputs each have an internal 1m w pullup resistor. if FX375lg or ls package styles are used pin 5 (serial enable 2) should be externally connected to v ss .
3 pin number function FX375j FX375lh 15 16 17 18 19 20 21 22 23 24 25 26 27 28 FX375lg FX375l 12 13 14 15 16 17 18 19 20 21 22 23 24 1 tx tone output : this is the buffered, programmed ctcss tone sinewave output in tx. during rx and notone operation this output is held at v bias . see note "g," page 7 with reference to capacitive load limits of this output. v bias : this bias pin is set internally to v dd /2. it must be externally decoupled using a capacitor, c 8 , of 1.0 m f (minimum) to v ss , see figure 2. filter output : the input audio bandpass filter output, this pin must be connected to the balanced modulator input via a capacitor, c 6 , and decoupled to v ss by c 10 , see figure 2. balanced modulator input : the input to the balanced modulator, this pin must be connected to the filter output via a capacitor, c 6 , see figure 2. rx audio output : outputs the received audio from a buffered output stage and is held at v bias when in tx. tx audio output : the output of the audio path in the tx mode and is held at v bias 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, c 4 , 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, c 3 , 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 = '1', tx = '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 3. in rx this input could be taken from the rx tone decode output. this input has an internal 1m w pullup resistor. rx tone input : the received tone input to the on-chip ctcss decoder and should be a.c. coupled via capacitor c 5 , see figure 2. v dd : positive supply rail. a single +5v power supply is required.
4 operational information component unit value r 1 1m r 2 820k r 3 330k c 1 33p c 2 33p c 3 0.1 m c 4 0.1 m c 5 0.1 m c 6 1.0 m c 7 0.1 m c 8 1.0 m c 9 1.0 m c 10 1.0n d 1 small signal x 1 4mhz c 6 c 4 c 3 c 5 decode comparator input rx tone detect output rx tone input rx tone decode output serial clock input C serial data input C serial enable 1 C serial enable 2 C xtal/clock FX375j control tx audio input rx audio input tx audio output rx audio output balanced modulator input tx tone output v dd v bias c 8 c 7 r 2 d 1 d 2 d 3 load/latch 20 15 16 17 18 19 21 22 23 24 25 26 27 28 6 1 2 3 4 5 7 8 9 10 11 12 13 14 ptl private enable filter output r 3 d 5 d 4 d 1 d 0 rx/tx v ss data inputs v ss c 9 c 10 recommended xtal components v ss xtal/clock 1 2 FX375j c 1 c 2 xtal r 1 x 1 xtal notone output receive operation C (rx/tx = '1') in the rx mode tx tone and tx audio paths are held at bias. private rx tone rx tone receive signal path d 0 C d 5 notone enable ptl detect decode state condition tone 1 0 1 0 1 bias x tone 1 0 0 0 1 open not inverted tone 1 0 x 1 0 open inverted notone 0 0 x x 0 open not inverted tone 1 1 1 0 1 bias x tone 1 1 0 0 1 open not inverted tone 1 1 x 1 0 open not inverted notone 0 1 x x 0 open not inverted transmit operation C (rx/tx ='0') in the tx mode the rx audio path is held at bias and the rx tone detect output at logic '0.' private transmitted tone transmit signal path d 0 C d 5 notone enable ptl state phase state condition tone 1 0 1 active 0 open inverted tone 1 0 0 active 180 open inverted notone 0 0 x bias x open not inverted tone 1 1 1 active 0 open not inverted tone 1 1 0 active 180 open not inverted notone 0 1 x bias x open not inverted notes 1. the pre- and de-emphasis circuits remain in the transmit path during clear and private operation. 2. power remains applied to the ctcss tone decoder at all times. 3. carrier frequency = 3333hz during private operation (tx or rx). 4. during clear operation the carrier frequency is turned off to reduce spurious emissions. 5. under rx-notone conditions the notone output can be used to operate squelch circuitry. 6. the functions in this table are applicable when the device is connected as recommended in figure 2. table 1 functions and outputs component tolerances resistors 10% capacitors 20% fig.2 external component connections operational truth table C table 1 (below) illustrates the output paths and logic functions of the FX375 private squelch circuit in both receive and transmit modes.
