Pulse or digital communications – Multilevel – Disparity reduction
Reexamination Certificate
1997-11-13
2001-06-26
Luther, William (Department: 2664)
Pulse or digital communications
Multilevel
Disparity reduction
C375S376000
Reexamination Certificate
active
06252913
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the invention
The present invention relates to a method of transmitting a frequency-modulated asynchronous NRZ signal, associating a transmitting frequency called center frequency to a given logic value and a frequency deviation which is sometimes positive and sometimes negative relative to the center frequency to the reverse logic value.
The invention further relates to a transmitter device for transmitting frequency-modulated asynchronous NRZ data and a receiver device for such data.
“Asynchronous” is understood to mean the fact that the signal may change from one logic state to another at any moment, irrespective of any clock.
2. Description of Related Art
There is a difficulty with a DC component, when one wishes to transmit a frequency-modulated asynchronous NRZ signal, due to the fact that the signal comprises an unequal number of bits in either one of the logic states. Let us suppose, for example, that a high-frequency oscillator is used, directly modulated by a two-level signal for which the frequency is more often deviated than centered: under the influence of its feedback loop, the oscillator drifts so that the deviated frequency approaches the nominal frequency. A partial solution is provided by a signal which is sometimes positive and sometimes negative for a given logic value. Such a signal is known to the expert: for example, the publication “Techniques de l'ingénieurs, Electronique” chapter E7100, page 4, shows a signal called bipolar signal whose “one” bits are alternately positive and negative. The problem is, however, not completely solved by utilizing such a signal, because an unbalance may continue to exist and, furthermore, with an asynchronous signal, the absence of a clock linked with the signal does not permit of counting its bits for generating alternately positive and negative bits. This problem could be solved, for example, by working at a low frequency with an oscillator without feedback, then transposing the produced frequency to higher values which would necessitate a relatively complex device.
SUMMARY OF THE INVENTION
It is an object of the invention to solve this problem in a simple manner.
For this purpose, the deviation has a positive value during a first period of time and then a negative value during a second period of time, these periods of time being determined in analog manner, so that the cumulated sum of the first periods and the cumulated sum of the second periods are practically equal in the course of time.
The invention is thus based on the idea of “fairly” distributing the positive and negative modulations based on time measurements.
Preferably, a period in which the frequency is the center frequency is always situated between a period having a positive frequency deviation and a period having a negative frequency deviation.
In this manner, the risk of bringing about an artefact is avoided: without this arrangement, as the signal mandatorily passes through the center frequency during a sign change of the frequency deviation, at the receiving end this could for a brief moment lead to the creation of an “error” bit corresponding to this center frequency.
Advantageously, the frequency modulation being provided by an oscillator having a phase-locked loop, the time constant of this loop is higher than said periods of time.
Thus, the phase locking of the oscillator does not run the risk of excessively impeding the frequency deviation.
A transmitter device according to the invention, comprising a frequency control voltage generator which produces three control voltage values called low, median, high voltage, associating the median voltage to a given logic value, and sometimes the high voltage and sometimes the low voltage to the reverse logic value, further comprises means for producing the high voltage during a first period of time and the low voltage during a second period of time, so that the cumulated sum of the first periods and the cumulated sum of the second periods are practically equal in the course of time.
For this purpose, the device comprises an analyzer element for analyzing the incoming asynchronous NRZ signal, which analyzer comprises an analog integrator for measuring time and actuates a switch for the selection of one of the three control voltage values.
In an advantageous embodiment, the analyzer element comprises a D-flipflop to whose clock input is fed the asynchronous NRZ signal, a resistance-capacitance feedback network between the output Q and the input D of the flipflop, and means for connecting its output {overscore (Q)} to its input D only when the NRZ signal has a given logic value.
For processing a demodulated signal, a receiver device according to the invention comprises a comparator whose threshold voltage is set so that it is activated by a negative signal, another comparator whose threshold voltage is set so that it is activated by a positive signal, and a logic circuit connected to the outputs of the comparators, whose output, intended to produce the reconstructed NRZ signal, is activated when either comparator is activated.
Advantageously, this device further includes a low-pass filter or analog integrator of the demodulated signal, for producing a voltage called mean voltage which represents the average value of this signal, a shift circuit for producing on its output a shifted voltage relative to this mean voltage, which output is connected to one of the comparators for supplying thereto its threshold voltage, called first threshold voltage.
This makes it possible to compensate the possible DC current drifts of the demodulated signal.
The device further includes a balanced voltage creating circuit for producing on the output a practically balanced voltage of the first threshold voltage relative to the mean voltage, which output is connected to the other comparator for supplying its threshold voltage thereto.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.
REFERENCES:
patent: 5200930 (1993-04-01), Rouquette
“Techniques de l'Ingenieurs, Electronique”, Chapter E7100, p. 4.
Luther William
Piotrowski Daniel J.
U.S. Philips Corporation
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