1. Field of the Invention
This invention relates to a direct conversion receiver, and more particularly to a direct conversion receiver suitable for use with a communication system wherein a transmission frequency and a reception frequency are different from each other and transmission and reception are performed simultaneously, and a transceiver (i.e., transmitter-receiver) that includes a direct conversion receiver of the type mentioned.
2. Description of the Relates Art
In recent years, attention is attracted to a direct conversion receiver for use with a communication system of the W-CDMA (Wide Band Code Division Multiple Access) system or the like wherein a radio frequency signal received by an antenna is directly converted into a baseband signal.
In the following, a direct conversion receiver will be described with a case wherein it is used in a W-CDMA system taken as an example. However, before the direct conversion receiver is described, the W-CDMA system itself will be described.
In the W-CDMA system, transmission and reception are performed simultaneously, and a transmission frequency and a reception frequency are different from each other. The difference between the transmission and reception carrier frequencies is referred as a carrier frequency interval. According to the existing standards of the WCDMA system, a W-CDMA terminal apparatus, i.e., a transceiver, has the following frequency configuration:
Reception carrier frequency fr:                2,110 MHz to 2,170 MHz,        
Transmission carrier frequency ft:                1,920 MHz to 1,980 MHz,        
Carrier frequency interval fs (=fr−ft):                190 MHz.        
First, a conventional example of configuration of a radio unit (i.e., transceiver) used as a terminal apparatus in a W-CDMA system will be described. FIG. 1 shows an example of conventional configuration that adopts a single super-heterodyne system as a configuration of a receiver.
In the transceiver shown in FIG. 1, duplexer 16 for separating a transmission signal and a reception signal is connected to antenna 1. Low noise amplifier (LNA) 2, band-pass filter 3 for removing an image frequency signal, mixer 4, band-pass filter 5 for intermediate frequency (IF) and variable gain amplifier (VGA) 6 are connected in series in this order to the output of duplexer 16 for a reception signal, and an output of variable gain amplifier 6 is supplied to quadrature demodulator 7. The transceiver includes first local oscillator 17 for generating a first local signal and second local oscillator 18 for generating a second local signal. The first local signal is supplied to mixer 4.
Quadrature demodulator 7 includes amplifier 8 for amplifying an input signal to quadrature demodulator 7, phase separation circuit 11 for receiving the second local signal as an input thereto and generating an inphase (I) component and a quadrature (Q) component whose phase is displaced by 90 degrees (i.e., π/2) from that of the in-phase component, multiplier 9 for multiplying an output of amplifier 8 and the in-phase component from phase separation circuit 11, and multiplier 10 for multiplying the output of amplifier 8 and the quadrature component from phase separation circuit 11. Here, π is, of course, the ratio of circumference of circle to its diameter. An output of multiplier 9 is supplied as reception baseband I signal 14 to the outside through band-pass filter 12 for baseband. An output of multiplier 10 is supplied as reception baseband Q signal 15 to the outside through band-pass filter 13 for baseband.
Meanwhile, on the transmission side, quadrature modulator 26, variable gain amplifier 25 for intermediate frequency, band-pass filter 24 for the intermediate frequency, mixer 23, band-pass filter 22 for removing image components and power amplifier (PA) 21 are connected in series in this order. An output of power amplifier 21 is supplied to a terminal for a transmission signal of duplexer 16. Transmission baseband I signal 35 is supplied to quadrature modulator 26 through band-pass filter 33 for a baseband, and also transmission baseband Q signal 36 is supplied to quadrature modulator 26 through band-pass filter 34 for baseband. Quadrature modulator 26 includes frequency divider 31 for dividing the frequency of the second local signal from second local oscillator 18 by 2 to produce a third local signal, phase separation circuit 30 for receiving an output of frequency divider 31 as an input thereto and generating an in-phase (I) component and a quadrature (Q) component whose phase is displaced by 90 degrees (π/2) from that of the in-phase component, multiplier 29 for multiplying an output of band-pass filter 33 and the in-phase component from phase separation circuit 30, multiplier 28 for multiplying an output of band-pass filter 34 and the quadrature component from phase separation circuit 30, and adder 27 for adding outputs of multiplier 28 and multiplier 29 and outputting a result of the addition as an output of quadrature modulator 26. The transmission baseband I signal has a peak value of a component whose phase is the same as that of the third local signal, and the transmission baseband Q signal has a peak value of a component having a quadrature phase to that of the third local signal.
In the transceiver, a reception signal is received by antenna 1 and separated by duplexer 16 which removes a transmission signal component from the reception signal. The separated reception signal is amplified by low noise amplifier 2 and inputted to band-pass filter 3 for removing the image component. The image component is a frequency component at a symmetrical position on the frequency axis to the reception signal with respect to the local frequency. The image component must be removed sufficiently by band-pass filter 3 because it otherwise leaks into the frequency band same as that of the signal when the signal is down converted by mixer 4. The reception signal from which the image component has been removed is mixed with the first local signal and down converted by mixer 4 to produce a reception intermediate frequency signal.
