Ask modulator and communication device using the same

Details Ask modulator and communication device using the same Ask modulator and communication device using the same

2000-04-28

2001-09-18

Mis, David (Department: 2817)

Modulators

Pulse or interrupted continuous wave modulator

Pulse amplitude modulator

C332S106000, C332S116000, C332S149000, C332S178000, C375S295000, C375S300000

Reexamination Certificate

active

06292067

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an ASK modulator; particularly, the invention relates to an ASK modulator used for an electronic toll collection (ETC) system.
2. Description of the Related Art
FIG. 5
shows a conventional amplitude shift keying (ASK) modulator. In an ASK modulator
1
shown in
FIG. 5
, the drain of a field effect transistor
2
(FET
2
) having a negative pinch-off voltage of −1 V is connected to a power-source terminal
4
via an inductor
3
. The drain is also connected to a modulation-signal output terminal
6
via a capacitor
5
. A positive voltage is applied to the power-source terminal
4
. The source of the FET
2
is grounded via a resistor
7
and a capacitor
8
connected in parallel. The gate of FET
2
is connected to a carrier-signal input terminal
9
. A data-signal input terminal
10
is grounded via resistors
11
and
12
which are connected in series, and a node where the resistors
11
and
12
are connected is connected to the gate of the FET
2
.
FIGS. 6A-6C
show waveforms of signals that are inputted to the carrier-signal input terminal
9
and the data-signal input terminal
10
and a signal that is outputted from the modulation-signal output terminal
6
. Referring to these waveforms, the operation of the ASK modulator
1
is described below. The amplitudes of the individual signals in the figures are not necessarily drawn to scale. As an example, for reference, the amplitude of the carrier signal may be smaller than that of the data signal, and the amplitude of the modulation signal may be greater than that of the carrier signal.
First, a carrier signal with a sine waveform as shown in
FIG. 6A
is inputted to the carrier-signal input terminal
9
. On the other hand, a digital data signal as shown in
FIG. 6B
is inputted to the data-signal input terminal
10
. The potential of the data signal is assumed to be 0 V at a high (H) level, and is assumed to be −5 V at a low (L) level. As a result, the data signal with the carrier signal overlapped is inputted to the gate of the FET
2
.
When the data signal is at the H level, the voltage between the gate and the source of the FET
2
is higher than the pinch-off voltage, thereby causing the FET
2
to perform an amplifying operation. The carrier signal is thus amplified, and the amplified signal is outputted from the modulation-signal output terminal
6
. In contrast, when the data signal is at the L level, since the voltage between the gate and the source of the FET
2
is lower, the FET
2
does not perform the amplifying operation. Therefore, the modulation-signal output terminal
6
outputs no signal.
Thus, as shown in
FIG. 6C
, depending on the condition, the modulation-signal output terminal
6
either outputs the carrier signal amplified in relation to the data signal or does not output any signal. It is apparent that this is attributable to ASK-modulation (amplitude modulation) wherein the data signal digitally modulates the carrier signal. In this way, the ASK modulator
1
implements the ASK-modulation operation.
With the ASK modulator
1
, to perform the modulation operation, however, at least an L-level potential of the data signal must be the same as or lower than the pinch-off voltage of the FET
2
, that is, it must be a negative potential. This requires a negative-voltage power source for supplying the negative potential, in addition to the power source for supplying the positive voltage that is applied to the power-source terminal
4
. This produces problems in that the provision of the additional negative power source makes it difficult to implement miniaturization of the ASK modulator
1
and of a module using it, and makes cost reduction difficult as well.
SUMMARY OF THE INVENTION
In view of the above problems, the present invention provides an ASK modulator that can be operated with only a positive-voltage power source and a communication device using it.
To achieve the object, according to one aspect of the present invention, an ASK modulator of the present invention comprises a field effect transistor (FET) having a negative pinch-off voltage, a carrier-signal input terminal and a data-signal input terminal that are connected to the gate of the FET, a modulation-signal output terminal connected to the drain of the FET, a resistor connected between the source and the ground, and a source-voltage switching circuit that is connected to the source of the FET and applies a positive voltage to the source of the FET when the data signal is at an L level. The operation of the source-voltage switching circuit is associated with the data signal inputted to the data-signal input terminal. In the described ASK modulator, the source-voltage switching circuit may comprise a positive-voltage power source and a second resistor connected between the positive-voltage power source and the source of the FET.
According to another aspect of the invention, a communication device of the present invention uses the above-described ASK modulator.
According to the described ASK modulator of the present invention, a modulating operation can be implemented without using a negative-voltage power source. The ASK modulator is configured such that a the gate of a FET having a negative pinch-off voltage is connected to a carrier-signal input terminal and a data-signal input terminal, the drain is connected to a modulation-signal output terminal, and the source is grounded via a resistor and is connected to a source-voltage switching circuit that applies a positive voltage to the source of the FET when the data signal is at an L level. The operation of the source-voltage switching circuit is associated with the data signal inputted to the data-signal input terminal. Accordingly, miniaturization and cost reductions can be implemented for the ASK modulator and a module having such an ASK modulator.
Also, the configuration of the source-voltage switching circuit can be significantly simplified by using the positive-voltage power source, and a second resistor connected between the positive-voltage power source and the source of the FET. This allows further dimensional and cost reductions to be achieved.
Furthermore, since the communication device of the present invention uses the ASK modulator of the present invention, miniaturization and cost reduction can be implemented.
Other features and advantages of the present invention will become apparent from the following description of embodiments of the invention which refers to the accompanying drawings.


REFERENCES:
patent: 5680078 (1997-10-01), Ariie
patent: 6087904 (2000-07-01), Wen
patent: 6194978 (2001-02-01), Romano
patent: 0977408 (2000-02-01), None

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