Non-linear squelch circuit

Abstract: Method and apparatus for non-linearly filtering impulse and low frequency noise received over a network so as to avoid false activation of equipment is disclosed. A squelch circuit assures that only multiple transitions of at least three half-cycles in the received waveform of a proper frequency and amplitude will activate the receive while a single half-cycle, although of a proper frequency and exceeding the threshold, will not falsely activate the receiver. RC circuits provide a variable delay in the squelch portion of the filter to suppress impulses. In an alternative embodiment, double half-cycles will not falsely activate the receiver. Emitter-coupled logic (ECL) circuitry is preferably employed to provide speed adequate for ten-million bit per second transmissions.

Description: This invention relates to interconnection of electronic equipment in a local area network and, more particularly, to a filtering and squelch circuit which eliminates impulse and other low frequency noise from falsely activating equipment located on the network. BACKGROUND OF THE INVENTION Interconnection of electronic equipment located at a premises is commonly performed by coaxial cable meeting rigorous standards. Shielded coax cable allows interconnection in a local area network (LAN) covering up to 300 linear feet distance. This is because external radio frequency (RF) energy is not induced in shielded coax cable and because crosstalk energy between parallel cables is suppressed by the use of such cable. Installing coax cable in a premises is costly and disruptive to the work being performed. In many cases, holes must be made in walls and/or floors and, possibly, an entire subflooring must be installed, to permit routing of coax cable. Once a particular cabling arrangement is established, it can be changed only with difficulty. Most buildings have an extensive telephone wiring system already in place. It would be highly desirable to use these twisted-pair (TP), unshielded, wires to interconnect electronic equipment, such as computers, workstations, data entry terminals, and the like. Even in buildings being planned, it would be desirable to use ordinary TP telephone wires rather than coax cable because of the formers' ease of installation and lower cost per foot. But these unshielded TP wires act as an antenna and are susceptible to external radio frequency (RF) energy. This RF noise can swamp a signal carried by the TP wires and thus they provide very poor signal-to-noise (S/N) ratios. Particularly, when long distances such as 300 feet are involved, as would be typical of a LAN. S/N ratios can be improved by using very high signal transmission power levels, but FCC regulations specify maximum tansmission levels and high power tansmission is costly. A better solution is to provide a totally differential balanced receiver and to filter at transmission to fall within the FCC regulations, and filter at receiver to 40-100 megahertz (MHz) which then eliminates bulk of external RF noise susceptibility. Known in the prior art are linear bandpass filters which are used to attenuate all signals outside of the 40-100 MHz band of frequencies. Such filters are expensive and do not provide the impulse rejection needed to avoid false activation of a receiver employed a LAN. Because the LAN to be employed with the instant invention is to be interconnected by TP unshielded wires, such impulse rejection is especially important. What is required is a fast-attack discriminating circuit which provides both bandpass filtering and "squelching" of impulses. EP-A-0 132 307 discloses a non-linear filter comprising: filter means responsive to a received signal for generating filtered differential signals representing said received signals having a frequency between a first and a second predetermined frequency; squelch means responsive to said filtered signals for generating a squelch signal at a first level when a noise pulse is detected in the filtered signal and at a second level when no noise pulse is detected, and logic means responsive to said filtered signals and said squelch signal for suppressing said filtered signals upon reception of said first level of said squelch signal and for not suppressing said filtered signals upon reception of said second level of said squelch signal. However, no details are given as to how the first and second levels of the squelch signal are generated. IBM Technical Disclosure Bulletin, 6 , No. 8, January 1964, pages 112-113 and R.C.A Technical Notes, No. 1332, September 1983, pages 1-4 both disclose that noise can be eliminated or reduced by suppressing input signals having less than a predetermined duration or not having a "normal" pulse width, respectively. According to a first aspect of the invention there is provided a non-linear filter comprising: filter means responsive to a received signal for generating filtered differential signals representing said received signals having a frequency between a first and a second predetermined frequency; squelch means reponsive to said filtered signals for generating a squelch signal at a first level when a noise pulse is detected in the filtered signal and at a second level when no noise pulse is detected, and logic means responsive to said filtered signals and said squelch signal for suppressing said filtered signals upon reception of said first level of said squelch signal and for not suppressing said filtered