Coded data generation or conversion – Analog to or from digital conversion – Analog to digital conversion
Reexamination Certificate
1999-09-20
2001-11-20
JeanPierre, Peguy (Department: 2819)
Coded data generation or conversion
Analog to or from digital conversion
Analog to digital conversion
C341S143000
Reexamination Certificate
active
06320529
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an A/D conversion apparatus and a radio apparatus to convert a received analog signal to a digital signal of predetermined frequency.
BACKGROUND OF THE INVENTION
In case of demodulation of digital processing in a receiver, a receiving signal input from an antenna is passed through a first filter. In the first filter, the receiving signal is converted to the predetermined frequency lower than a frequency of input modulation signal. After eliminating any unnecessary signals from the converted analog signal by a second filter, an A/D converter converts this analog signal to a digital signal. A digital demodulator demodulates the digital signal to regenerate the original baseband signal.
In a radio apparatus, a bandwidth of the signal is narrow in comparison with the carrier frequency. As mentioned-above, a modulation signal as the receiving signal is down-converted to a frequency of low signal bandwidth and digitized by the A/D converter. As a result, sampling frequency of the A/D converter is low, and a low-cost, low speed A/D converter is used. In this case, in order to correctly obtain the demodulation output, characteristics of low distortion and low noise are required for a mixer of the frequency converter. Furthermore, input to the mixer is the demodulation signal received through a wireless transmission path, and the input level widely varies. However, the mixer is generally comprised of a non-linear circuit. Therefore, it is difficult to realize the mixer of characteristics of low distortion over a wide range of the input level.
As a solution means of this problem, the radio (wireless) receiver shown in
FIG. 1
is proposed. In
FIG. 1
, an RF signal (modulation signal) received by the antenna
11
is input to the A/D conversion apparatus
10
through a preamplifier
12
. The A/D conversion apparatus
10
converts the RF signal to a digital signal of predetermined frequency below the frequency of the RF signal. The digital signal is demodulated by the demodulator
22
.
The A/D conversion apparatus
10
comprises a subtractor
13
, a loop mixer
14
, a mixer
15
, a reference signal generator
16
, a filter
17
, an A/D converter
18
, a D/A converter
19
, a mixer
20
, and a reference signal generator
21
. As a whole, this component is formed as a negative feedback loop. The mixer
15
and the reference signal generator
16
comprise a first frequency converter (down converter) to convert an output signal from the subtractor
13
to a predetermined frequency below the frequency of input signal Sin. The mixer
20
and the reference signal generator
21
comprise a second frequency converter (up converter) to convert a feedback signal Sf to the subtractor
13
to a frequency nearly equal to the frequency of the input signal Sin.
The subtractor
13
subtracts the feedback signal Sf from the input signal Sin. An output signal from the subtractor
13
is input to the mixer
15
through the loop filter
14
and multiplied with a reference signal from the reference signal generator
16
to convert it to a predetermined frequency below the frequency of the input signal Sin, i.e., a frequency of bandwidth of baseband signal or an intermediate frequency. Output from the mixer
15
(output from the first frequency converter) includes an unnecessary frequency component whose frequency is above or below the predetermined frequency. The output from the mixer
15
is input to the filter
17
to pass the predetermined frequency component by eliminating the unnecessary frequency component and converted to a digital signal by the A/D converter
18
.
Output from the A/D converter
18
is supplied to the demodulator
22
and the D/A converter
19
as an output signal Sout. The D/A converter
19
converts the output signal Sout to the analog signal. Output from the D/A converter
19
is input to the mixer
20
, and multiplied with the reference signal from the reference signal generator
21
to convert to a frequency nearly equal to the frequency (career frequency) of the input signal Sin. Output from the mixer
20
(output from the second frequency converter) is supplied to the subtractor
13
as the feedback signal Sf.
In
FIG. 1
, the A/D conversion apparatus
10
is adopted as a noise shaping type (&Dgr;&Sgr; type A/D converter), and formed as the negative feedback loop as a whole. The first frequency converter consisting of the mixer
15
and the reference signal generator
16
is included in the main signal path of the negative feedback loop, i.e., a signal path from input to output of the A/D conversion apparatus
10
. Accordingly, less than perfect operation (characteristics of distortion and noise) of the mixer
15
is relieved by operation of the negative feedback loop. In a theory of the negative feedback circuit, it is proved that distortion and noise existed in the main signal path are suppressed.
On the other hand, the second frequency converter consisted of the D/A converter
19
, the mixer
20
and the reference signal generator
21
is included in feedback signal path of the feedback loop. The noise and distortion generated from the D/A converter
19
and the mixer
20
affects accuracy of the feedback signal Sf. Accordingly, excellent characteristics of low distortion and low noise are required for the D/A converter
19
and the mixer
20
.
As a result, in
FIG. 1
, a number of bits of the A/D converter
18
and the D/A converter
19
are relatively set small, and dynamic range of the input signal to the D/A converter
19
and the mixer
20
is set narrow in order to easily comprise the D/A converter and the mixer of low distortion and low noise. Especially, if the number of bits of the A/D converter
18
and the D/A converter
19
is
1
, a high accurate mixer can easily be realized. However, in such composition, the number of bits of the A/D converter
18
is small and a quantized noise generated from the A/D converter
18
is large. As a result, a noise chracteristics of the A/D conversion apparatus
10
falls.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an A/D conversion apparatus and a radio apparatus to easily realize a frequency conversion function of high accuracy without characteristics of low distortion and low noise for wide range of input level while the number of quantized bits of the A/D converter is large.
According to the present invention, there is provided an A/D conversion apparatus including subtraction means for subtracting a first signal from a second signal, and for outputting a third signal as a subtraction result; first conversion means for converting the third signal to a digital signal of predetermined frequency different from a frequency of the second signal; and second conversion means for converting the digital signal to an analog signal of a frequency nearly equal to the frequency of the second signal; and for supplying the analog signal as the first signal to said subtraction means. Further, the second conversion means includes a plurality of current sources, a filter means for calculating a number of use of each of the plurality of current sources, and a selection means for selecting the current source having a lower value of the number of use. The number of selected current sources corresponds to a value of the digital signal.
Further in accordance with the present invention, there is also provided an A/D conversion apparatus including subtraction means for subtracting a first signal from a second signal, and for outputting a third signal as a subtraction result; first conversion means for converting the third signal to two digital signals whose frequency are orthogonal to each other and are different from a frequency of the second signal; and second conversion means for converting the two digital signals to two analog signals, and for adding the two analog signals as the first signal whose frequency is nearly equal to the frequency of the second signal. Further, the second conversion means includes a plurality of current sources, a filter means for c
Jean-Pierre Peguy
Kabushiki Kaisha Toshiba
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
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