Modulator

Details Modulator Modulator

1998-07-21

2001-02-06

Kinkead, Arnold (Department: 2817)

Modulators

Phase shift keying modulator or quadrature amplitude modulator

C332S104000, C455S110000, C455S118000, C375S295000, C375S298000, C375S261000, C375S271000

Reexamination Certificate

active

06184756

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to modulators, and more specifically to a modulator for modulating a carrier wave with a transmission signal to be transmitted.
2. Description of the Background Art
First, a modulator disclosed in Japanese Patent Laying-Open No. 6-152675 (hereinafter referred to as “first modulator”) is described in reference to
FIG. 12
which shows its block structure. In
FIG. 12
, the first modulator is a device for modulating a carrier wave with a transmission signal to generate a modulated signal. It includes a quadrature base band generator
71
, a quadrature polar coordinate converter
72
, a phase accumulator
73
, an adder
74
, a sine wave table memory
75
, a multiplier
76
, and a D/A converter
77
.
The quadrature base band generator
71
converts an inputted digital signal into a quadrature base band signal. When a modulated signal is represented as the synthesis of two carriers orthogonal to each other, the quadrature base band signal represents the amplitude and the phase of these carriers. The quadrature polar coordinate converter
72
converts the quadrature base band signal into a phase modulating signal and an amplitude modulating signal. When the modulated signal is represented in a polar coordinate system, the phase modulating signal and the amplitude modulating signal represent the phase and the amplitude of the modulated signal. The adder
74
adds the phase modulating signal to an output of the phase accumulator
73
. The sine wave table memory
75
outputs a carrier signal of a sine wave based on an output from the adder
74
. The multiplier
76
multiplies the carrier signal by the amplitude modulating signal. Thus, the first modulator generates a modulated signal with prescribed variations in phase and amplitude.
Next, a modulator disclosed in Japanese Patent Laying-Open No. 6-244883 (hereinafter referred to as “second modulator”) is described referring to
FIG. 13
which shows its block structure and
FIGS. 14
a
to
14
d
showing the output waveforms from each of its components.
In
FIG. 13
, the modulator includes a signal point arranging circuit
81
, a complex coefficient BPF (Band Pass Filter)
82
, a latch
83
, a D/A converter
84
, and an analog BPF
85
, and performs modulation without generating a trigonometric function.
The signal point arranging circuit
81
outputs a quadrature base band signal. The quadrature base band signal is a signal sampled at a sampling rate f
c
/2, and has harmonic components of an integral multiple of f
c
/2, as shown in
FIG. 14
a
. The complex coefficient BPF
82
converts the quadrature base band signal into a complex band signal for selecting a prescribed frequency band. Therefore, as shown in
FIG. 14
b
, only the components of the quadrature base band signal within the prescribed frequency band are selected. The latch circuit
83
and the D/A converter
84
multiply a real signal component of the complex band signal by a pulse whose duty ratio is smaller than 1 to perform a pulse amplitude modulation. The analog band pass filter
85
extracts a desired harmonic component from the output of the D/A converter
84
. Since the D/A converter
84
converts only the real signal component, the output signal from the D/A converter
84
causes aliasing components as shown in FIG.
14
C. These aliasing components are the result of the signal shown in
FIG. 14
b
being folded at an operating frequency of the complex coefficient BPF
82
, f
s
. The analog BPF
85
only extracts a prescribed harmonic component of the output signals from the D/A converter
84
to generate a modulated signal as shown in
FIG. 14
d.
As described above, the first modulator has one trigonometric function generating portion composed of the phase accumulator
73
and the sine wave table memory
75
. The trigonometric function generating portion operates with a clock rate of at least more than double the frequency of the above carrier signal (the center frequency of the modulating signal). As a result, it is necessary for the first modulator to operate the trigonometric function generating portion, a complex circuit, with a frequency of at least more than double the modulating wave frequency, which disadvantageously results in an increase in circuit size and power consumption.
As in the second modulator, it is possible to construct a modulator without using a trigonometric function generating portion. However, in this case, since an analog band pass filter is required, it is not suitable for integration as a circuit. Further, in the second modulator, the center frequency of the modulating signal is limited to an integer multiple of fc/2, it cannot be changed easily.
SUMMARY OF THE INVENTION
Therefore, an object of the present invention is to provide a modulator allowing for a small circuit size and low power consumption, capable of simple variations to the center frequency of a modulating signal and further suitable for integration as a circuit.
A first aspect of the present invention is a device for modulating a carrier wave with a transmission signal to be transmitted, including:
a discrete signal generating portion for sampling the transmission signal inputted from an outside source at a first sampling rate and generating a discrete signal; and
a band pass portion operating at a second sampling rate which is higher than the first sampling rate, when the discrete signal generated in the discrete signal generating portion is inputted and executing a band selection for selecting a signal which exists in a prescribed frequency band.
In the first aspect of the invention, since the band pass portion operates at the second sampling rate, inputting a discrete signal sampled at the first sampling rate is the same as inputting a discrete signal obtained by sampling the first discrete signal at the second sampling rate. The band pass portion executes band selection with respect to the discrete signal, selecting only a signal which exists in a prescribed frequency band as a modulated signal (a signal obtained by modulating a carrier with the transmission signal to be transmitted).
As described above, the band pass portion operates at the second sampling rate and simply executes the band selecting operation. Now assume that the trigonometric function generating portion of the conventional modulator operates at the second sampling rate. The structure of the trigonometric function generating portion is more complex than that of the band pass portion. Thus, power consumption of the band pass portion is small compared with that of the trigonometric function generating portion. Further, since the discrete signal generating portion preceding the band pass portion operates at the first sampling rate which is relatively low, it is possible to make the power consumption low.
Further, in the first aspect, unlike the conventional modulator, it is possible to generate a modulating signal without using the trigonometric function generating portion whose circuit structure is complicated.
Therefore, in accordance with the first aspect, it is possible to make the modulator's circuit size small and its power consumption low.
According to a second aspect of the invention, which further refines the first aspect, the discrete signal generating portion includes a low pass filter operating at the first sampling rate when the transmission signal is inputted from an outside source and passing only a signal which exists in low frequencies.
As described above, the discrete signal generating portion generates a discrete signal using only a low pass filter which operates at the first sampling rate. Therefore, in accordance with the second aspect, it is possible to make the modulator's circuit size small and its power consumption low.
According to a third aspect of the invention, which further refines the first aspect, the transmission signal is previously band-limited. The discrete signal generating portion includes an interpolating filter for, when the transmission signal is inputted from an outside source,

Affiliated with

Also associated with

Rating

Modulator does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Modulator, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Modulator will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-2601794