5 operational information the logical inputs (d 0 C d 5 ) are used to programme the FX375 tone frequency (rx/tx) as shown in table 2 (below). loading of data is carried out in either serial or parallel formats. nominal frequency (hz) FX375 frequency (hz) d fo (%) tone data programme inputs d 0 d 1 d 2 d 3 d 4 d 5 67.0 67.05 + 0.07 111111 71.9 71.9 0 111110 74.4 74.35 - 0.07 011111 77.0 76.96 - 0.5 111100 79.7 79.77 + 0.09 101111 82.5 82.59 + 0.1 011110 85.4 85.38 - 0.2 001111 88.5 88.61 + 0.13 011100 91.5 91.58 + 0.09 110111 94.8 94.76 - 0.04 101110 97.4 97.29 - 0.11 010111 100.0 99.96 - 0.04 101100 103.5 103.43 - 0.07 001110 107.2 107.15 - 0.05 001100 110.9 110.77 - 0.12 110110 114.8 114.64 - 0.14 110100 118.8 118.8 0 010110 123.0 122.8 - 0.17 010100 127.3 127.08 - 0.17 100110 131.8 131.67 - 0.10 100100 136.5 136.61 + 0.08 000110 141.3 141.32 + 0.02 000100 146.2 146.37 + 0.12 111010 151.4 151.09 - 0.2 111000 156.7 156.88 + 0.11 011010 162.2 162.31 + 0.07 011000 167.9 168.14 + 0.14 101010 173.8 173.48 - 0.19 101000 179.9 180.5 + 0.14 001010 186.2 186.29 + 0.05 001000 192.8 192.86 + 0.03 110010 203.5 203.65 + 0.07 110000 210.7 210.17 - 0.25 010010 218.1 218.58 + 0.22 010000 225.7 226.12 + 0.18 100010 233.6 234.19 + 0.25 100000 241.8 241.08 - 0.30 000010 250.3 250.28 - 0.01 000000 serial input mode x x clk data 0 1 notone 000011 table 2 tone programming load load/latch and control loading mode of :- configuration logic logic d 0 C d 5 rx/tx, private enable parallel 1 0 transparent transparent parallel 0 0 latched latched parallel 1 1 transparent transparent parallel 0 1 latched transparent serial 0 1 load data in transparent serial 0 C 1 C 0 0 latch data in latched notes glossary C transparent data at the device inputs acts directly. latched in this position data and/or functions are latched in. '0 C 1 C 0' is a strobe pulse as shown in figures 3 and 4 (timing). table 3 load/latch and control functions load/latch and control functions C the load/latch function regulates the loading of the FX375 tone frequency (d 0 C d 5 , table 2) in either the serial or parallel modes. the control input enables the flexible use of the rx/tx and private enable functions, its use is illustrated in table 3.