signals upon reception of said second level of said squelch signal, characterized in that the squelch signal at said first level is generated upon reception of no more than a predetermined number of consecutive half-cycles of said filtered signals within a preselected frequency range and within a preselected amplitude range, and at said second level upon reception of more than said predetermined number of consecutive half-cycles of said filtered signals within said preselected frequency and said preselected amplitude range. A low-cost filter and non-linear squelch circuit is provided by the present invention which, employed in a receiver in a LAN, eliminates low frequency noise and impulses present by virtue of un-shielded TP wires interconnecting elements on the LAN. The low pass filter and non-linear squelch circuit assures that only signals having the right frequency - 3 MHz to 15 MHz in the case of the IEEE 802.3 standards - and which are not transient impulses, will be passed to the receiver, causing its activation. The resulting non-linear filter provides an inexpensive (approximately 50 cents) solution to the attenuation of error-causing transient "spikes" in the LAN and employs emitter-coupled logic (ECL) circuitry for speed. The invention assures that only a multiple transition of at least three half-cycles of a received waveform, at a proper frequency, each half-cycle exceeding a threshold level will activate the receiver. While a single half-cycle, although at a proper frequency and exceeding the threshold will not falsely activate the receiver. Employing certain circuit values, even a double half-cycle will not falsely activate the receiver. The non-linear squelch portion of the circuit employs ECL comparators and logic elements providing fast suppression of transient impulses so as to avoid false activation of the receiver. Each of a pair of ECL comparators receive the positive or negative portion of the filtered differential signal generated by the low pass filter. A threshold value is also supplied to each comparator and a signal is generated by each comparator if the filtered signal it receives exceeds the threshold. An RC circuit is connected to the output terminal of each comparator to provide a time constant for the decay of the signal generated by the comparator. This time constant is chosen to cut off all frequencies lower than 3 MHz. An ECL NAND gate receives the signals generated by the comparator. In a preferred embodiment, a second RC circuit is connected to the output terminal of the NAND gate to avoid double-cycle false activation. A final ECL comparator generates a squelch signal at a level appropriate for suppressing signals generated by the low-pass filter. BRIEF DESCRIPTION OF THE DRAWINGS Fig.1 is a logic diagram of the non-linear squelch circuit of the instant invention. Fig.2 is a waveform diagram illustrating the operation of the squelch circuit of the instant invention. DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT With reference to Fig. 1, a pair of terminals 50 and 52 carries the signals received on one TP wire by a transceiver. These wires comprise one-half of the full-duplex interconnection commonly employed in telephone equipment. The signals carried thereon follow IEEE 802.3 LAN standard Protocol. A transformer 54 receives at its primary winding the signals present at terminals 50 and 52. A secondary winding of transformer 54 generates signals to the low-pass filter (LPF) 56 of the present invention. LPF 56 includes resistors 58 and 60 connected in series between the secondary windings of transformer 54. A first terminal of a capacitor 62 is connected to the common point of the resistors 58 and 60. The second terminal of capacitor 62 is connected to ground. A pair of resistors 64 and 66 have a first terminal connected to each end of the secondary windings of transformer 54. The second terminals of resistors 64 and 66 are connected to a first terminal of inductors 68 and 70, respectively. A capacitor 72 is connected between the second terminals of resistors 64 and 66. A capacitor 74 is connected between the second terminals of inductors 68 and 70. Also connected between the second terminals of inductors 68 and 70 is a series connection of resistors 76 and 78. The second terminals of inductors 68 and 70 is also connected to an inverting and a non-inverting input, respectively, of an amplifier (amp) 82. The common point of resistors 76 and 78 receives a reference voltage generated by a voltage source 83. The signals generated by amp 82 are differential signals passed by the low pass filter section 56 of the instant invention. These signals are conducted to the squelch portion 84 of the non-linear squelch circuit of the present invention. Squelch portion includes ECL comparators 86 and 88 receiving at a non-inverting and an invorting terminal, respectively, the differential signals generated by amp 82. Inverting and non-inverting terminal, respectively, of comparators 86 and 88 are connected to the common point of a series connection of resistors 90 and 92. The resistors 90 and 92 serve as a voltage-divider and are connected between the reference voltage source 83 and ground. The output terminals of comparators 86 and 88 are connected to the inputs of an ECL AND gate 94. A capacitor 96 and a resistor 98 are connected between the output terminal of comparator 86 and ground. A capacitor 100 and a resistor 102 are connected between the output terminal