Receiver circuit and receiver apparatus including the same

In a receiver circuit, an analog signal processor frequency-converts an input high frequency signal into a baseband signal, and performs low pass filtering at a cutoff frequency below a desired-wave band. An ADC converts an output of the analog signal processor into a digital signal. A digital signal processor compensates an output of the ADC for a signal component in the desired-wave band which has been attenuated by the filtering operation of the analog signal processor.

BACKGROUND

The present disclosure relates to receiver circuits, and more particularly to receiver circuits which frequency-convert high frequency signals used in mobile phones, broadcasting systems, etc. into baseband signals.

Generally, a high frequency signal input to a receiver circuit is amplified by a low noise amplifier (LNA) first, and then is frequency-converted into a baseband signal by a frequency converter. Then, after the baseband signal is amplified by an automatic gain control (AGC) amplifier, an analog filter performs low-pass filtering for attenuating the interference wave adjacent to the desired-wave band. In some cases, an equalizer is provided in a succeeding stage of the analog filter, and thus compensation is performed for a phase change due to a filtering operation of the analog filter. Some receiver circuits having such a configuration seek to reduce the power consumption by setting a relatively high cutoff frequency to the analog filter and by halting operation of the equalizer if no interference waves are present (see, e.g., Japanese Patent Publication No. 2008-005259).

In order to reduce the power consumption of a receiver circuit, reduction of the operating voltage is effective. However, reducing the operating voltage of a receiver circuit causes the distortion characteristic of each signal processing block, especially of the AGC amplifier which amplifies the baseband signal, to degrade significantly, thereby posing a problem in that the interference wave immunity of the receiver apparatus is reduced. That is, a conventional receiver circuit is not capable of sufficiently reducing a distortion component due to an interference wave only by reducing the operating voltage, and thus has limitations on the interference wave immunity. In addition, reducing the operating voltage of a receiver circuit also poses a problem in that if the reception level of the input desired wave is too high, the signal to noise plus distortion ratio (SNDR) is reduced due to a distortion component caused by the desired wave. This is a serious problem for a receiver circuit of an analog broadcast signal which requires a high SNDR such as an NTSC or PAL signal.

SUMMARY

A receiver circuit according to the present disclosure is advantageous in that even low voltage operation keeps preferred interference wave immunity and a preferred SNDR.

A receiver circuit according to one example includes an analog signal processor configured to frequency-convert an input high frequency signal into a baseband signal, and to perform low pass filtering at a cutoff frequency below a desired-wave band, an analog-to-digital converter (ADC) configured to convert an output of the analog signal processor into a digital signal, and a digital signal processor configured to compensate an output of the ADC for a signal component in the desired-wave band which has been attenuated by the filtering operation of the analog signal processor.

According to this, even if the distortion characteristic of each signal block of the receiver circuit significantly degrades due to low voltage operation, the filtering characteristic of the analog signal processor allows the distortion component to be reduced, and the compensator compensates the signal component of the desired wave, thereby allowing the interference wave immunity and the SNDR of the receiver circuit to be improved.

For example, the analog signal processor includes a first amplifier configured to perform low noise amplification on the high frequency signal, a local oscillator configured to generate a local oscillator signal, a frequency converter configured to frequency-convert an output of the first amplifier using the local oscillator signal, and to perform the low pass filtering on a frequency-converted signal at the cutoff frequency below the desired-wave band, a second amplifier configured to amplify an output of the frequency converter, and an analog filter configured to attenuate a signal component outside of the desired-wave band contained in an output of the second amplifier. The digital signal processor includes a digital filter configured to remove a signal component outside of the desired-wave band contained in the output of the ADC, and a compensator configured to filter an output of the digital filter with an inverse characteristic relative to that of the frequency converter. In addition, filtering characteristics of the frequency converter and of the compensator may be variable, and the receiver circuit may include a control unit configured to control the respective filtering characteristics of the frequency converter and of the compensator depending on a reception level of the high frequency signal.