6 timing information ...... control instructions are input to the FX375 by serial (figure 3) or parallel (figure 4) means, using data inputs and load/latch as shown in the diagrams below. fig.3 serial load timing min. typ. max. unit serial figure 3 serial mode enable set up time C (t sms ) 250 C C ns clock 'high' pulse width C (t pwh ) 250 C C ns clock 'low' pulse width C (t pwl ) 250 C C ns data set up time C (t ds ) 150 C C ns data hold time C (t dh )50CCns load/latch set up time C (t ll ) 250 C C ns load/latch pulse width C (t llw ) 150 C C ns parallel figure 4 data valid time C (t vp ) 200 C C ns load time C (t l ) 150 C C ns fall time C (t f )CC50ns data hold time C (t h )50CCns serial loading sequence : with load/latch at logic '0' serial data is loaded in the sequence :- d 5 , d 4 , d 3 , d 2 , d 1 , d 0 , rx/tx, private enable. when these 8 bits have been clocked in on the rising clock edge, data is latched by strobing the load/latch input C " 0 C 1 C 0 " (figure 3). table 4 timing fig.4 parallel load timing load data data latched load/latch t vp t l t f t h - data inputs - d0 C d5 rx/tx private enable serial clock serial mode enable d 4 d 5 serial data input load/latch t sms t pwh t pwl t ds t dh t ll t llw
7 specification electrical characteristics absolute maximum ratings exceeding the maximum rating can result in device damage. operation of the device outside the operating limits is not implied. supply voltage - 0.3 to 7.0v input voltage at any pin (ref v ss = 0v) - 0.3 to (v dd + 0.3v) sink/source current (supply pins) +/- 30ma (other pins) +/- 20ma total device dissipation @ t amb 25 c 800mw max. derating 10mw/ c operating temperature range: FX375j - 30 c to + 85 c (ceramic) FX375lg/ls - 30 c to + 70 c (plastic) storage temperature range: FX375j - 55 c to + 125 c (ceramic) FX375lg/ls - 40 c to + 85 c (plastic) operating limits all device characteristics are measured under the following conditions unless otherwise specified: v dd = 5.0v, t amb = 25 c, xtal/clock f 0 = 4.0 mhz, audio level 0db ref: = 300mv rms. composite input signal = 0db, 1khz tone in C12db (6khz band limited) gaussian white noise with a C20db ctcss tone. characteristics see note min. typ. max. unit static values supply voltage 4.5 5.0 5.5 v supply current : rx /tx (operating) C 8.0 C ma rx standby (no decode) C 2.8 C ma rx only (decoding) C 5.0 C ma analogue input impedance C 0.5 C m w analogue output impedance C 0.5 C k w tone input impedance C 1.0 C m w digital input impedance C 1.0 C m w input logic '1' 3.5 C C v input logic '0' C C 1.5 v output logic '1' (i = 0.1ma) 4.0 C C v output logic '0' (i = 0.1ma) C C 1.0 v dynamic values maximum input level C + 10.5 C db decoder tone input signal level 1,4 - 20 C C db response time 1,4,6 C C 250 ms de-response time 1,4,6 C C 250 ms selectivity 4 0.5 C 3.0 %?o encoder tone output level (relative 775mvrms) - 3.0 0 + 3.0 db tone frequency accuracy - 0.3 C + 0.3 %?o tone harmonic distortion C 2.0 5.0 % tone output load current 2 C C 5.0 ma output level variation between tones C 0.1 C db rise time (to 90% nominal level) (?o >100hz) 5 C 15 C ms (?o <100hz) 5 C 45 C ms
8 specification frequency characteristics characteristics see note min. typ. max. unit rx clear total harmonic distortion 3 C 2 5 % output noise level 7 C - 43 C db passband gain (300hz C 3033hz) C 0 C db passband ripple (300hz C 3033hz) 3 C C 3 db audio stopband attenuation (?in > 3333hz) C 20 C db (?in > 3633hz) C 45 C db (?in < 250hz) C 42 C db rx invert carrier frequency C 3333 C hz total harmonic distortion 3,8 C 4 10 % baseband breakthrough C - 40 C db carrier breakthrough C - 40 C db output noise level 7 C - 37 C db passband ripple (300hz C 3033hz) 8 C C 5 db audio stopband attenuation (?in > 3333hz) C 50 C db (?in > 3633hz) C 60 C db (?in < 250hz) C 60 C db tx clear total harmonic distortion 3 C 3 5 % output noise level 7 C - 43 C db passband gain (300hz C 3033hz) 3 C 0 C db passband ripple (300hz C 3033hz) 3 C C 4 db audio stopband attenuation (?in > 3333hz) C 20 C db (?in > 3633hz) C 45 C db (?in < 250hz) C 42 C db pre- and de-emphasis C C 6 db/octave tx invert carrier frequency C 3333 C hz total harmonic distortion 3,8 C 4 10 % baseband breakthrough C - 40 C db carrier breakthrough C - 40 C db output noise level 7 C - 37 C db passband ripple (300hz C 3033hz) 3,8 C C 5 db audio stopband attenuation (?in > 3333hz) 8 C 50 C db (?in > 3633hz) 8 C 60 C db (?in < 250hz) 8 C 60 C db pre- and de-emphasis C C 6 db/octave notes 1. these values are obtained using the external integrator components as detailed in figure 2. 2. an emitter follower output. 3. with an input signal of 1khz @ 0db. 4. under composite signal test conditions. 5. any programmed tone with rl = 600, cl = 15pf. including any response to a phase reversal instruction. 6. ?o > 100hz, (for 100hz > ?o > 67hz : t = [100/?o (hz)] x 250ms). 7. input a.c. short circuit, audio path enabled, measured in a 30khz bandwidth. 8. due to frequency inversion, this figures reflects the difference from the expected ideal response.