The first local signal is generated by first local oscillator 17 as described above, and in the example shown, frequency (first local frequency fLO1) of the first local signal is lower substantially by transmission reception carrier frequency interval fs (=190 MHz) than transmission carrier frequency ft. In other words,
first local frequency fLO1 (≈ft−fs):                1,730 MHz to 1,790 MHz.Accordingly, central frequency frm of the down converted intermediate frequency signal is given byfrm=fr−fLO1≈fr−ft+fs=fs+fs=2·fs,and is substantially equal to 380 MHz which is twice the transmission-reception carrier frequency interval.        
The intermediate frequency signal is subjected to band-limitation by band-pass filter 5 and then amplified to a level necessary for quadrature demodulation by variable gain amplifier 6. The amplified signal is subjected to quadrature demodulation by quadrature demodulator 7 with the second local signal to produce a set of two reception baseband signals of an in-phase component (reception baseband I signal) and a quadrature component (reception baseband Q signal).
The second local signal is generated by second local oscillator 18 as described hereinabove, and in the present example, the frequency (second local frequency fLO2) of the second local signal is substantially equal to twice the transmission-reception carrier frequency interval fs (190 MHz), that is,
second local frequency fLO2≈2·fs=380 MHz, and is substantially equal to central frequency frm of the intermediate frequency signal.
In the inside of quadrature demodulator 7, phase separation circuit 11 generates an in-phase component and a quadrature component using the second local signal, and the in-phase component and the quadrature component are multiplied by the intermediate frequency signal by multipliers 9, 10 to generate respective reception baseband signals.
The reception baseband signals are subjected to band-limitation by band-pass filters 12, 13 and sent as reception baseband I signal 14, reception baseband Q signal 15, respectively, to a signal processing circuit (not shown) in the following stage so that data decoding of the reception signal is performed by the signal processing circuit.
While the configuration and operation of the reception side are described, the frequency configuration of the reception side is associated closely with the configuration of the transmission side, and therefore, also operation of the transmission side will be described.
On the transmission side, a set of transmission baseband I signal 35 and transmission baseband Q signal 36 which are baseband signals produced by processing transmission data by means of a signal processing circuit (not shown) in the preceding stage are inputted and passed though band-pass filters 33, 34 for transmission baseband. Band-pass filters 33, 34 limit the frequency band of transmission baseband I signal 35 and transmission baseband Q signal 36, respectively. The band-limited transmission baseband signals are inputted to quadrature modulator 26 in which the quadrature modulation of those signals is performed.
Quadrature modulator 26 uses the third local signal produced by dividing the frequency of the second local signal (=380 MHz) by 2 by means of divider 31 therein. Frequency fLO3 of the third local frequency isfLO3=fLO2/2≈2·fs/2=fs (=190 MHz).In the inside of quadrature modulator 26, phase separation circuit 30 uses the third local signal to generate an in-phase component and a quadrature component. The in-phase component and the quadrature component are multiplied by the transmission baseband I signal and the transmission baseband Q signal by multipliers 29, 28, respectively, and resulting signals are added by adder 27 to generate a transmission intermediate frequency signal. Central frequency ftm of the transmission intermediate frequency signal is substantially equal to fs, that is,ftm≈fs=190 MHz.The transmission intermediate frequency signal is amplified to a necessary level by variable gain amplifier 25 and then, after unnecessary waves outside the transmission band are removed from the transmission intermediate frequency by band-pass filter 24, it is supplied to mixer 23. Mixer 23 mixes the first local frequency and the transmission intermediate frequency signal to effect up conversion of the transmission intermediate frequency signal up to the transmission frequency band. First local frequency fLO1 is originally set to fLO1≈ft−fs, where ft is the transmission carrier frequency and fs is the transmission-reception carrier frequency interval, and apparently a correct transmission signal can be obtained if first local frequency fLO1 is added to central frequency ftm≈fs of the transmission intermediate frequency to effect frequency conversion.
The transmission signal produced by the up conversion by mixer 23 is supplied to band-pass filter 22, by which unnecessary waves outside the transmission frequency band such as an image frequency component generated inadvertently by mixer 23 are removed from the transmission signal. Then, the transmission signal from band-pass filter 22 is amplified up to a predetermined transmission output level by power amplifier 21 and transmitted through duplexer 16 and antenna 1.
It is to be noted that, where first local frequency fLO1 and second local frequency fLO2 are set in such a manner as described above, only two local oscillators 17, 18 can be used to generate all local signals necessary for transmission and reception. While the present specification uses the term “substantially” and the symbol “≈” like a case wherein first local frequency fLO1 is set so as to be “substantially” equal to ft−fs, this is because, as obvious to those skilled in the art, it is not necessary to set fLO1 accurately to ft−fs and, in order to adjust the frequency to a prescribed transmission frequency band or reception frequency band, strictly the frequency band of the baseband signal itself must be taken into consideration. Therefore, in the present specification, displacement of the frequency is permitted as far as modulation, demodulation and frequency conversion can be performed in accordance with the scheme recited in the present specification.
The conventional configuration which uses the single super-heterodyne system is such as described above. Although the conventional configuration operates sufficiently, it has the following problems if it is intended to proceed with development of an LSI (large scale integration) in order to reduce the cost and the number of parts of a radio unit in the future.