of comparator 88 and ground. A complementing output terminal of AND gate 94 is connected to an inverting input of an ECL comparator 104. Accordingly, AND gate 94 functions as a NAND gate, and will be referred to thusly hereinafter. A capacitor 106 and a resistor 108 are connected between the output terminal of NAND gate 94 and ground. A voltage source 110 is connected to a non-inverting input of comparator 104. The signal generated by comparator 104 is a squelch generated by squelch portion 84 of non-linear squelch circuit of the instant invention. The voltage generated by voltage source 84 as divided by resistors 90 and 92 provides a reference voltage so that the squelch signal generated by comparator 104 has a predetermined threshold value. A resistor 111 is connected between the output terminal of comparator 104 and the common point of connection of resistors 90 and 92. Resistor 111 provides a hysteresis effect which avoids off-on-off-on cycling of the squelch circuit at the threshold. The signals generated by amp 82 are also conducted to an inverting and a non-inverting input terminal of an amplifier 112. The input terminals of amp 112 are connected to ground via resistors 114 and 116, respectively. The signals generated by op amp 112 are differential signals conducted to a first input terminal of ECL AND gate 118 and 120, respectively. Second input terminals of gates 118 and 120 receive the squelch signal generated by comparator 104. The first input terminals of AND gates 118 and 120 are connected to ground via resistors 122 and 124, respectively. Signals generated at the output terminals of AND gates 118 and 120 represent the differential signals actually applied to data terminal equipment connected to the LAN via the Pod including the receiver section including low-pass filter 56 and the squelch 84, comprising the non-linear squelch circuit of the instant invention. With reference now to Fig. 2, the operation of the non-linear squelch circuit of the instant invention will now be described. The differential signals applied at terminals 50 and 52 are passed through LPF 56 which acts a linear differential-mode rejection filter preferably having flat response at 3 MHz and a 3 dB attenuation at 15 MHz. Common-mode rejection is provided by transformer 54, and LPF56. A pod and Hub connected via twisted-paid unshielded wires in a LAN are sensitive to signals in the frequency range 3-15 MHz in accordance with the IEEE 802.3 LAN 10 base 5 standard power spectrum. LPF 56 attenuates much of the impulse and high-frequency noise associated with the use of unshielded TP wires. But, despite LPF 56, spurious transient signals representing high-frequency impulse noise or large low-frequency crosstalk noise in the LAN may falsely activate the DTE connected to the LAN because the LPF 56 will pass such noise if it is in the bandpass frequency 50KHz - 15MHz. Squelch circuit 84 receives differential signals Rx+ and Rx- generated by amp 82 of LPF 56, as shown as waveform 200 of Fig. 2. Because of the application of Rx+ and Rx- to inverting and non-inverting terminals of comparators 86 and 88, respectively, the difference (Rx+) - (Rx-) signal is depicted as waveform 200. The signals generated by comparators 88 and 86 are shown as waveforms 202 and 204, respectively. Each waveform experiences a sharp rise 206, 208, 210 and 212 at successive transitions of waveform 200 and a smooth drop 214, 216, 218 and 220 determined by the RC time constant determined by the values of resistor-capacitor combination 98 and 96, and 102 and 100 connected to the output terminal of comparator 86 and 88, respectively. The magnitude of the rise and overall drop shown for waveforms 202 and 204 is determined by a reference voltage generated by voltage source 83 and voltage divider comprising resistors 90 and 92. The logical product of waveforms 202 and 204 is generated by NAND gate 94 and is shown as waveform 222. As shown, it comprises a signal reaching a HIGH level 224 and 226 upon every rising transition of waveform 200. In one embodiment of the present invention, capacitor 106 is omitted, and a corresponding waveform 228 is generated by comprator 104 representing the squelch signal applied to AND gates 118 and 120. A single one-half cycle transition of waveform 200 will accordingly cause waveform 228 to produce a LOW logic level signal such as shown at 230 and 232 in Fig. 2. Hence, AND gates 118 and 120 will not pass the differential signal passed by LPF 56 as generated by amp 112. A single positive-going impulse even within the 3-15 MHz bandpass frequency of LPF 56 will be suppressed. Accordingly, false activation of the DTE connected to the LAN via a Pod including the non-linear squelch circuit of the instant invention, will be avoided. Two or more such pulses occurring within 100 nanoseconds (ns), however, will not cause suppression of the received and filtered signals. This is because the RC time constants provided by resistor-capacitor combinations connected to the output terminals of comparators 86 and 88 are choosen to cause the decay of signals shown in waveforms at 202 and 204 at 214, 216, 218 and 220 too slowly to allow decline to below the level corresponding to a logic HIGH as received by NAND gate 94. Such level is set by voltage source 83 in conjunction with the values of resistors 90 and 92. Two or more pulses occurring close in time, then, will result in NAND gate 94 generating at its complemented output, a logic