9 receive condition the signal path will only be inverted when the programmed ctcss tone is received. although other logic actions will enable the receive path, privacy of the conversation is maintained at all times. pre- and de-emphasis (6db/octave) filters are included on-chip in the transmit path, so that the use of this device will produce natural sounding audio (clear or private modes) when installed in modern radio communication transceivers, with or without existing audio processing circuitry. the recommended layout is shown in block form below. figure 5 shows the recommended positioning of the private squelch circuit ...... application notes the FX375 private squelch circuit utilizes audio frequency inversion and continuous tone controlled squelch system (ctcss) techniques to provide secure voice communication on a common radio channel. clear/private switching is controlled by the logic state of the private enable input. table 1 shows that, in the fig.5 the private squelch circuit installed within a typical audio stage amplifier pre emphasis band pass filter band pass filter de-emphasis pre-emphasis modulator to transmitter stages demodulator band pass filter band pass filter de-emphasis amplifier from receiver stages clear clear FX375 (shaded areas) when installed within the audio stages of a typical transceiver system. the accompanying waveform diagrams indicate the relative "voice band amplitudes" at each stage of the receive or transmit process. installation recommendations C care should be taken on the design and layout of the printed circuit board taking into consideration the points noted below. (a) all external components (as recommended in figure 2) should be kept close to the package. (b) tracks should be kept short, particularly the audio and v bias inputs. (c) xtal/clock and digital tracks should be kept well away from analogue inputs and outputs. (d) inputs and outputs should be screened wherever possible. (e) a "ground plane" connected to v ss will assist in eliminating external pick-up on input and output pins. (f) it is recommended that the power supply rails have less than 1mv rms of noise allowed. (g) tx tone output loading C large capacitive loads could cause this pin to oscillate. if capacitive loads in excess of 100pf are unavoidable, a resistor of 1k w or greater put in series with the load should minimise this effect.
package outlines the FX375 is available in the package styles outlined below. mechanical package diagrams and specifications are detailed in section 10 of this document. pin 1 identification marking is shown on the relevant diagram and pins on all package styles number anti-clockwise when viewed from the top. handling precautions the FX375 is a cmos lsi circuit which includes input protection. however precautions should be taken to prevent static discharges which may cause damage. cml does not assume any responsibility for the use of any circuitry described. no circuit patent licences are implied and cml reserves the right at any time without notice to change the said circuitry. ordering information FX375j 28-pin cerdip dil (j5) FX375lg 24-pin encapsulated bent and cropped (l1) FX375ls 24-lead plastic leaded chip carrier (l2) not to scale max. body length 10.25mm max. body width 10.25mm FX375lg 24-pin quad plastic encapsulated bent and cropped (l1) not to scale max. body length 10.40mm max. body width 10.40mm FX375ls 24-lead plastic leaded chip carrier (l2) not to scale max. body length 37.05mm max. body width 13.36mm FX375j 28-pin cerdip dil (j5)
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