A gain of at least one of the first or the second amplifier may be variable, and for example, if the reception level of the high frequency signal is low, the control unit increases the variable gain and the cutoff frequency of the frequency converter, and if the reception level of the high frequency signal is high, the control unit decreases the variable gain and the cutoff frequency of the frequency converter. Alternatively, the control unit may detect a presence or absence of a high level of interference wave adjacent to the desired-wave band from a level difference between an input and the output of the digital filter, and if the interference wave is not detected, the control unit may increase the cutoff frequency of the frequency converter, and if the interference wave is detected, the control unit may decrease the cutoff frequency of the frequency converter.

Alternatively, for example, the analog signal processor includes a first amplifier configured to perform low noise amplification on the high frequency signal, a local oscillator configured to generate a local oscillator signal, a frequency converter configured to frequency-convert an output of the first amplifier using the local oscillator signal, a second amplifier configured to amplify an output of the frequency converter, and an analog filter configured to perform the low pass filtering on an output of the second amplifier at the cutoff frequency below the desired-wave band. The digital signal processor includes a digital filter configured to remove a signal component outside of the desired-wave band contained in the output of the ADC, and a compensator configured to filter an output of the digital filter with an inverse characteristic relative to that of the analog filter. In addition, a gain of the second amplifier may be variable, and the receiver circuit may include a control unit configured to control the gain of the second amplifier depending on a reception level of the high frequency signal. The analog signal processor may further include a third amplifier configured to amplify an output of the analog filter with a variable gain, and the control unit may control a gain of the third amplifier depending on the reception level of the high frequency signal.

The receiver circuit may include a demodulator configured to demodulate a signal output from the digital signal processor. For example, the demodulator evaluates signal quality after demodulation, and the control unit adjusts the filtering characteristic of the compensator based on an evaluation result of the demodulator.

DETAILED DESCRIPTION

First Embodiment

FIG. 1illustrates a configuration of a receiver circuit according to the first embodiment. A high frequency signal (RF signal) received by an antenna1is converted into a differential RF signal, composed of two signals which are 180 degrees out of phase relative to each other, by a balun2. The RF signal is not limited to a radio signal, but may be a wired signal input through a cable. An analog signal processor10A performs quadrature conversion on the differential RF signal, thereby generates I and Q baseband signals. More specifically, in the analog signal processor10A, a low noise amplifier (LNA)11performs low noise amplification on the differential RF signal. The gain of the LNA11may be either variable or fixed. Frequency converters12A frequency-convert the output of the LNA11respectively using I and Q local oscillator signals. Such local oscillator signals are obtained by applying phase shifts to a local oscillator signal generated by a local oscillator13using a 90-degree phase shifter14so as to introduce a phase shift of 90 degrees relative to each other. Moreover, the frequency converters12A perform low pass filtering on the frequency-converted signals.

FIG. 2illustrates an example configuration of the frequency converters12A. The frequency converters12A include an amplifier121which amplifies the input differential RF signal, a switch unit122which frequency-converts the differential output of the amplifier121using the local oscillator signals, and an analog filter123which performs low pass filtering on the frequency-converted signals at a variable cutoff frequency. The analog filter123according to this embodiment utilizes a capacitor bank configuration, in which a control signal controls the capacitance value. However, the resistance value may be variable instead. The filtering characteristic of the analog filter123may be a fixed filtering characteristic dependent on the desired-wave band. Furthermore, the analog filter123may be a higher-order Butterworth filter, Chebyshev filter, elliptic filter, etc. Note that, in any case, the cutoff frequency of the analog filter123is below the desired-wave band, and preferably is about half the desired-wave band.

Returning toFIG. 1, in the analog signal processor10A, amplifiers15amplify the respective outputs of the frequency converters12A. The amplifiers15may be an AGC amplifier. Alternatively, the gain of the amplifiers15may be fixed. Analog filters16attenuate signal components outside of the desired-wave band contained in the outputs of the amplifiers15. The analog filters16may be a low pass filter (LPF) having a cutoff frequency above the desired-wave band.