1) In order for the receiver to remove an image component before the input of mixer 4, a steep image removing filter is required as band-pass filter 3. To this end, it cannot be avoided to use a passive element such as a SAW (surface acoustic wave) filter or a dielectric filter. Therefore, band-pass filter 3 is not suitable for formation of an LSI chip.
2) Also band-pass filter 5 used in the intermediate frequency stage performs channel selection, and a steep passive element such as a SAW filter or a dielectric filter must be used also for band-pass filter 5. Therefore, band-pass filter 5 is not suitable for formation of an LSI chip.
3) Variable gain amplifier 6 in the intermediate frequency stage is a high frequency circuit, and therefore, it is difficult to incorporate variable gain amplifier 6 in an LSI chip in order to integrate it with the baseband unit.
One of possible countermeasures to overcome the problems described above is adoption of direct conversion for the receiver. An example of it will be described with reference to FIG. 2. FIG. 2 is a block diagram showing a configuration of a transceiver which adopts direct conversion for the reception side.
The transceiver shown in FIG. 2 has a modified configuration to the transceiver shown in FIG. 1 in that it does not include mixer 4, band-pass filter 5 for intermediate frequency and variable gain amplifier 6 for intermediate frequency but instead includes variable gain amplifiers 19, 20 provided on the output side of band-pass filters 12, 13 for baseband, respectively, and a reception signal having passed through band-pass filter 3 for removing an image frequency component is inputted as it is to quadrature demodulator 7. Also, frequency fLO1 of the first local signal generated by first local oscillator 17 is different, and not the second local signal but the first local signal is supplied to phase separation circuit 11 of quadrature demodulator 7. The configuration of the transmission side and the configurations of second local oscillator 18, antenna 1, duplexer 16 and low noise amplifier 2 are same as those of the transceiver shown in FIG. 1. However, the frequency of the first local signal on the transmission side is different.
In particular, the transceiver shown in FIG. 2 is different from that shown in FIG. 1 in that, after a reception signal passes through moderate band-pass filter 3, it is converted into a reception baseband signal immediately by quadrature demodulator 7. Here, a steep image removing filter is not required. Quadrature demodulator 7 uses the first local signal as a local signal for generation of a reception baseband signal.
The first local signal is produced by first local oscillator 17 similarly as in the transceiver shown in FIG. 1. However, in the example shown in FIG. 2, frequency fLO1 of the first local signal is substantially equal to reception carrier frequency fr. In particular,
first local frequency fLO1 (≈fr):                2,110 MHz to 2,170 MHz.In the inside of quadrature demodulator 7, the first local signal is used to generate an in-phase component and a quadrature component by means of phase separation circuit 11, and the in-phase component and the quadrature component are multiplied by the reception signal by multipliers 9, 10, respectively, to generate reception baseband signals. Accordingly, the signals outputted from quadrature demodulator 7 have a peak value of a component of the reception signal which has a phase same as that of the internal local signal and a peak value of another component of the reception signal which has a quadrature phase to that of the internal local signal.        
The reception baseband signals are subjected to band-limitation by band-pass filter 12, 13 for baseband and amplified up to levels required for variable gain amplifiers 19, 20 and sent as reception baseband I signal 14 and reception baseband Q signal 15, respectively, to a signal processing circuit (not shown) in the following stage so that decoding of the reception data may be performed by the signal processing circuit.
On the transmission side, the configuration itself is similar to that shown in FIG. 1. However, since frequency (first local frequency fLO1) of the first local signal is replaced by reception carrier frequency fr, mixer 23 operates in the following manner. In particular, since first local frequency fLO1 is substantially equal to fr and central frequency ftm of the transmission intermediate frequency signal is substantially equal to transmission-reception carrier frequency interval fs, mixer 23 extracts a frequency of the difference between them. In particular, although fLO1−fs≈fr−fs, since fs originally is fs=fr−ft, also in this instance, it is apparent that a transmission signal produced by up conversion by mixer 23 is a correct transmission signal.
Although the configuration of a conventional direct conversion receiver is described above, it has the following problems. The problems listed arise from the fact that the frequency of the first local signal generated by the first local oscillator is substantially equal to the reception carrier frequency. Accordingly, where direct conversion is involved, these problems cannot be eliminated.
4) The first local signal may possibly be radiated through duplexer 16, antenna 1. The radiated first local signal has a bad influence on another receiver.
5) The first local signal may possibly leak into a reception signal. In this instance, an unstable dc offset occurs with the reception baseband signal outputted from quadrature demodulator 7 and causes saturation of a variable gain amplifier or data decoding error.
6) A reception signal sometimes has a very high intensity at a place immediately below a base station of the other party of communication, and operation of the first local oscillator may possibly be rendered unstable by interference of the intense reception signal.
As described hereinabove, an ordinary direct conversion receiver has problems in radiation of unnecessary waves to the outside of the apparatus by leak of a local signal, disturbance to a local oscillator by an intense reception signal from the outside and occurrence of a dc offset at an output of a quadrature demodulator by leak of a local signal into a reception signal.