Telecommunications – Receiver or analog modulated signal frequency converter – Noise or interference elimination
Reexamination Certificate
1997-06-23
2002-12-24
Trost, William (Department: 2683)
Telecommunications
Receiver or analog modulated signal frequency converter
Noise or interference elimination
C375S345000, C375S319000, C327S307000
Reexamination Certificate
active
06498929
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to a receiver built in a potable radio terminal for use in a radio communication system, and a radio communication system using the same. More specifically, the invention relates to a small receiver having a function of removing a direct current offset (which will be hereinafter referred to as a “DC offset” if necessary), which is an undesired DC component produced in a direct conversion receiving system or a superheterodyne receiving system, and a radio communication system using the same.
In recent years, with the rapid progress of radio communication systems, it has been greatly required to decrease the size and price of radio terminals. As a receiving system complying with such a request, the direct conversion receiving system has been attracted. Referring to the drawings, the construction and operation principle of a direct conversion receiver will be described below.
FIG. 1
shows a basic construction of a direct conversion receiver. A high frequency signal received by an antenna
2
of a receiving section
1
is amplified by a RF amplifier
11
in an analog signal processing circuit
10
, and then, divided into signals of two lines. The signals of two lines are mixed with carrier waves, which are supplied from a local oscillator
18
and which have substantially the same frequency as that of the received signal, by means of mixers
16
and
17
, respectively, and then, directly frequency-converted into base frequency bands (base bands). The local oscillator
18
is directly connected to the mixer
16
, and connected to the mixer
17
via a &pgr;/2 phase-shifter
19
. Therefore, the signals of two lines frequency-converted into the base bands have phases shifted by &pgr;/2 from each other. The base band signals of two lines pass through low-pass filters (each of which will be hereinafter simply referred to as a “LPF”)
22
and
23
serving as channel selectors to remove undesired frequency components, respectively. Thereafter, the base band signals are amplified to desired signal levels by means of base band amplifiers
26
and
27
, respectively. Then, the amplified base band signals are analog-to-digital converted by means of an analog/digital (which will be hereinafter simply referred to as an “A/D”) converted
3
, and then, demodulated to original data by means of a digital signal processing circuit
40
serving as detection/demodulation means.
In this direct conversion receiving system, since the received signals are directly frequency-converted into the base bands, there is no intermediate frequency, and there is no image response in theory. Therefore, there is an advantage in that it is not required to provide a very steep filter for removing image although such a filter is essential to the superheterodyne system. In addition, since the LPFs
22
and
23
for channel selection can be formed as large scale integrated circuits (LSIs), there is an advantage in that it is possible to decrease the size and price of receivers with the rapid progress of LSIs in recent years.
Although the direct conversion receiving system is suitable to decrease the size and price, there are the following problems. Referring to
FIGS. 2A and 2B
, such problems will be described below.
In
FIG. 2A
, a reference carrier wave necessary for the frequency conversion in the mixer
16
(or
17
) is supplied from the local oscillator
18
. Although it is desired that the isolation between the local port
16
a
and the RF port
16
b
of the mixer
16
is infinity, it is about 30 dB in fact. Therefore, the reference carrier wave inputted from the local port
16
a
is leaked toward the RF port
16
b
, and a part thereof is reflected on the output side of the RF amplifier
11
to be a reflected wave
32
to be inputted to the mixer
16
again. Alternatively, the part of the leaked reference carrier wave passes through the RF amplifier
11
to be leaked into the antenna
2
to be emitted from the antenna
2
to the outside as shown by reference number
34
, and then, reflected on a reflector
36
to be inputted to the antenna
2
again to be a reflected wave
35
which is inputted to the mixer
16
again. In the mixer
16
, these reflected waves
32
and
35
are mixed with the reference carrier waves outputted from the local oscillator
18
(self mixing). Since the reflected waves
32
and
35
have the same frequency as that of the reference carrier wave, the reflected waves appear as direct current output components (which will be hereinafter referred to as “DC offset”) in the output of the mixer
16
by the self-mixing.
FIG. 2B
shows such a DC offset on the axis of frequency. That is, in the direct conversion system, since a desired wave is frequency-converted into a base frequency band containing a DC component by nature, a DC offset component
7
produced by reflection is suppressed on a desired wave
6
. It is known that the DC offset of this type causes the deterioration of receive error rate particularly in delay detection. As a ratio of D (=desi red wave) to U (=DC offset component) (which will be hereinafter referred to as “D/U”) in order to obtain a desired receiver error rate, it is required to attenuate the DC offset component, for example, until about 20 to 30 dB can be obtained. However, the reference carrier wave supplied from the local oscillator
18
usually has about 0 dBm, so that the produced reflected waves
32
and
35
usually have a higher level than a desired wave level to be received by nature. Therefore, it is required to provide means for removing only the DC offset component on the output side of the mixer
16
in order to obtain a desired D/U.
As an example of conventional methods for removing the DC offset, there is a method using AC coupling means (which will be hereinafter referred to as an “AC couple”)
30
and
31
in the output of the mixers
16
and
17
, as shown in FIG.
1
. This method is effective when the DC offset is always constant. However, when the DC offset fluctuates with time, there is the following problem. FIG.
3
(
a
) shows the output of the mixer
16
having no AC couple when a DC offset
104
is superposed on a desired wave
108
. It is assumed that the DC offset
104
is changed to a DC offset
105
at time t′
103
. This corresponds to the fact that the circuit operation conditions of the RF amplifier
11
in
FIG. 2A
is changed. For example, this corresponds to the fact that the output impedance of the RF amplifier
11
varies to allow the reflected mount
32
to fluctuate in a case where the gain of the RF amplifier
11
is switched at time t′
103
on the basis of a control signal
33
outputted from the digital signal processing circuit
4
.
At the time, a difference 106 between the DC offset
104
before time t′ 103 and the DC offset
105
after time t′ 103 in FIG.
3
(
a
) causes the fluctuation of DC offset. In such a case, the state that the AC couple
30
is applied to the output of the mixer
16
is shown in FIG.
3
(
b
). That is, a transient response
109
corresponding to a time constant of a AC couple
30
occurs over a period of time 107 under the influence of the difference 106 of DC offsets. When a signal to be received incomes before the transient response
109
falls, the desired wave is under the influence of the DC offset, so that the deterioration of the receive characteristic occurs. That is, even if the AC couple
30
is used, the influence of DC offsets can not be removed. In addition, in a case where the AC couple
30
is used, a part of the desired wave is removed with respect to a desired signal containing a DC component as shown in FIG.
3
(
b
) by the frequency characteristic
8
of the AC couple shown in
FIG. 2B
, so that there is a disadvantage in that the receive characteristic is deteriorated.
The aforementioned method for providing means for switching the gain of the RF amplifier
11
is often used in order to increase the receive dynamic range particularly n a direct conversion receiver. In addition
Fujimoto Ryuichi
Horiguchi Hiroshi
Itakura Tetsuro
Tanimoto Hiroshi
Tsurumi Hiroshi
Kabushiki Kaisha Toshiba
Sobutka Philip J.
Trost William
LandOfFree
Receiver having DC offset decreasing function and... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Receiver having DC offset decreasing function and..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Receiver having DC offset decreasing function and... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2960883