DIGITAL LINE RECEIVER

Abstract: A stream of high-speed bipolar signals (18) (19) transmit­ ted along standard twisted pair (13) telephone wiring are detected by a circuit (10) that includes a pair of differential amplifiers (20) (21) each having a signal input (11) (12) connected directly to one conductor (14) (15) of the twisted pair (13) and a reference input (22) (23) connected through a peak voltage detector (29) to the opposite conductor (14) (15). The amplifiers (20) (21) function as an input signal comparator which generates a corresponding bit stream of logical ones in response to the bipolar signals that exceed 50% of the average peak voltage input. A threshold bias voltage corresponding to positive going signals on each conductor (14) (15) is generated by the detector (29) and charges separate capacitors (37) (39) that connect each reference input (22) (23) to its conductor (14) (15). The bias voltage and bipolar signals input to each amplifier (20) (21) are algebraically summed which doubles the differential input signal and results in an increased signal to noise ratio of about six decibels.

Description: This invention relates to a digital signal receiver and more particularly to a digital line receiver for detecting a stream of bipolar input signals and generating a corresponding bit stream of logical ones in response thereto. User demand has demonstrated a need for increasingly higher speed digital access in new office communication controllers as daily routines become more complex and greater demands are placed on improving productivity. Current Local Area Network (LAN) solutions that provide this higher speed are beset by high cost and congestion. Moreover, these solutions are usually poorly suited to integrating voice services. An alternative to the traditional LAN approach is to integrate all voice and data services into a centralized controller and to utilize inexpensive standard in-building twisted pair wiring to provide the required interconnects. A commonly experienced problem in this approach is the restriction to low baud rates for transmission. Notwithstanding the limited transmission capability of standard twisted pair wiring, a viable centralized controller is expected to execute both ends of a 2.56 Mb/s digital line interface over such wiring. A tri-level serial data stream and its inverted form are received and separated into data and clock for use with standard bi-level logic is disclosed in US Patent 4380080, in which the incoming tri-level signal and its inverted form are separately biased to permit a true differential comparison and signal detection. High common mode rejection of noise that is developed between a transmitter and receiver is thus provided. A digital signal receiving circuit is furthermore disclosed in JP-A-59-193617, which comprises comparator means peak detector means coupled to a signal terminal to receive a predetermined portion of the bipolar signal, and a corresponding output producing a threshold voltage in response thereto and coupling the voltage to a corresponding reference terminal of said operational amplifier; and capacitor means receiving a charge corresponding to the threshold voltage. Accordingly, a principal object of the present invention is to provide a digital line receiver that is operable as an interface over standard twisted pair wiring at transmission rates up to at least 2.56 Mb/s. Another provision of the invention is a digital line receiver having differential input amplifiers that provide common-mode noise rejection while maximizing eye opening to a data detector. Still another provision of the invention is a digital line receiver that provides some immunity to high frequency noise pulses that could otherwise disturb threshold operating levels of the differential amplifiers. Yet another provision of the invention is a digital line receiver that responds principally to a predetermined differential to achieve maximum signal dynamic range and a minimum mixmatch between positive and negative peaks of a bipolar input signal. The problems associated with the prior art may be substantially overcome and the foregoing objectives achieved by recourse to the present invention which provides a digital receiver, comprising: comparator means having a differential input including a pair of signal terminals connectable to a source of bipolar signals, a pair of corresponding reference terminals each cross-coupled to an opposite one of the signal terminals, and an output producing a chain of pulses in response to a predetermined differential in the bipolar signals; characterized by including peak detector means having a pair of input terminals each coupled to corresponding ones of the signal terminals to receive a predetermined