LOW coincident with the second pulse. This LOW will be applied to the inverting terminal of comparator 104 which will, in turn, generate a logic HIGH signal. This signal, when applied to AND gates 118 and 120 will not suppress the differential signals passed by LPF 56 and these differential signals will be applied to the DTE and will correctly activate it. In the embodiment without capacitor 106, multiple low-frequency half-cycles of noise less than 3MHz will produce undesirable false deactivation and activation of the squelch circuit. In an alternative embodiment of the invention, consecutive double half-cycles, representing one complete cycle of a spurious transient impulse can be suppresed so as to avoid false activation of the DTE equipment connected to the LAN via the Pod. In this embodiment, capacitor 106 is included in the squelch circuit 84. The value for capacitor 106 is selected so that even though two consecutive impulses cause AND gate 94 to generate the LOW logic signal as described hereinabove, the signal so-generated decay as shown in waveform 234 of Fig. 2. Accordingly, by selecting the value of capacitor 106 large enough, a decay slow enough to maintain a HIGH logic signal, applied to the inverting input terminal of comparator 104, long enough through two consecutive impulses or low-frequency noise of the signal passed by LPF 56. Throughout this time, then, gates 118 and 120 will effectively be OFF and no false activation of DTE will be made. The value of capacitor 106 will however be chosen so that three or more consecutive half-wave pulses will occur over a sufficiently long time to assure ample decay of the waveform 234 to permit generation by comparator 104 of a HIGH logic level. This, in turn, places AND gates 118 and 120 in an "ON" mode thereby permitting passage of received signals to the DTE. The elements 86, 88, 94 and 104 of squelch circuit 84 are preferably ECL monolithic circuits which afford speed to permit fast suppression of impulse noise. Yet, because of the simplicity of the squelch circuit of the present invention, it provides a non-linear filter function at low cost. In another embodiment of the invention, open collector transistor-tansistor logic (TTL) circuitry could be employed to provide the requisite speed.

Claims: A non-linear filter comprising: filter means (56) responsive to a received signal for generating filtered differential signals representing said received signals having a frequency between a first and a second predetermined frequency; squelch means (84) responsive to said filtered signals for generating a squelch signal at a first level when a noise pulse is detected in the filtered signal and at a second level when no noise pulse is detected, and logic means (118,120) responsive to said filtered signals and said squelch signal for suppressing said filtered signals upon reception of said first level of said squelch signal and for not suppressing said filtered signals upon reception of said second level of said squelch signal, characterised in that the squelch signal at said first level is generated upon reception of no more than a predetermined number of consecutive half-cycles of said filtered signals within a preselection frequency range and within a preselected amplitude range, and at said second level upon reception of more than said predetermined number of consecutive half-cycles of said filtered signal within said preselected frequency and said preselected amplitude range. A non-linear filter according to claim 1, further including means (83) for generating a first reference-voltage, said squelch means (84) comprising: first comparator means (86) responsive to a first of said filtered differential signals and said first reference voltage for generating at an output a binary-valued signal indicative of whether said first filtered differential signal exceeds said first reference voltage; second comparator means (88) responsive to a second of said filtered differential signals and said first reference voltage for generating at an output a binary-valued signal indicative of whether said first reference voltage exceeds said second filtered differential signal; first and second passive means (96,98,100,102) coupled to said first and second comparator means output, respectively, for delaying by a first selected amount the time said signal generated at said first and second comparator means, respectively, changes from a first said binary value to a second said binary value; and peak-detection means (94,110) receiving said signals generated at said first and second comparator means outputs for generating at an output said two-level squelch signal when both said received signals exceed in absolute value a second predetermined reference voltage within said first selected delay time, indicating said reception of no more than said predetermined number of consecutive half-cycles of said filtered signal. A non-linear filter according to claim 2, wherein said peak-detection means comprises: a NAND gate (94) having a first and a second input terminal connected to said first and second comparator means (86,88) output, respectively, generating a signal therefrom at an output terminal; means (110) for generating said second predetermined reference voltage; and third comparator means (104) responsive to said second predetermined reference voltage and to said signal generated by said NAND gate for generating at an output said two-level squelch signal indicative