ADCs20respectively convert the I and Q baseband signals output from the analog signal processor10A into digital signals. A digital signal processor30A compensates the digital signals for the signal components in the desired-wave band attenuated by the filtering operation of the analog signal processor10A. More specifically, in the digital signal processor30A, digital filters31remove the signal components outside of the desired-wave band contained in the input digital signals. Compensators32filter the outputs of the digital filters31with an inverse characteristic relative to that of the frequency converters12A. The I and Q baseband signals, which have been converted into digital signals and output from the compensators32, are combined by a digital operation, and then the result is output from the digital signal processor30A. There may be provided only one of the digital filters31and one of the compensators32, and the process may be implemented in such a way that the I and Q baseband signals, which have been converted into digital signals in the ADCs20, are combined first, and then the combined signal is input to the digital filter31.

FIGS. 3A-3Eillustrate output frequency spectra of respective signal processing blocks.FIG. 3Aillustrates an output frequency spectrum of the LNA11. The high frequency signal shown here is, for example, a digital broadcast wave, and a high level of interference wave is present adjacent to the desired wave.FIG. 3Billustrates an output frequency spectrum of the frequency converters12A. Each of the frequency converters12A performs frequency conversion using, for example, a local oscillator signal having a center frequency of the desired-wave band (direct conversion method). The filtering operations of the frequency converters12A significantly attenuate the interference waves, allowing the levels of distortion components generated in the amplifiers15and in the analog filters16to be significantly reduced.FIG. 3Cillustrates an output frequency spectrum of the analog filters16. The filtering operations of the analog filters16further attenuate the interference waves. It is assumed here that each of the ADCs20has a sufficiently wide dynamic range to perform analog-to-digital conversion over a range from the minimum amplitude level of the attenuated desired wave to the maximum amplitude level of the remaining interference wave.FIG. 3Dillustrates an output frequency spectrum of the digital filters31. The filtering operations of the digital filters31remove the interference waves.FIG. 3Eillustrates an output frequency spectrum of the compensators32. The filtering operations of the compensators32compensate the signal levels in the desired-wave band so as to be equalized. As described above, if the dynamic range of the ADCs20is wide enough, the desired-wave band can be compensated without degradation in signal quality. Thus, degradation in signal quality in the demodulator40(described later) can be prevented.

Returning toFIG. 1, a demodulator40demodulates the output of the digital signal processor30A, thereby generates a video or audio signal. In the case of an analog broadcast signal, the demodulator40performs digital-to-analog conversion on the output of the digital signal processor30A before demodulation. A control unit50controls the variable gain or the variable filtering characteristic of each of signal processing blocks, that is, the LNA11, the frequency converters12A, the amplifiers15, and the compensators32, depending on a reception level of the high frequency signal input to the receiver circuit. The control unit50can detect the reception level of the high frequency signal by, for example, comparing the output level of the digital signal processor30A with a reference level. The reference level is assumed to be high enough for the demodulator40to perform demodulation. A level comparator may be additionally provided so that the reception level of the high frequency signal is detected by a result of comparison between the output level of each of the signal processing blocks and the reference level.

The variable gain or the variable filtering characteristic of each of the signal processing blocks is controlled specifically as follows. If the reception level of the high frequency signal is low, the gains of the LNA11and of the amplifiers15are increased. In addition, the cutoff frequency of the frequency converters12A is increased, and a filtering characteristic dependent on the change in the cutoff frequency is set to the compensators32. The cutoff frequency of the frequency converters12A may be increased when the gain of the LNA11is controlled so as to be a maximum value. Meanwhile, if the reception level of the high frequency signal is high, the gains of the LNA11and of the amplifiers15are decreased. In addition, the cutoff frequency of the frequency converters12A is decreased, and a filtering characteristic dependent on the change in the cutoff frequency is set to the compensators32. A decrease of the cutoff frequency causes the NF of the frequency converters12A to be decreased, thereby improving the interference wave immunity of the amplifiers15and of the analog filters16provided in succeeding stages of the frequency converters12A. Moreover, the lowest receive sensitivity of the receiver circuit can be prevented from being reduced.