portion of the bipolar signals and a pair of corresponding outputs each producing a threshold voltage in response thereto and coupling the voltage to a corresponding reference terminal, and capacitor means cross-coupling each reference terminal to its opposite signal terminal and receiving a charge corresponding to the threshold voltage, whereby bipolar signals input to the signal terminals are algebraically summed with the corresponding threshold voltage thereby exceeding the predetermined differential and effectively increasing the signal to noise ratios of the bipolar signals. The invention will now be more particularly described with reference to an embodiment thereof shown, by way of example, in the accompanying drawings in which: Fig.1 is a schematic diagram of a digital line receiver in accordance with the present invention; and Fig.2 illustrates idealized wave forms of voltages appearing in the circuit of Fig. 1. A schematic diagram of a digital line receiver 10 is shown in Fig. 1 with a pair of input signal termials 11 and 12 which are connected to a telephone line shown as a standard twisted pair 13 comprising individual conductors 14 and 15. The pair 13 are employed in the transmission of a signal stream composed of alternate mark inverted encoded pulses 16 and 17 which appear in Fig. 1 in an idealized form. Typically signals input to the terminals 11 and 12, following modification by the capacitive, resistive and inductive effects of the pair 13, appear as bipolar signals 18 and 19 as shown in Fig. 2 a . The receiver 10 functions to detect logical ones whenever the terminals 11 and 12 differ in voltage by more than 50% of the average peak voltage applied thereto. Having regard to Fig. 2 a the signal 18 typifies the input to the terminal 11 whereas the signal 19 is representative of the input to the opposite terminal 12. It will be observed that the terminals 11 and 12 are connected to a pair of positive inputs of a comparator circuit that comprises two differential amplifiers 20 and 21. The amplifiers 20 and 21 also include a pair of reference terminals 22 and 23 which are connected to negative inputs as shown. Although not indicated in Fig. 1, it will be understood that the amplifiers 20 and 21 have considerably lower sensitivity to a common-mode voltage applied to the terminals 11 and 12 than to a differential voltage applied thereto. Common-mode voltage refers to voltages having the same polarity whereas differential voltages are of opposite polarity. In the receiver 10, it is intended that the signals 18 and 19 be mainly differential to achieve a maximum signal dynamic range and a minimum mismatch between positive and negative going peaks. A voltage divider 24, consisting of like valued resistors 25 and 26 and a lower valued resistor 27, is connected across the terminals 11 and 12. In this arrangement the complete incoming bipolar signal is applied to the positive inputs of the amplifiers 20 and 21 whereas a portion of the input signals appears at the junctions of the resistors 25, 26 and 27. Thus, at the junction of the resistors 25 and 27 the signal voltage is slightly greater than 50% of the peak voltage at the terminal 11 relative to the terminal 12. Similarly the signal voltage at the junction of the resistors 26 and 27 is slightly greater than 50% of the peak voltage at the terminal 12 relative to the terminal 11. The reduced signal voltages of the divider 24 are input to a peak detector 29 which comprises a pair of differential amplifiers 30 and 31. Each amplifier 30 and 31 includes a pair of input terminals 32 and 33 which are connected to the junctions of the resistors 25 and 27 and 26 and 27, respectively. A second pair of input terminals 34 and 35 serve as reference inputs for their respective amplifiers and are cross-connected to an opposite input terminal. Thus, the terminal 34 is shown connected to the terminal 12 via a resistor 36 in series with a bias capacitor 37. Similarly, the terminal 35 is cross-connected to the terminal 11 via a resistor 38 in series with a bias capacitor 39. Thus, except for stray capacitances intermediate circuit conductors and power rails, together with the common-mode range of the amplifiers 20 and 21, the terminal 11 may be considered as a ground reference for the operation of the amplifier 31 in the peak detector whereas the terminal 12 may be considered as the ground reference for the amplifier 30 of the peak detector. The purpose of the resistor 27 is to compensate for the aforenoted loss mechanisms in the peak detector 29 and for the absence of a very high common-mode rejection in the amplifiers 30 and 31. Accordingly, a positive going signal 18 input to the terminal 32 produces an output at the amplifier 30 which is