of whether said signal generated by said NAND gate exceed said second reference voltage, indicating said reception of only one of said consecutive half-cycles of said filtered signal at said first level or of at least two of said consecutive half-cycles of said filtered signal at said first level. A non-linear filter according to claim 3, wherein said peak detection mean further comprises: third passive means (106,108) coupled to said NAND gate (94) output terminal for delaying by a second selected amount the time said signal generated at said NAND gate output terminal changes from a first to a second level, whereby said squelch signal generated by said third comparator means indicates said reception of only two of said consecutive half-cycles of said filtered signal at said first level and of at least three said consecutive half-cycles of said filtered signal at said second level. A non-linear filter according to any preceding claim, wherein said logic means comprise: a first AND gate (118) receiving at a first input a first of said filtered differential signals and at a second input said squelch signal and generating a first differential filtered, squelched signal at an output terminal; and a second AND gate (120) receiving at a first input a second of said filtered differential signals and at a second input said squelch signal and generating a second differential filtered, squelched signal at an output terminal. A non-linear filter according to any of claims 2 to 5, wherein said first passive delay means comprises a first capacitor (96) having a first capacitance connected between said first comparator means (86) output and ground and a first resistor (98) having a first resistance connected between said first comparator means output and ground; wherein said second passive delay means comprises a second capacitor (100) having said first capacitance connected between said second comparator means (98) output and ground and a second resistor (102) having said first resistance connected between said second comparator means output and ground; and wherein said first selected delay is the RC "time-constant" provided by said first capacitance and said first resistance. A non-linear filter according to any of claims 4 to 6, wherein said third passive delay means comprises a third capacitor (106) having a second capacitance connected between said NAND gate (94) output terminal and ground and a third resistor (108) having a second resistance connected between said NAND gate output terminal and ground, wherein said second selected delay is the RC "time-constant" provided by said second capacitance and said second resistance. A non-linear filter according to any of claims 3 to 7, wherein said first and said second comparator means (86,88) each have at least a first input terminal coupled to said first reference voltag (83), said first input terminals connected together, said non-linear filter (56) further including resistance means (92) connected between said third comparator means output and said common connection of said first input terminals of said first and second comparator means, for effecting hysteresis within said squelch means (84). A method of non-linearly filtering a received signal comprising the steps: a) filtering said received signals by attenuation outside a predetermined frequency pass-band; and b) generating a squelch signal at a first level upon reception of no more than a predetermined number of consecutive half-cycles of said filtered signal within a preselected frequency range and within a preselection amplitude range and at a second level upon reception of more than said predetermined number of consecutive half-cycles of said filtered signal within said preselected frequency range and said preselected amplitude range; and suppressing said filtered signals upon reception of said first level of said squelch signal and not suppressing said filtered signals upon reception of said second level of said squelch signal. A non-linear filtering method according to claim 9, wherein step (b) includes: (b1) comparing said filtered signals against a first and a second predetermined reference voltage; (b2) generating at a first level a first bi-level signal when said filtered signals exceed said first reference voltage and generating at a first level a second bi-level signal when said filtered signals are less than said second reference voltage; (b3) delaying said first and said second signals a selected amount in the time said first and said second signals change from said first level to a second level; (b4) generating a first level of a bi-level squelch signal when said first and said second signals exceed a third predetermined reference voltage within said selected delay time; and (b5) suppressing said filtered signals upon generation of said first level of said squelch signal. A non-linear filtering method according to claim 10, wherein step (b4) further includes delaying generation of said squelch signal a selected amount in the time said squelch signal changes from said first level to a second level. A non-linear filtering method according to claims 10 or 11, wherein said delay at step (b3) is selected so that said filtered signals are suppressed when not sustained for at least one half-cycle.

Inventors: HUANG FRED G, HUANG, FRED G.
Applicants: HEWLETT PACKARD CO, HEWLETT-PACKARD COMPANY
Application year: 1988
Main classification: H04L1
IPC codes: H04B1, H03H11, H04L25, H03K5, H04L1