Pulse or digital communications – Equalizers – Automatic
Reexamination Certificate
1997-11-18
2001-01-30
Vo, Don (Department: 2734)
Pulse or digital communications
Equalizers
Automatic
C375S316000
Reexamination Certificate
active
06181740
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a sampling system suited for frequency-converting a modulated signal into a low frequency and a radio receiving device using the sampling system.
2. Description of Related Art
FIG. 1
shows the typical structure of a receiving device of a portable telephone and other mobile communication radio equipment. It is a radio receiving device of a double super-heterodyne system, wherein a modulated signal received by an antenna
11
is amplified by a low noise amplifier (LNA)
12
and is then input to a band-pass filter
13
having a relatively moderate characteristic for suppressing an image signal. The signal output from the band-pass filter
13
is frequency-converted in a first mixer
14
by a local signal from a local oscillator
15
into a signal having a first intermediary frequency. The signal frequency-converted by the first mixer
14
is input to a second mixer
17
via an IF filter
16
to be frequency-converted into a signal having a second intermediary frequency which is lower than the first one by a local signal from a local oscillator
18
. The signal frequency-converted by the second mixer
17
is input to an A/D converter
20
via an IF filter
19
to be converted into a digital signal. The digital signal output from the A/D converter
20
is then demodulated by a demodulator
21
.
Here, it is very difficult to reduce the size of the circuit because the mixers
14
and
17
are composed of analog devices in general. Although mixer circuits which are suitable for the miniaturization of the circuit have been proposed and put into the practical use so far, all of them have had a problem that the power consumption is large or the circuit is complicated because they are intended to attain an enough dynamic range and performances such as low distortion characteristics.
Further, the IF filters
16
and
19
are required to have a highly accurate frequency selectivity in order to select a signal having the frequency of the received channel and to remove the signal having the image frequency in the double super-heterodyne system as shown in FIG.
1
. Because an IF filter needs to be composed of passive parts such as a ceramic filter in many cases to enhance the frequency selectivity, the size of the filter becomes large and it is not suited for the use of the radio receiving device of portable equipment which are required to be miniaturized.
Meanwhile, there is a down-sampling system as shown in
FIG. 2
as another method for constructing the radio receiving device. A modulated signal received via an antenna
21
is amplified by a low noise amplifier (LNA)
22
and the unnecessary frequency component is removed by a band-pass filter
23
. Then, it is down-sampled in a sampling frequency which is lower than a center frequency of the modulated signal by an A/D converter
24
to be converted into a digital signal. The digital signal converted by the A/D converter
24
is demodulated by a demodulator
25
.
Although the structure of the whole system may be miniaturized because no mixer nor IF filter are necessary in the down-sampling system shown in
FIG. 2
, there has been a problem that the band-pass filter
23
is required to have a high frequency selectivity and the size of the filter becomes also large because the noise signal (hereinafter referred to as aliasing noise) aliased due to the sampling in the A/D converter
24
needs to be removed by the band-pass filter
23
. There has been also another demerit that the noise figure (NF) becomes worse because the A/D converter
24
is required to use a high down-sampling ratio in this arrangement.
As described above, the radio receiving device of the prior art double super-heterodyne system has had the problems that (1) because the mixer is required to have the wide dynamic range and the low distortion characteristic, the mixer requires the large quantity of power consumption and complicates the circuit structure and (2) the IF filter (e.g. the ceramic filter) having the high frequency selectivity is required to select the frequency signal of the received channel and to remove the signal of the image frequency, increasing the dimension of the filter required here.
The radio receiving device of the system of down-sampling the modulated signal directly by the A/D converter has had the problems that the dimension of the filter increases and the noise figure becomes worse due to the high down-sampling ratio, i.e. the generated noise increases, because the filter having the high frequency selectivity such as the ceramic filter or a dielectric filter is required in the radio frequency signal processing section in order to remove the aliasing noise generated during the down-sampling.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to solve the above-mentioned problems of the prior art radio receiving device by providing a sampling system which converts a modulated signal into a signal having a low frequency by a simpler circuit structure without using the small-scale, high precision and expensive mixer circuit and the filtering circuit
The inventive sampling system is characterized basically in that a received modulated signal is sampled in a frequency belonging to a higher band than the frequency band of the modulated signal in a first sampling circuit and the modulated signal is converted into a certain low frequency signal by a second sampling circuit after passing the sampled signal through an analog decimation filter to suppress the signal which may turn out to be aliasing noise. Here, the analog decimation filter has a function of removing the signal component of the frequency band which causes the aliasing noise from the input signal component in order to suppress the occurrence of an aliasing noise signal which occurs due to the down-sampling of the signal. Typically, the analog decimation filter is composed of a FIR filter.
According to this sampling system, the mixer circuit which is required to have the highly accurate characteristic like the prior art heterodyne receiving system becomes unnecessary and the function of the IF filter having the sharp characteristic for suppressing an image signal may be given to the analog decimation filter, so that the circuit scale of the radio equipment may be miniaturized.
Further, as for the output signal of the analog decimation filter, it may have a sampling frequency lower than the sampling frequency used in the pre-stage of the analog decimation filter and it is permitted to operate at the slower clock frequency, so that the radio equipment may be realized readily and the electric power consumed in the radio equipment may be reduced.
It is also possible to sequentially sample the modulated signal by composing the first sampling circuit by a plurality of sampling elements and by using those sampling elements selectively. Such a structure allows the radio equipment to be realized more readily because the individual sampling element may be operated at the slower sampling frequency as compared to the case of sampling the modulated signal by one sampling circuit.
When the first sampling circuit is composed of the plurality of sampling elements and when the predetermined weighting is implemented on the output signal of each sampling element and the output signals are added, a filtering characteristic may be given to the added signal, so that the analog decimation filter may be realized by this structure.
The individual sampling element composing the first sample and hold circuit may be composed of a plurality of switches and a plurality of capacitors connected respectively in series to those switches. In this case, the output of each sampling element may be weighted by the capacity of the capacitors.
When the radio equipment is constructed by disposing a &Dgr;&Sgr; modulator at the latter stage of the second sampling circuit, the addition of the signals weighted in accordance to the degree of the capacity of the capacitor contained in the sampling circuit may be implemented
Kabushiki Kaisha Toshiba
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Vo Don
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