Pulse or digital communications – Transmitters – Amplitude modulation
Reexamination Certificate
1999-07-27
2004-09-14
Phu, Phuong (Department: 2731)
Pulse or digital communications
Transmitters
Amplitude modulation
C375S268000, C332S149000
Reexamination Certificate
active
06792050
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an ASK (Amplitude Shift Keying) modulation apparatus, particularly to an ASK modulation apparatus and method for a mobile terminal used for a microwave band with reduced unintended radiation and improved on/off ratio.
Recently, in the data communication system which uses a microwave band radio signal for duplex high speed data communication between a master station and a plurality of small mobile terminal, particularly, because of small size and low power consumption, a binary ASK modulation system is used as a modulation system in the mobile terminal.
In such a data communication system, data is communicated between the master station and mobile terminals by using two or more ASK modulated channels in the non-operating section with a certain frequency interval.
If a rectangular pulse as shown in
FIG. 18A
is used as a modulation pulse, in a general ASK modulator, the frequency spectrum of a transmission signal as shown in
FIG. 18B
is that a rectangular pulse (modulating wave) is developed as Fourier series.
But, this frequency spectrum includes an odd high level higher harmonic component in addition to a modulation component that is necessary for data transfer. This higher harmonic component raises a problem of adjacent-channel interference with other communication channels or failure to satisfy the occupied frequency bandwidth defined by the radio law.
On the other hand, the binary ASK modulated wave is obtained by turning on/off a carrier wave, but if a carrier leak is large while the carrier is turned off, the carrier detection circuit of the demodulator in the receiver side of the master station may malfunction, and the carrier leak must be sufficiently lowered.
A method has been proposed to reduce the unintended out-band radiation due to the higher harmonic component and carrier leak power.
For example, to reduce the higher harmonic component, the JP-A-No.167645/1992 discloses the method to apply a modulation pulse of rectangular wave to an ASK modulator through an integration circuit composed of a resistor and a capacitor.
FIG. 19
is a block diagram showing the ASK modulating circuit mentioned in the above reference.
A data signal is applied to the base of a transistor
403
through a base resistor
407
via an integration circuit
404
composed of a resistor
405
and a capacitor
406
. The collector of the transistor
403
is connected to a transmission line
402
that input the output of a carrier signal generator
401
.
When the data signal is applied through the integration circuit
404
, a smooth modulated pulse is generated to reduce the unnecessary higher harmonic component level.
The method of the above reference decreases the higher harmonic component to a certain extent, but a desired characteristic can not be obtained in the data communication system that the present invention is applied. The reasons are described as below.
FIG. 20
shows the primary required characteristics of the ASK modulation apparatus adopted in the data communication system of the present invention. In
FIG. 20
, a carrier frequency is 6 GHz and a data modulation rate is 2 Mbps. The present modulation appratus performs binary ASK modulation under this condition. As the adjacent channel is allocated at 10 MHz away from the center frequency (6 GHz), the D/U ratio of the maximum power (non-modulated carrier power) to the adjacent channel leak power (hereinafter, referred to a D/U ratio) needs to be 40 dB or higher depending on the condition not to deteriorate the code error rate when transmitting data through the adjacent channel.
The on/off ratio of the maximum power when the carrier is off (non-modulated carrier power) to the carrier leak power (hereinafter, simply referred to an ON/OFF ratio) needs to be 20 dB or higher.
Further, a 99% occupied frequency bandwidth (hereinafter called an occupied bandwidth) must satisfy severe 8 MHzp-p or lower that is four times of 2 Mbps.
When using the ASK modulating circuit having the primary integration circuit described in the above patent, it is theoretically impossible to obtain the D/U ratio of 40 dB or higher while satisfying the occupied-bandwidth. Besides, it is also impossible to satisfy the required on/off ratio, because a high frequency component leaks to the output through the transmission line even while the carrier is off. This is the same even if the integration circuit is replaced by a secondary simple low-path filter.
With the primary or secondary simple integration circuit, for a step-like input signal such as a digital signal, the response waveform on the time axis for output signal rapidly changes, fails to smooth the signal completely, attenuates the high frequency component of a base band signal, and provides no effect to limit the frequency band. Further, as the input impedance of the band limit filter is not constant over a wide range of frequency, it is apt to cause ringing, overshoot and waveform disturbance against a base band signal like a digital signal including a high frequency component. This will unnecessarily expand the occupied frequency band. Thus, the desired characteristic can not be obtained.
As a method of decreasing the unnecessary out-band radiation due to the higher harmonic component, a digital filter is used because it is excellent in the band limit characteristic and is relatively easy to change data.
With the ASK modulation apparatus using a digital filter, the characteristic of the out-band higher harmonic component is satisfied, but a D/A converter and a memory to store data to meet the required spurious characteristic become necessary, the compactness as a mobile terminal will not be realized, and the battery driven low power consumption requirement will not be achieved.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a microwave band ASK modulation apparatus and method which satisfies compactness, low power consumption and required modulation characteristics such as D/U ratio, occupied bandwidth and on/off ratio by using a passive element and not by using a large complex circuit such as a digital filter.
The objective of the present invention is achieved by an ASK modulation apparatus which modulates a rectangular high speed data signal within a microwave band comprising:
a band limiting means for waveform shaping to eliminate the inclination of an output waveform at rising of data signal by using a passive element, and
an ASK modulating means for inputting carrier wave of a microwave band and ASK-modulating said carrier wave by turning off the drive voltage of a high frequency amplifier based on the output of said band limiting means.
The ASK modulation apparatus of the present invention uses a band limiting means to raise a rectangular data signal waveform with zero inclination. This decrease the higher harmonic component included in the rectangular pulse of the data signal. The high frequency amplifier of the ASK modulation means is operated on the band limited output from the band limiting means, and when the rectangular wave is at a logical level “1”, it becomes in the operating region and amplifies the microwave band carrier. If the rectangular wave is at a logical level “0”, it becomes in the cut-off state and cuts off the carrier output. As a result, it provides a preferable ASK modulated wave.
The ASK modulation apparatus further includes a drive means comprising a buffer amplifier for driving said high frequency amplifier in the output of said band limiting means.
With the drive means, the bank limiting circuit is stably operated.
The band limiting means comprises the N sets (N is an integer greater than 2) of basic circuit which are connected in cascade to data signal input and the grounded resistor R being connected to the output of the Nth stage of the said basic circuit. Each of the basic circuit comprises a coil L provided between input and output and a serial circuit being one end connected to the input of said coil L and other end connected to the ground, and the serial circuit having a resis
Shiikuma Kazumi
Yamasaki Toyosaka
McGinn & Gibb PLLC
Phu Phuong
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