FIG. 4illustrates the relationships between the cutoff frequency of the frequency converters12A and IM3 and NF, respectively. The horizontal axis represents the ratio of the cutoff frequency to the desired-wave band. The left vertical axis represents the third-order distortion component (IM3) generated in the amplifiers15. The right vertical axis represents the NF of a circuit part including the frequency converters12A and the amplifiers15. It is assumed that the gain of the frequency converter12A is 18 dB, and that the LPF characteristic is a second-order characteristic. It is also assumed that the gain of the amplifiers15is 21 dB, IIP3=0 dBm, and NF=25 dB. The graph shown as an example illustrates a case in which an interference wave of −30 dBm is input adjacent to the desired-wave band. The NF and IM3 values change in the opposite way with respect to the horizontal axis. At a value of about 0.5 along the horizontal axis, the NF degradation can be reduced to about 0.5 dB while the IM3 can be reduced by as much as about 20 dB. Therefore, as described above, the cutoff frequency of the frequency converters12A is preferably about half the desired-wave band.

Thus, according to this embodiment, even if the distortion characteristic of each of the signal processing blocks significantly degrades due to low voltage operation, the filtering characteristic of the frequency converters12A allows the distortion components generated in the amplifiers15and in the analog filters16to be reduced. Moreover, compensation of the signal components in the desired-wave band by the compensators32allows the interference wave immunity of the receiver circuit to be improved.

Second Embodiment

FIG. 5illustrates a configuration of a receiver circuit according to the second embodiment. The differences from the first embodiment will be described below.

In an analog signal processor10B, frequency converters12B only frequency-convert the output of the LNA11respectively using the I and Q local oscillator signals, and have no low pass filtering characteristics. Instead, the analog filters16perform low pass filtering on the outputs of the frequency converters12B at a cutoff frequency below the desired-wave band. In addition, amplifiers17are provided in succeeding stages of the analog filters16. This is to apply a lower gain to the amplifiers15, and to sufficiently amplify the signals filtered by the analog filters16because amplification of the outputs of the frequency converters12B at a high gain in the amplifiers15may degrade the distortion characteristic due to a high level of interference wave. The gains of the amplifiers15and17are controlled by the control unit50.

FIGS. 6A-6Cillustrate output frequency spectra of respective signal processing blocks.FIG. 6Aillustrates an output frequency spectrum of the frequency converters12B. The high frequency signal shown here is, for example, a digital broadcast wave, and a high level of interference wave is present adjacent to the desired wave.FIG. 6Billustrates an output frequency spectrum of the analog filters16. The filtering operations of the analog filters16significantly attenuate the interference waves, allowing the levels of distortion components generated in the amplifiers17to be significantly reduced.FIG. 6Cillustrates an output frequency spectrum of the compensators32. The filtering operations of the compensators32compensate the signal levels in the desired-wave band so as to be equalized.

Thus, according to this embodiment, even if the distortion characteristic of each of the signal processing blocks significantly degrades due to low voltage operation, the filtering characteristic of the analog filters16allows the distortion components generated in the amplifiers17to be reduced. Moreover, compensation of the signal components in the desired-wave band by the compensators32allows the interference wave immunity of the receiver circuit to be improved.

Note that, similarly to the first embodiment, the control unit50may control the gain of the LNA11. Other configurations may also be used in which the amplifiers17are removed, and the amplifiers15each have a higher gain, although the interference wave immunity will degrade in some degree.

Third Embodiment

FIG. 7illustrates a configuration of a receiver circuit according to the third embodiment. The receiver circuit of this embodiment differs from the receiver circuit ofFIG. 1in that an image rejection unit33is added. The differences from the first embodiment will be described below.

In the analog signal processor10A, the local oscillator13generates a local oscillator signal having a frequency below or above the desired-wave band. That is, the analog signal processor10A performs frequency conversion of the low-intermediate frequency (IF) method, and thus the high frequency signal is converted into a baseband signal in the IF band. The IF has, for example, a value of half the desired-wave band. Furthermore, in a digital signal processor30B, the image rejection unit33is provided in the preceding stage of the digital filters31. The image rejection unit33receives the I and Q baseband signals respectively output from the ADCs20, and removes image signal components contained in the respective signals.

