Digital slicer having a pulse-width locked loop
First Claim
1. An adaptive slicer for a bit stream, which bit stream includes level indicating pulses having a predetermined nominal duration and finite rise and fall times, the slicer comprising:
- controllable comparator means including a first input terminal and a second input terminal, said first input terminal being coupled for receiving said bit stream and for comparing the instantaneous amplitude of said bit stream with a reference slicing level signal applied to said second input terminal for generating bilevel signals representing bit decisions, the duration of said bilevel signals during said level indicating pulse being indicative of the instantaneous slicing level;
pulse separating means coupled to said controllable comparator means for separating said level indicative pulse portion of said bilevel signals from the remainder of said bit stream to produce a separated level indicating pulse;
controllable start-stop oscillator means coupled to said pulse separating means and responsive to said separated level indicating pulse for generating oscillations at a rate such that a plurality of cycles of said oscillations occur during an interval equal to said nominal duration;
bistable means coupled to said controllable start-stop oscillator means and responsive to each cycle of said oscillations for changing state to form an umprocessed raw control signal, said unprocessed control signal having a first state prior to receipt of each level indicating pulse, and having said first state after receipt of said level indicating pulse if said controllable start-stop oscillator means produces an even number of said cycles of said oscillations in response to said separated level indicating pulse, and having a second state after receipt of said level indicating pulse if said controllable start-stop oscillator means produces an odd number of said cycles of said oscillations in response to said separated level indicating pulse, whereby the average level of said unprocessed control signal varies in the interval between said level indicating pulses depending upon the slicing level;
first filter means coupled to said bistable means for receiving said unprocessed control signal therefrom for filtering said unprocessed control signal to produce a first control signal;
second comparator means coupled to said first filter means for comparing said first control signal with a predetermined reference level to generate a bilevel compared output signal which changes state when said first control signal passes through a value corresponding to said predetermined reference level;
second filter means coupled to said second comparator means and to said second input terminal of said controllable comparator means for filtering said bilevel compared output signal to form said reference slicing level, and for applying said reference slicing level to said second input terminal of said controllable comparator means to form a pulse width locked control loop for maintaining said slicing level near a predetermined value.
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Abstract
A bit-decision circuit or slicer for a digital communications receiver includes a first comparator which compares the received signal level with a reference slicing level. In order to have the lowest error rate, the reference slicing level must be controlled. Control is provided by periodic level indicating pulses in the transmitted signal. The level indicating pulses have predetermined nominal duration and controlled rise and fall times. A pulse width locked loop at the receiver responds to the level indicating pulse at the output of the comparator. The pulse width locked loop includes a start-stop oscillator which produces an odd or an even number of oscillations in response to each level-indicating pulse depending upon the ON time of the oscillator. A flip-flop coupled to the oscillator changes state during each oscillation, so that an even number of oscillations causes the flip-flop output to remain in the same state after the level indicating pulse as before, while an odd number of oscillations causes a change of state. The average D.C. level at the output of the flip-flop therefore changes significantly for a change of duration of the level indicating pulse corresponding to one clock oscillation cycle or a portion of a clock oscillator cycle. The flip-flop output level is filtered and applied as an input signal to a second comparator. The second comparator switches state in response to excursion of the filtered flip-flop signal above and below a reference level. The output of the second comparator is filtered to form the reference slicing level for the first comparator. This closes a pulse width locked loop which takes excursions between two extreme values of pulse width. The data pulses occurring in the interval between level indicating pulses are compared with the same reference level for low error rate.
29 Citations
20 Claims
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1. An adaptive slicer for a bit stream, which bit stream includes level indicating pulses having a predetermined nominal duration and finite rise and fall times, the slicer comprising:
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controllable comparator means including a first input terminal and a second input terminal, said first input terminal being coupled for receiving said bit stream and for comparing the instantaneous amplitude of said bit stream with a reference slicing level signal applied to said second input terminal for generating bilevel signals representing bit decisions, the duration of said bilevel signals during said level indicating pulse being indicative of the instantaneous slicing level; pulse separating means coupled to said controllable comparator means for separating said level indicative pulse portion of said bilevel signals from the remainder of said bit stream to produce a separated level indicating pulse; controllable start-stop oscillator means coupled to said pulse separating means and responsive to said separated level indicating pulse for generating oscillations at a rate such that a plurality of cycles of said oscillations occur during an interval equal to said nominal duration; bistable means coupled to said controllable start-stop oscillator means and responsive to each cycle of said oscillations for changing state to form an umprocessed raw control signal, said unprocessed control signal having a first state prior to receipt of each level indicating pulse, and having said first state after receipt of said level indicating pulse if said controllable start-stop oscillator means produces an even number of said cycles of said oscillations in response to said separated level indicating pulse, and having a second state after receipt of said level indicating pulse if said controllable start-stop oscillator means produces an odd number of said cycles of said oscillations in response to said separated level indicating pulse, whereby the average level of said unprocessed control signal varies in the interval between said level indicating pulses depending upon the slicing level; first filter means coupled to said bistable means for receiving said unprocessed control signal therefrom for filtering said unprocessed control signal to produce a first control signal; second comparator means coupled to said first filter means for comparing said first control signal with a predetermined reference level to generate a bilevel compared output signal which changes state when said first control signal passes through a value corresponding to said predetermined reference level; second filter means coupled to said second comparator means and to said second input terminal of said controllable comparator means for filtering said bilevel compared output signal to form said reference slicing level, and for applying said reference slicing level to said second input terminal of said controllable comparator means to form a pulse width locked control loop for maintaining said slicing level near a predetermined value. - View Dependent Claims (2, 3, 4)
