Direct conversion receiver capable of canceling DC offset...

Details Direct conversion receiver capable of canceling DC offset... Direct conversion receiver capable of canceling DC offset...

1998-03-03

2001-01-16

Maung, Nay (Department: 2744)

Telecommunications

Receiver or analog modulated signal frequency converter

Frequency modifying or conversion

C375S319000

Reexamination Certificate

active

06175728

ABSTRACT:

BACKGROUNDS OF THE INVENTION
1. Field of the Invention
The present invention relates to a direct conversion receiver, and more particularly to a direct conversion receiver known as a zero intermediate frequency (IF) receiver, for use in a portable telephone or other radio terminal devices.
2. Description of the Related Art
Recently, there have been rising the demands for downsizing, power consumption reduction, and cost reduction of a radio according to the prevalence of a portable telephone and other radio portable terminals. As a radio receiving method for realizing such demands, a direct conversion is an attractive architecture in radio design.
FIG. 9
shows a block diagram of the typical direct conversion receiver of this kind. With reference to
FIG. 9
, radio frequency signals are respectively supplied to quadrature mixers
73
a
and
73
b
through an antenna
71
and a radio frequency amplifier
72
. One is multiplied by a local oscillation frequency of a local oscillator
74
in the mixer
73
a
, and the other is multiplied by the quadrature component of the local oscillation frequency in the mixer
73
b
, then to be supplied as I-component and Q-component. The quadrature component of the local oscillation frequency of the local oscillator
74
is generated by a 90 degrees phase shifter
75
. The respective output signals of the mixers
73
a
and
73
b
consist of the sum component of input carrier frequency and local oscillation frequency and the difference component relative to the zero frequency (in case of ± expression of spectrum). LPFs
76
a
and
76
b
extract only the difference component with the zero frequency fixed as a reference (hereinafter, referred to as a zero frequency component). The extracted zero frequency component is further processed and demodulated by respective baseband amplifiers
77
a
and
77
b.
The above-mentioned direct conversion method directly converts the input frequency into the baseband frequency. This corresponds to the case where the intermediate frequency is zero in the superheterodyne method. Because no image frequency component exists, no radio frequency filter is required. The baseband signal has such a form as turning back at the zero frequency and a LPF may be used as a channel filter. This makes IC fabrication easier compared with a channel filter of BPF type for use in the superheterodyne method. The direct conversion method has been recently recognized as a suitable circuit for 1 chip receiver because of requiring fewer external parts and making LSI fabrication easier compared with the superheterodyne method.
However, in order to use a direct conversion receiver in a radio system such as a portable telephone, it is necessary to eliminate dc (direct-current) offset voltage generally existing on the order of several mV to several 10 mV in a mixer, which is a serious practical problem. In order to get a good sensitivity in PDC (Personal Digital Cellular-phone) system and PHS (Personal Handy-phone System), the amplification degree of the baseband amplifiers
77
a
and
77
b
must be set at an extremely high value, for example, at several 10 dB. The baseband amplifiers, however, are saturated with the dc offset voltage occurring in the mixer as mentioned above, and never function as a receiver.
In order to solve the above problem, placing dc blocking condensers
78
a
and
78
b
behind the mixers
73
a
and
73
b
may be considered as illustrated in FIG.
10
. This method, however, also blocks the zero frequency component and therefore this method can be applied only to the FSK modulation, for example, for use in a pager system. Even in this case, another additional circuit for shortening the charge/discharge time of a capacitor is generally needed, thereby increasing the circuit size disadvantageously, since a pager with an intermittent receiving function is generally designed in order to extend a battery life.
Another conventional technique for eliminating dc offset voltage is disclosed in, for example, Japanese Patent Publication Laid-Open (Kokai) No. Heisei 3-220823, “Direct Conversion Receiver”. The same publication describes a method of eliminating dc offset voltage by a negative feedback loop using AD and DA converters as illustrated in FIG.
11
. In the block diagram of
FIG. 11
, the same reference numerals are respectively attached to the same components as those of the block diagram of FIG.
9
.
In a direct conversion receiver as shown in
FIG. 11
, dc offset voltage is extracted from the output signals of the baseband amplifiers
77
a
and
77
b
by use of the AD converters
81
a
and
81
b,
and according to the extracted result, the dc offset voltage is suppressed by use of a data processing circuit
82
and DA converters
83
a
and
83
b
that are closed-loop controlling means.
This method is applicable to &pgr;/4QPSK modulation for use in PDC and PHS because of blocking off none of the zero frequency component in signal information. However, this method, by use of a closed-loop, is not effective in the change of offset voltage at the earlier time because of the restriction of the loop convergence time. More specifically, in case of controlling the average value of the offset voltage, for example, to be zero during a long period of one second, some effects can be expected to an extent. However, it is not effective in the offset voltage such as would vary at a speed of time slot of the digital portable telephone system (for example, 0.625 msec in PHS). Further, it is defective in increasing the circuit size because of using AD and DA converters.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a direct conversion receiver with an offset voltage canceling circuit capable of canceling finest offset voltages at a high speed.
Another object of the present invention is, in addition to the above object, to provide a direct conversion receiver with reduced size compared with conventional circuits and suitable for a portable telephone or the other portable radio terminal device.
According to the first aspect of the invention, a direct conversion receiver comprises
frequency mixing means for mixing a received radio frequency signal and a local oscillation signal in frequency,
signal converting means for converting the output signal of the frequency mixing means into a baseband signal,
offset voltage detecting means for detecting offset voltage from the output signal of the signal converting means,
offset canceling means for supplying offset canceling voltage for canceling the offset voltage in the output signal of the frequency mixing means, correspondingly to the offset voltage detected by the offset voltage detecting means, and
switching means for supplying the output signal of the frequency mixing means to the offset voltage detecting means during a given period based on a predetermined rule, and supplying the offset canceling voltage sent from the offset canceling means to the output side of the frequency mixing means during a period other than the given period.
In the preferred construction, when it is applied to a communication system of Time Division Multiplex Access method, the switching means switches connection so as to supply the output signal of the frequency mixing means to the offset voltage detecting means during a period other than an assigned time slot of the present receiver, or so as to supply the offset canceling voltage sent from the offset canceling means to the output side of the frequency mixing means during a period of the assigned time slot of the present receiver.
In the preferred construction, when it is applied to a communication system of Frequency Division Multiplex Access method, the switching means switches connection so as to supply the output signal of the frequency mixing means to the offset voltage detecting means during a period corresponding to a pilot signal part of a received frame, or so as to supply the offset canceling voltage sent from the offset canceling means to the output side of the frequency mixing means during the other period.
In the pre

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