applied to the base of an NPN transistor 42, turning the transistor on and charging the capacitor 37 through the resistor 36 to a voltage Vc. The voltage Vc is the peak voltage at the terminal 11 relative to the terminal 12 less voltage drops in the amplifier 30, the transistor 42 and the resistor 36. Vc should be 50% of the peak voltage at the terminal 11 relative to the terminal 12. The voltage is less, however, due to the aforenoted voltage drops as well as to stray circuit capacitance along with the emitter-base junction capacitance of the transistor 42. These losses are corrected by adjusting the value of the resistor 27 to give an appropriately increased voltage at the terminal 32 relative to the terminal 12. It will be understood from the foregoing description that the amplifier 30 and the transistor 42 function together as a diode equivalent having a low offset voltage. Together, the resistor 36 and the capacitor 37 act as a low-pass filter to reduce sensitivity to short noise spikes such as may be produced by logic circuitry. The capacitor 37 also operates to store the detected peak voltage which is applied to the amplifier 20 via the terminal 22 as the threshold bias voltage Vc. Referring next to Fig. 2 d , the effect of the bias voltage Vc on the signal 18 will be seen having regard to Figs. b and c . Thus, the signal at the terminal 11 rises from zero to an effective maximum amplitude of +Vc as shown at 18'. An equal and opposite peak at the terminal 22 simultaneously varies from +Vc to zero as indicated at 18''. During the interval of the signal 19, the signal at the terminal 11 swings from zero to an effective minimum amplitude of -Vc as shown at 19'. An equal and opposite peak at the terminal 22 simultaneously varies from +Vc to +2Vc as indicated at 19''. The result is a differential input at the terminals 11 and 22 which varies from -3Vc to +Vc as shown by the waveform 18''' in Fig. 2 d . The effect of using the opposite input as a reference for each amplifier 20 and 21 is to increase the input signal to noise ratio by nearly 6 decibels while maintaining noise and interference signals unchanged as compared to the conventional method of using an electrical mid-point between the terminals 11 and 12 as a ground reference. The operation of the other half of the peak detector 29 is the same as that described for the first half except that the ground reference point is taken to be the terminal 11. Accordingly, the amplifier 31 together with an NPN transistor 44 function as a diode equivalent to charge the capacitor 39 to the voltage Vc'. Applied to the terminal 23 as a threshold bias, the voltage Vc' operates in the same manner as the voltage Vc when connected to the terminal 22 since the circuit of the receiver 10 is symmetrical. Waveforms 46 and 47 constitute the outputs of the amplifiers 20 and 21, respectively, which are input to an OR gate 48 that, in turn, produces a corresponding bit stream of logical ones at its output 49. In order that the voltages Vc and Vc' may follow the fluctuations in the peaks of the bipolar input signals, it is required to bleed the capacitors 37 and 39 at a predetermined rate. This is achieved by recourse to a known solid-state switch which functions to alternately charge and discharge a small-valued dumping capacitor. In the case of the capacitor 37, a P channel Field Effect Transistor (FET) 50 is connected in parallel with a complementary N channel FET 50' such that the drains 51 and 51' thereof are connected to the terminal 22 and their respective sources 52 and 52' are connected to drains 53 and 53' of a complementary pair of N channel and P channel FETs 54 and 54'. These latter FETs are likewise connected in parallel so that their sources 55 and 55' are connected to the terminal 12. A dumping capacitor 56 is connected between the sources and drains of the FETs 54 and 54'. Trigger signals shown as clock pulses CLK and CLK are applied to gates 57, 57' and 58, 58', alternately, to time the operation of the foregoing FET switch. Appropriate CLK and CLK pulses turn on the FETs 50 and 50' which permits the capacitor 37 to discharge therethrough to charge the capacitor 56. The FETs 50 and 50' are subsequently switched off by reversing the polarity of the clock pulses and the FETs 54 and 54' are switched on by application of the CLK and CLK pulses to their gates 58 and 58'. The capacitor 56 is therefore discharged during the time that the FETs 54 and 54' are switched on. Periodic partial discharge of the capacitor 39 occurs in the same manner as described for the capacitor 37 using corresponding P and N channel FETs 60 and 60' and corresponding N and P channel FETs 61 and 61' configured as a solid-state switch together with a dumping capacitor 62.

