Ask Modulator

Details Ask Modulator Ask Modulator

1998-12-30

2003-03-04

Chin, Stephen (Department: 2634)

Pulse or digital communications

Transmitters

Amplitude modulation

C332S149000

Reexamination Certificate

active

06529562

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to ASK (Amplitude Sift Keying) modulators that are suitably applicable to wireless communication systems.
2. Description of Related Art
In performing a wireless data communication, an ASK modulation system in a wireless communication system transmits radio waves only during periods in which the transmission data is at a logic “H” level and halts a radio wave transmission during periods in which the transmission data is at a logic “L” level.
For example, modulation systems of wireless units of ETC (Electronic Toll Collection; non-stop automatic toll collection) systems adopt this system. Here, the ETC system performs a non-stop automatic toll collection at a tollbooth, wireless-communicating between a wireless beacon installed on the road side and an on-vehicle equipment. ETC systems use a high frequency band of 5.8 GHz.
FIG. 1
is a block diagram showing an exemplary configuration of a conventional ASK modulator that is applied to such an ETC system. In what follows, with reference to
FIG. 1
, the configuration and operation of the conventional ASK modulator will be explained.
As shown in
FIG. 1
, a 5.8 GHz band wireless carrier signal generated by the local oscillator
1
is amplified by the buffer amplifier
2
and is supplied to the input terminal IN of the gallium arsenic (GaAs) high speed switch
3
. On the other hand, a data signal supplied to the data signal input terminal a is sent to the control input terminal CONT of the GaAs high speed switch
3
.
FIG. 2
explains the operation characteristic of the GaAs high speed switch
3
. In FIG.
2
(A), when the data signal supplied to the control terminal CONT is a logic “H” signal, the logic “H” signal is input to the gate of the MOS-FET
302
via the resistor
310
. As a result, the region between the drain and source of the MOS-FET
302
is turned on. On the other hand, when a logic “L” signal is input to the MOS-FET
305
via the inverse device
307
and resistor
306
, the region between the drain and source of the MOS-FET
305
is turned off. As a result, the 5.8 GHz wireless carrier signal input to the input terminal IN is supplied to the output terminal OUT via the capacitance device
301
, MOS-FET
302
, and capacitance device
304
.
When the data signal supplied to the control terminal CONT is a logic “L” signal, the logic “L” signal is supplied to the gate of the MOS-FET
302
. As a result, the region between the drain and source of the MOS-FET
302
is turned off. On the other hand, a logic “H” signal is supplied to the gate of the MOS-FET
305
. As a result, the region between the drain and source of the MOS-FET
305
is turned on. Therefore, the 5.8 GHz wireless carrier signal input to the input terminal IN is supplied to the ground GND via the capacitance device
301
, MOS-FET
305
, and capacitance device
308
in this order but not to the output terminal OUT.
Therefore, as shown in FIG.
2
(B), the GaAs high speed switch
3
supplies the 5.8 GHz wireless carrier signal to the power amplification circuit
4
when the data signal sent from the data signal input terminal a is a logic “H” signal. On the other hand, when the data signal sent from the data signal input terminal a is a logic “L” signal, the GaAs high speed switch
3
does not supply the 5.8 GHz wireless carrier signal to the power amplification circuit
4
.
By this operation, the GaAs high speed switch
3
outputs an ASK modulation signal in the 5.8 GHz band, that is, a 5.8 GHz band wireless carrier signal that intermittently operates by the same timings as the logic level of the data signal.
The power amplification circuit
4
then amplifies to a prescribed power level the ASK modulation signal in the 5.8 GHz band that has passed through the GaAs high speed switch
3
. The band-pass filter
5
shown in
FIG. 1
removes spurious radiation components of the ASK modulation signal, which are not needed for communication. The ASK modulation signal is then sent to an antenna not shown in the drawing via the antenna terminal
6
and is emitted as a radio wave.
FIG.
3
(A) shows the waveform of the original signal to be supplied to the conventional ASK modulator. FIG.
3
(B) shows the waveform of a Manchester signal transformed from the original signal to be supplied to the data signal input terminal of the conventional ASK modulator. FIG.
3
(C) shows the waveform of a 5.8 GHz band ASK modulated signal generated by the conventional ASK modulator. In this example of the prior art, the original data signal is transformed into a Manchester data signal so that the clock component of the original data signal can be easily reproduced on the receiver side. The original data signal having the waveform shown in (A) is transformed into a Manchester data signal having the waveform shown in (B). When this data signal shown in (B) is supplied to the data signal input terminal a, the GaAs high speed switch
3
outputs an ASK modulation signal in the 5.8 GHz band shown in (C). This is a 5.8 GHz band wireless carrier signal that intermittently operates with exactly the same timings as the Manchester data signal shown in (B).
However, the above-described conventional ASK modulator has the following problem.
10×log
10
[(sin
4
(&pgr;f/2×10
6
))/(&pgr;f/2×10
6
)
2
]  (1)
The above-provided expression (1) is a general formula of a data signal transformed into a Manchester signal. Data signals transformed into Manchester signals have rectangular waveforms as shown in FIG.
3
. The frequency spectrum of such a signal extends to infinity.
In other words, the conventional ASK modulator transforms a data signal having a frequency spectrum that extends to infinity into a 5.8 GHz band wireless carrier signal and obtains an ASK modulated signal by intermittently operating this 5.8 GHz band wireless carrier signal. Therefore, the frequency spectrum of this ASK modulation signal is centered at 5.8 GHz and also spreads to infinity, which is a problem.
A band restricting low pass filter can be inserted into FSK (Frequency Shift Keying) modulators and PSK (Phase Shift Keying) modulators to restrict the frequency band of the data signal as well as the frequency band of the modulated 5.8 GHz band wireless carrier signal.
However, the GaAs high speed switch
3
of the conventional ASK modulator is turned on and off at about an intermediate voltage between the drain voltage and source voltage of the MOS-FET. Therefore, even if a band restricting low pass filter is inserted into the conventional ASK modulator to restrict the frequency band of the data signal, the frequency spectrum of the ASK modulated signal obtained on the output side of the GaAs high speed switch
3
is centered at 5.8 GHz and still spreads to infinity. Therefore, inserting a low pass filter into the conventional ASK modulator does not solve the problem.
For this reason, ASK modulators capable of reducing the frequency band width of the ASK modulation signal are in demand.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an ASK modulator to solve the problem.
To solve the problem, according to the first embodiment of the present invention, an ASK modulator which modulates an input signal and generates an amplitude shift keying (ASK) modulated signal in accordance with a logic level of the input signal is provided. This ASK modulator has an oscillating means for generating a carrier signal, a frequency filter means which passes a prescribed frequency component of the input signal and outputs a frequency limited signal having the prescribed frequency component, and a modulator which modulates the input signal based on the carrier signal and the frequency limited signal and outputs the ASK modulated signal having a limited frequency component.
According to one aspect of the first embodiment of the present invention, the frequency filter means may be a low pass filter which passes a low frequency component of the input signal.
According to another aspect of the first embodiment

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