Data signal zero crossing detection process

Details Data signal zero crossing detection process Data signal zero crossing detection process

1997-05-19

1999-01-19

Chin, Stephen

Pulse or digital communications

Receivers

375316, 375317, 327 79, H03K 900

Patent

active

058621857

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF THE INVENTION

The present invention concerns a process for detecting the zero crossings of a data signal by means of a comparator.
Such a process is disclosed, for example, in the book by U. Tietze and Ch. Schenk "Halbleiter-Schaltungstechnik" Semiconductor Circuitry 3.sup.rd edition, 1976, pages 296 and 297. In this known process, a difference amplifier receives a reference voltage at one input and at another input it receives an input voltage to be monitored. In this known process, a maximum voltage appears at the output of the difference amplifier when the input voltage is higher than the reference voltage, and a minimum output voltage occurs when the input voltage is lower than the reference voltage, so a zero crossing of the input voltage can be detected due to the change in output voltage if the reference voltage is selected accordingly.
The aforementioned process does not lend itself to accurate detection of zero crossings of a data signal if the data signal has been transmitted over a system with filtering characteristics. Such a system exists, for example, with symmetrical lines in communications technology, where transformers are normally used. The transformers result in high-pass characteristics. As a result, the individual pulses transmitted have a rather marked pulse tilt even in the case of data signals with a constant signal level. The full swing of the next pulse of the data signals begins at the end of the pulse tilt of each individual pulse of the data signal, so it is impossible to accurately detect the zero crossing with a comparator having a fixed threshold because of the significant rise time of the individual pulses of the data signal. This then results in a phase jitter that is random because it depends on the data content. This is not negligible in accurate measurements and therefore must be suppressed.


SUMMARY OF THE INVENTION

An object of this invention is to propose a process for detecting the zero crossings of a data signal by means of a comparator with which data signal zero crossings can be detected accurately.
To achieve this object according to this invention, an auxiliary voltage corresponding to half the value of the difference between the maximum value determined for the data signal and the minimum value determined is generated, and an additional voltage is formed by comparing a reference quantity that is proportional to the auxiliary voltage with the instantaneous value of the data signal where the characteristic of the additional voltage reflects an offset-like change in the data signal; the auxiliary voltage and the additional voltage are added, forming a total voltage which is used to form the reference voltage of the comparator.
Such a device is known from the journal "Journal of Lightwave Technology", vol. 7 (1989), November, no. 11, p. 1634-1640, which teaches the automatic setting of a threshold value within the context of data regeneration. However, this device has a total of three decision modules with which an input signal can be tested for an average threshold value that is lower by a predetermined deviation. A threshold value generator is controlled by means of two deviation detecting circuits downstream from the decision modules over a downstream control signal generator so that the number of pulses delivered by the deviation detecting circuits is the same, so an average threshold value adapted to the respective input signal is set.
The essential advantage of the process according to this invention is that the reference voltage of the comparator is adapted to offset-like changes in the data signal variation over time, so a fixed reference voltage is not used for the comparator. Another advantage of the process according to this invention is that it can be carried out at comparatively little cost because only an easily generated auxiliary voltage and the additional voltage need to be produced to form the reference voltage.
The auxiliary voltage can be generated according to a measured peak-to-peak swing of the data signal or it can be a fix

REFERENCES:
patent: 3727079 (1973-04-01), Garrett
patent: 4590600 (1986-05-01), Beeman et al.
patent: 4759035 (1988-07-01), McGary et al.
patent: 5051616 (1991-09-01), Stuchbury
Braquet, Zero-Crossing Detector, IBM TDB vol. 18, No. 1, p. 144, Jun. 1975.
Journal of Lightwave Technology, Bd. 7, Nr. 11, Nov. 1989, New York, US, pp. 1634-1640.
U. Tietze, Ch. Schenk, "Halbleiter-Schaltungstechnik", 3. Auflage, 1976, Seiten 296-297.

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