Claims: A digital receiver (10), comprising: comparator means (20) (21) having a differential input including a pair of signal terminals (11) (12) connectable to a source of bipolar signals (18) (19), a pair of corresponding reference terminals (22) (23) each cross-coupled to an opposite one of the signal terminals (11) (12), and an output (49) producing a chain of pulses in response to a predetermined differential in the bipolar signals; characterized by including peak detector means (29) having a pair of input terminals (32) (33) each coupled to corresponding ones of the signal terminals (11) (12) to receive a predetermined portion of the bipolar signals (18) (19) and a pair of corresponding outputs each producing a threshold voltage in response thereto and coupling the voltage to a corresponding reference terminal (22) (23), and capacitor means (37) (39) cross-coupling each reference terminal (22) (23) to its opposite signal terminal (12) (11) and receiving a charge corresponding to the threshold voltage, whereby bipolar signals (18) (19) input to the signal terminals (11) (12) are algebraically summed with the corresponding threshold voltage thereby exceeding the predetermined differential and effectively increasing the signal to noise ratios of the bipolar signals (18) (19). A receiver (10) as claimed in Claim 1 further comprising means (50 (50') (54) (54') (56) (60) (60') (61) (61') (62 for periodically discharging a portion of the charge acquired by the capacitor means (37)(39) to permit following fluctuations in bipolar signal peaks. A receiver (10) as claimed in Claim 2 wherein the capacitor means (37)(39) comprises a pair of separate bias capacitors (37)(39). A receiver (10) as claimed in Claim 3 wherein the means for discharging a portion of the charge acquired by the capacitor means (37)(39) comprises: a pair of dumping capacitors (56)(62) each having one terminal connected to a terminal of a corresponding bias capacitor (37)(39); and switch means (50)(50') serially connected with the free terminal of each dumping capacitor (56)(62) and the other terminal of the bias capacitor (37)(39) and operably responsive to a source of trigger signals to enable momentary connection of each dumping capacitor (56) (62) across its corresponding bias capacitor (37)(39) to discharge a portion of the charge thereon. A receiver (10) as claimed in Claim 4 wherein the switch means (50)(50')(60)(60') further comprises means (54)(54')(61)(61') for alternately shorting each dumping capacitor (56)(62) when disconnected from its corresponding bias capacitor (37)(39). A receiver (10) as claimed in Claim 5 wherein the switch means (50)(50')(60)(60') comprises a first pair of parallel connected complementary N and P channel Field Effect Transistors (50)(50')(60)(60') serially connected with a second pair of parallel connected complementary N and P channel Field Effect Transistors (54)(54')(61)(61') with the first and second pairs of transistors bridging the bias capacitor (37)(39) and one of the pairs of transistors bridging the dumping capacitor (56)(62). A receiver (10) as claimed in Claim 6 wherein the input terminals (32)(33) of the peak detector (29) are coupled to the signal terminals (11)(12) via a resistive bridge comprising first and second resistors (25)(26) of the same value, each resistor having one end connected to respective ones of the input terminals (11)(12), and a third resistor (27) of lesser value connected intermediate the free ends of the first and second resistors (25)(26) and across the input terminals (32)(33) of the peak detector (29). A receiver (10) as claimed in Claim 7 wherein the comparator means (20)(21) comprises a pair of first differential amplifiers (20)(21) each having a differential input of which a first pair of corresponding inputs comprise the pair of signal terminals (11)(12) and a second pair of corresponding inputs comprise the pair of reference terminals (22)(23). A receiver (10) as claimed in Claim 8 wherein the peak detector means (29) comprises a pair of second differential amplifiers (30)(31) each having a differential input of which a first pair of corresponding inputs comprise the pair of input terminals (32)(33) coupled to the signal terminals (11)(12) and a second pair of corresponding inputs (34)(35) are each coupled to corresponding ones of the peak detector outputs. A receiver (10) as claimed in Claim 9 wherein the outputs of the peak detector means (29) each comprise a transistor (42)(44) having a base electrode connected to an output terminal of one of the second differential amplifiers (30)(31), a collector electrode connected to a source of operating voltage and an emitter electrode connected to the reference terminal (22)(23) of the corresponding first differential amplifier (20)(21) via a current limiting resistor (36)(38). A receiver (10) as claimed in Claim 10 wherein the output of the comparator means (20)(21) comprises separate outputs from the first differential amplifiers (20)(21) connected as inputs to an OR gate (48) having an output (49) producing a bit stream of logical ones.

Inventors: HOGEBOOM, JOHN GORDON, THOMAS, TERENCE NEIL
Applicants: NORTHERN TELECOM LIMITED
Application year: 1986
Main classification: H03K5
IPC codes: H03M5, H04L25, H03K5