FIGS. 8A-8Cillustrate output frequency spectra of respective signal processing blocks.FIG. 8Aillustrates an output frequency spectrum of the LNA11. The high frequency signal shown here is, for example, an analog broadcast wave, and contains high levels of video and audio signals in the desired-wave band. As shown here, an analog broadcast wave has a predetermined level difference and a predetermined frequency difference between the video and audio signals. In general, the video signal has a higher level and a lower frequency. Thus, frequency conversion is performed using a local oscillator signal having a frequency below the desired-wave band. Meanwhile, if the level of the video signal is lower than that of the audio signal, then frequency conversion is performed using a local oscillator signal having a frequency above the desired-wave band.FIG. 8Billustrates an output frequency spectrum of the frequency converters12A. The video and audio signals are frequency-converted into the IF band, and are attenuated by the filtering operations of the frequency converters12A. The low-IF frequency conversion causes the level difference between the video and audio signals to increase, thereby causing the level of the audio signal to significantly attenuate as compared to that of the video signal. Thus, the levels of intermodulation distortion components of the video and audio signals generated in the amplifiers15and in the analog filters16can be significantly reduced.FIG. 8Cillustrates an output frequency spectrum of the compensators32. The filtering operations of the compensators32each compensate the level difference between the video and audio signals so as to be the same as the level difference before the frequency conversion.

Thus, according to this embodiment, even if the distortion characteristic of each of the signal processing blocks significantly degrades due to low voltage operation, the low-IF frequency conversion and the filtering characteristic of the frequency converters12A allow the distortion components generated in the amplifiers15and in the analog filters16to be reduced. Moreover, compensation of the signal components in the desired-wave band by the compensators32allows the SNDR characteristic of the receiver circuit to be improved. Note that a configuration in which the image rejection unit33is added to the receiver circuit ofFIG. 4also provides similar advantages.

Fourth Embodiment

FIG. 9illustrates a configuration of a receiver circuit according to the fourth embodiment. The differences from the first embodiment will be described below.

The control unit50receives the input signal to, and the output signal from, the digital filter31which filters the I signal, detects the presence or absence of a high level of interference wave adjacent to the desired-wave band from the level difference between the input and output signals, and controls the signal processing blocks based on the detection result. For example, if the level difference between the input and output signals is greater than a threshold, a high level of interference wave is considered to be present, while the level difference between the input and output signals is less than the threshold, a high level of interference wave is considered to be absent. If a high level of interference wave is not detected, the control unit50increases the gains of the LNA11and of the amplifiers15, increases the cutoff frequency of the frequency converters12A, and sets a filtering characteristic dependent on the change in the cutoff frequency in the compensators32. Meanwhile, if a high level of interference wave is detected, the control unit50decreases the gains of the LNA11and of the amplifiers15, decreases the cutoff frequency of the frequency converters12A, and sets a filtering characteristic dependent on the change in the cutoff frequency in the compensators32.

The demodulator40evaluates signal quality after demodulation. Examples of the signal quality include a BER characteristic, a C/N characteristic, etc. The control unit50adjusts the filtering characteristic of the compensators32based on the evaluation result of the demodulator40.

Thus, according to this embodiment, even if the distortion characteristic of each of the signal processing blocks significantly degrades due to low voltage operation, changing the filtering characteristic of the frequency converters12A depending on the presence or absence of an interference wave allows the distortion components generated in the amplifiers15and in the analog filters16to be reduced. Moreover, compensation of the signal components in the desired-wave band by the compensators32allows the interference wave immunity of the receiver circuit to be improved. Furthermore, monitoring the signal quality characteristic of the demodulator40allows the filtering characteristic of the compensators32to be adaptively controlled so that an optimum interference wave immunity is achieved.

In each of the above embodiments, the configuration may be modified such that the balun2is removed, and each signal processing block processes a non-differential signal.

EXAMPLE OF APPLICATION

FIG. 10illustrates an overview of a receiver apparatus according to any of the embodiments of the present invention. The receiver apparatus is, for example, a television receiver including a receiver circuit100according to any of the above embodiments, and a display panel101as an output device for displaying the output of the receiver circuit100. The receiver circuit100can also be applied to a television broadcast recorder including a storage as an output device.