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5. An adaptive slicer for a bit stream, which bit stream includes recurrent sync information and level indicating pulses having a predetermined nominal duration and finite rise and fall times, the slicer comprising:
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controllable comparator means including a first input terminal and a second input terminal, said first input terminal being coupled for receiving said bit stream and for comparing the instantaneous amplitude of said bit stream with a reference slicing level signal applied to said second input terminal for generating inverted and noninverted bilevel signals representing bit decisions, the duration of at least one of said inverted and noninverted bilevel signals during said level indicating pulse being indicative of the instantaneous slicing level; sync separating means coupled to said controllable comparator means for separating said sync information from one of said inverted and noninverted bilevel signals from the remainder of said bit stream to produce a generated sync pulse; pulse generating means coupled to said sync separating means for generating a window pulse having a predetermined duration in response to an edge of said generated pulse; coincidence means including a first input terminal coupled to said pulse generating means for receiving said window pulse and also including a second input terminal coupled to said controllable comparator means for receiving one of said inverted and noninverted bilevel signals for generating a bilevel coincidence signal in response to said level indicating pulses; controllable start-stop oscillator means coupled to said coincidence means and responsive to one level of said bilevel coincidence signal for generating oscillations at a rate such that a plurality of cycles of said oscillations occur during an interval equal to said nominal duration; bistable means coupled to said controllable start-stop oscillator means and responsive to each cycle of said oscillations for changing state to form a raw control signal, said raw control signal having a first state prior to receipt of each level indicating pulse, and having said first state after receipt of said level indicating pulse if said controllable start-stop oscillator means produces an even number of said cycles of said oscillations in response to said separated level indicating pulse, and having a second state after receipt of said level indicating pulse if said controllable start-stop oscillator means produces an odd number of said cycles of said oscillations in response to said separated level indicating pulse, whereby the average level of said raw control signal varies in the interval between said level indicating pulses depending upon the slicing level; first filter means coupled to said bistable means for receiving said raw control signal therefrom for filtering said raw control signal to produce a first control signal; second comparator means coupled to said first filter means for comparing said first control signal with a predetermined reference level to generate a bilevel compared output signal which changes state when said first control signal passes through a value corresponding to said predetermined reference level; second filter means coupled to said second comparator means and to said second input terminal of said controllable comparator means for filtering said bilevel compared output signal to form said reference slicing level, and for applying said reference slicing level to said second input terminal of said controllable comparator means to form a pulse width locked control loop for maintaining said slicing level near a predetermined value. - View Dependent Claims (6, 7, 8, 9, 10)
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11. A binary communications arrangement, comprising:
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a source of binary information; a source of level indicating pulses having rise and fall times which remain constant; multiplexing and transmitting means coupled to said source of binary information and to said source of level indicating pulse for multiplexing said binary information together with said level indicating pulses to form a transmitted signal; first comparator means coupled to said multiplexing and transmitting means for comparing said transmitted signal with a controllable reference voltage for generating compared pulses including compared level indicating pulses; gated oscillator means coupled to said first comparator means for generating bursts of oscillations in response to said compared level indicating pulses; bistable means coupled to said gated oscillator means for changing state in response to each of said oscillations in said bursts of oscillations, said bistable means producing an output signal having a first state before receipt by said gated oscillator means of said compared level indicating pulses and having a second state after receipt by said gated comparator means of said compared level indicating pulse if the duration of said compared level indicating pulse substantially corresponds to the duration of an odd number of said oscillations, and having said first state after receipt by said gated comparator means of said compared level indicating pulse if the duration of said compared level indicating pulse substantially corresponds to the duration of an even number of said oscillations; first filter means coupled to said bistable means for filtering said output signal of said bistable means for producing a first averaged signal; second comparator means coupled to said first filter means for comparing said first averaged signal with a reference voltage having a value intermediate the voltages represented by said first and second states of said output signal of said bistable means for generating an unfiltered control signal; and second filter means coupled to said first comparator means and to said second comparator means for filtering said unfiltered control signal to generate said controllable reference voltage, whereby the duration of said compared pulses is held substantially constant. - View Dependent Claims (12, 13, 14)
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15. A digital slicer for a bit stream, said bit stream including level indicating pulses, said slicer comprising:
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comparator means including first and second input terminals, said first input terminal being coupled for receiving said bit stream and said second input terminal being coupled for receiving a reference slicing signal, for comparing said bit stream with said reference slicing signal for generating compared pulses including compared level indicating pulses; gated oscillator means coupled to said comparator means for generating bursts of oscillations in response to said compared level indicating pulses; bistable means coupled to said gated oscillator means for changing state in response to each of said oscillations in said bursts of oscillations, said bistable means producing an output signal having a first state before receipt of each of said bursts of oscillators and having a second state after receipt of one of said bursts of oscillations containing an odd number of oscillations and having said first state after receipt of one of said bursts of oscillations containing an even number of oscillations; averaging and control signal processing means coupled to said bistable means for averaging said first and second states of said output signal of said bistable means and for processing the signal so averaged to generate said reference slicing level; and coupling means coupled to said averaging and control signal processing means and to said comparator means for coupling said reference slicing level to said comparator means with a polarity selected to maintain constant duration of said compared level indicating pulses. - View Dependent Claims (16, 17, 18, 19, 20)
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Specification