Pulse or digital communications – Systems using alternating or pulsating current – Plural channels for transmission of a single pulse train
Reexamination Certificate
1998-07-27
2001-11-20
Ghayour, Mohammad H. (Department: 2734)
Pulse or digital communications
Systems using alternating or pulsating current
Plural channels for transmission of a single pulse train
C375S278000, C375S285000, C332S104000, C332S107000, C332S123000
Reexamination Certificate
active
06320912
ABSTRACT:
BACKGROUND OF THE INVENTION
(i) Field of the Invention
The present invention relates to a digital modulator for use in a multi-level quadrature amplitude modulation (QAM) type digital radio communication system, especially to a digital modulator which can automatically set a modulation parameter suitable for a change of a system transmission parameter based on a control signal from the outside.
(ii) Description of the Related Art
A modulator whose modulation parameter is controlled at the time of malfunction or operation is heretofore known. For example, in a known technique, outside control data including the modulation parameter is entered into a modulator and controlled in a software manner. Examples of the outside control data include control data concerning the bit number of two orthogonal data strings or a cut-off frequency of a waveform shaping filter. In accordance with the control data an orthogonal data modulating section of the modulator and the operation frequency of the waveform shaping filter are controlled to automatically change the modulation parameter.
In the prior art described above, the control data is transmitted to the modulator in a software manner when the modulation parameter is changed. Here, when power supply is turned on in the modulator, the control data is set after modulator operation is stabilized. In a transient state from the turning on of the power supply till the setting of the control data, the modulator operation becomes unstable in some case. Furthermore, even when the control data is entered, in a transient state in which the previous modulation parameter is shifted to a new modulation parameter, the modulator operation becomes unstable in some case. In the transient states, if an output spectrum of the modulator is spread beyond a required band which is determined in the system, other adjoining circuits are disadvantageously interfered with. The problem will be described in detail with reference to the drawings.
FIG. 7
shows an RF spectrum waveform when an output frequency of a digital modulator is converted to an RF frequency. The spectrum waveform shown by a solid line in
FIG. 7
represents a modulation spectrum for obtaining required characteristics. Its central frequency is represented by f
0
, and its bandwidth is represented by f
r
. Channels adjoining the modulation spectrum are shown by dotted lines, and a central frequency of each of the channels is apart by ±f
r
from the central frequency f
0
.
When malfunction occurs immediately after the power supply turns on, and incorrect data with a double bandwidth is set, then the bandwidth with the central frequency f
0
is spread to 2f
r
. As a result, the modulation spectrum becomes an interference signal with the adjoining channels.
To solve the aforementioned problem of interference with the adjoining channels, for example, a system is disclosed in Japanese Patent Application Laid-open No. 77324/1989 in which adjoining circuits are prevented from being interfered with by lowering an output of a power amplifier. However, in the method, since the output is lowered, an input level of a next-stage transmission device is lowered and an apparatus failure alarm (ALM) is therefore generated. Furthermore, in a next-stage transmitter ALC circuit, full gain is provided. Therefore, the interference with the other adjoining circuits cannot be solved yet.
SUMMARY OF THE INVENTION
Wherefore, an object of the invention is to provide a digital modulator whose modulation parameters can be controlled in which modulation data or modulation signals are prevented from interfering with other adjoining circuits through control when power supply is turned on, when a system is changed or at another setting transient time.
To attain this and other objects, the invention provides a digital modulator which has a function of outside control of a modulation parameter and which comprises switching means for modulation with fixed data instead of transmission data for a predetermined time when power supply of the digital modulator is turned on or when a signal for the outside control is detected.
Furthermore, the switching means is provided with a power-on detecting circuit for detecting that the power supply of the digital modulator is turned on, a control signal latch circuit for detecting that a control signal for controlling the modulation parameter is entered, an AND circuit for detecting that an output of the power-on detecting circuit or the control signal latch circuit is generated, a timer circuit for delaying an output of the AND circuit by a predetermined time and a switch for switching the fixed data and the transmission data based on an output of the timer circuit when the power supply is turned on or when the modulation parameter is changed.
Another switching means is provided with a power-on detecting circuit for detecting that the power supply of the digital modulator is turned on, a control signal latch circuit for detecting that a control signal for controlling the modulation parameter is entered, an AND circuit for detecting that an output of the power-on detecting circuit or the control signal latch circuit is generated, a timer circuit for delaying an output of the AND circuit by a predetermined time and a flip-flop connected to the transmission data for outputting a held data value just before operation of the timer circuit based on an output of the timer circuit when the power supply is turned on or when the modulation parameter is changed.
Further switching means is provided with a power-on detecting circuit for detecting that the power supply of the digital modulator is turned on, a control signal latch circuit for detecting that a control signal for controlling the modulation parameter is entered, an AND circuit for detecting that an output of the power-on detecting circuit or the control signal latch circuit is generated and a timer circuit for delaying an output of the AND circuit by a predetermined time. Based on an output of the timer circuit, a D/A converter for digital-analog converting the transmission data is inhibited from sampling when the power supply is turned on or when the modulation parameter is changed.
REFERENCES:
patent: 5745049 (1998-04-01), Akiyama et al.
patent: 6181494 (2001-01-01), Fujiwara
patent: 6201837 (2001-03-01), Okamoto
patent: 0895363 A (1999-02-01), None
patent: 64-77324 (1989-03-01), None
patent: 11-146026 (1999-05-01), None
Ghayour Mohammad H.
NEC Corporation
Ostrolenk Faber Gerb & Soffen, LLP
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