High speed half duplex modem with fast turnaround protocol
First Claim
1. An improved method for locking a sampling signal onto a predetermined zero-crossing point of a reference signal having a characteristic frequency, said sampling signal having a sampling frequency and a phase with respect to said reference signal, comprising:
- (a) sampling said reference siagnal at said sampling frequency to obtain a plurality of sampling points, said sampling frequency being a multiple M of said characteristics frequency, M being an integer greater than three;
(b) designating one of said sampling points as a primary sampling point;
(c) using said primary sampling point and a secondary sampling point to determine in which quadrant of said reference signal that said primary sampling point lies;
(d) using said primary sampling point and said secondary sampling point to calculate a phase difference between said primary sampling point and said predetermined zero-crossing point; and
(e) adjusting said phase of said sampling signal by a phase step size which reduces said phase difference.
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Accused Products
Abstract
A phase locked loop circuit which eliminates the phase difference between an incoming reference signal and a sampling signal by sampling the incoming reference signal to produce a sampled signal. The sign of the sampled signal at two sample points is compared to determine in which quadrant a predetermined one of these sample points is located. The phase adjustment to the sampling signal is dependent upon the quadrant in which this sample point is located and the magnitude of this sample point. A large phase difference produces a large phase adjustment so that this sample point is quickly locked onto the zero-crossing points of the incoming reference signal. A small phase difference produces a small phase adjustment and prevents jitter. The lock onto the zero-crossing point of the incoming reference signal minimizes the data error rate of the modem.
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Citations
16 Claims
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1. An improved method for locking a sampling signal onto a predetermined zero-crossing point of a reference signal having a characteristic frequency, said sampling signal having a sampling frequency and a phase with respect to said reference signal, comprising:
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(a) sampling said reference siagnal at said sampling frequency to obtain a plurality of sampling points, said sampling frequency being a multiple M of said characteristics frequency, M being an integer greater than three; (b) designating one of said sampling points as a primary sampling point; (c) using said primary sampling point and a secondary sampling point to determine in which quadrant of said reference signal that said primary sampling point lies; (d) using said primary sampling point and said secondary sampling point to calculate a phase difference between said primary sampling point and said predetermined zero-crossing point; and (e) adjusting said phase of said sampling signal by a phase step size which reduces said phase difference. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9)
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10. An improved method for locking a sampling signal onto a predetermined zero-crossing point of a reference signal having a characteristic frequency, comprising:
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(a) sampling said reference signal at a first sample point and a second sample point to obtain a first sample and a second sample, respectively; (b) using said first sample and said second sample to determine in which quadrant of said reference signal that a selected point of said first sample point and said second sample point lies and to calculate a phase difference between said selected point and said predetermined zero-crossing point; (c) adjusting said first point and said second point in repsonse to said phase difference; and (d) repeating steps (a) through (c) at least until said phase difference has a magnitude which is less than a predetermined tolerance. - View Dependent Claims (11, 12, 13, 14, 15, 16)
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Specification