Patent Publication Number: US-8538363-B2

Title: Semiconductor integrated circuit and radio receiver

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2011-260505, filed on Nov. 29, 2011, the entire contents of which are incorporated herein by reference. 
     BACKGROUND 
     1. Field 
     Embodiments described herein relate generally to a semiconductor integrated circuit and a radio receiver. 
     2. Background Art 
     The superheterodyne system and the direct conversion system are generally known systems for converting a radio frequency (RF) signal to a baseband signal in the receiving circuit of a semiconductor integrated circuit used for radio communications of a cellular phone or the like. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram illustrating an example of the configuration of a radio receiver  1000  according to a first embodiment; 
         FIG. 2  is a block diagram illustrating an example of the configuration of the semiconductor integrated circuit  100  illustrated in  FIG. 1 ; 
         FIG. 3  is a circuit diagram illustrating an example of the configuration of the variable gain amplifier circuit VGA illustrated in  FIG. 2 ; 
         FIG. 4  is a circuit diagram illustrating an example of the configuration of the first low-pass filter LPF 1  illustrated in  FIG. 2 ; 
         FIG. 5  is a circuit diagram illustrating an example of the configuration of the second low-pass filter LPF 2  illustrated in  FIG. 2 ; 
         FIG. 6  is a circuit diagram illustrating an example of the configuration of the third low-pass filter LPF 3  illustrated in  FIG. 2 ; 
         FIG. 7  shows waveform simulation results of the signal SF 1  at the first terminal X and the signal SF 3  at the second terminal Y when a modulating signal is inputted under ideal conditions in which the signal path does not have a DC offset and DC offset variations are not found in the time of gain variations; 
         FIG. 8  is a waveform chart of a comparative example in which cut-off frequencies are switched at a high input signal level; and 
         FIG. 9  is a waveform chart of the output signal of the semiconductor integrated circuit  100  according to the first embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     A semiconductor integrated circuit according to an embodiment includes a first adder that outputs a first addition signal obtained by adding an input signal inputted to an input terminal and a first inverted signal obtained by inverting a first feedback signal outputted from a first terminal. The semiconductor integrated circuit includes a variable gain amplifier circuit that outputs an output signal obtained by amplifying the first addition signal to an output terminal, the variable gain amplifier circuit having a variable gain. The semiconductor integrated circuit includes a first low-pass filter that is fed with the output signal and outputs a signal obtained by filtering the output signal, the first low-pass filter having a variable cut-off frequency. The semiconductor integrated circuit includes a second low-pass filter that is fed with the output signal and outputs a signal obtained by filtering the output signal, the second low-pass filter having a lower cut-off frequency than that of the first low-pass filter. The semiconductor integrated circuit includes a second adder that outputs a second addition signal obtained by adding the input signal and a second inverted signal obtained by inverting a second feedback signal inputted to a second terminal. The semiconductor integrated circuit includes a third low-pass filter that is fed with the second addition signal and outputs a signal obtained by filtering the second addition signal, as the second feedback signal to the second terminal. The semiconductor integrated circuit includes a first control circuit that controls the gain of the variable gain amplifier circuit and the cut-off frequency of the first low-pass filter so as to keep constant a product of a transfer function of the variable gain amplifier circuit and a transfer function of the first low-pass filter. The semiconductor integrated circuit includes a second control circuit that compares a level of the second feedback signal and a preset threshold value after completion of gain control of the variable gain amplifier circuit, the second control circuit inputting the signal outputted from the first low-pass filter, as the first feedback signal to the first terminal as long as the level of the second feedback signal is not lower than the threshold value, the second control circuit inputting the signal outputted from the second low-pass filter, as the first feedback signal to the first terminal when the level of the second feedback signal falls below the threshold value. 
     For example, in the direct conversion system, flicker noise is generated and DC components are varied (DC offset) by self mixing of a mixer circuit and a mismatch of elements constituting the circuit. These factors in the direct conversion system may cause direct degradation of signals or may degrade circuit characteristics so as to indirectly degrade signals. 
     In order to solve the problem, generally, the low frequency components of signals are removed using high pass filters (HPFs). 
     In this case, an HPF having an extremely high cut-off frequency may lose information carried by a signal, leading to degradation of receiving performance. 
     Thus, a cut-off frequency is set at a low frequency that does not degrade receiving performance. 
     Furthermore, in a receiving circuit, high-speed gain control is demanded. The range of power (dynamic range) received by a receiver is extremely widened depending on, for example, a distance from the transmitter. When an RF signal is inputted to an AD converter circuit, the RF signal has to be adjusted to a predetermined signal amplitude. This is because a time for gain control is limited to obtain a high transmission rate. 
     In a typical receiver, a variation of gain changes the value of a DC offset generated by a mismatch of elements constituting a circuit. Thus, an extremely large DC offset may be obtained immediately after a change of gain. An HPF causes the DC offset to attenuate and converge after a while. At this point, a time constant is determined by the cut-off frequency of the HPF. 
     Thus, in the foregoing gain control at a low cut-off frequency, convergence of offset variations is extremely time-consuming, precluding high-speed gain control. 
     Hence, in a method of preventing degradation of receiving performance, the cut-off frequency of an HPF is set so high as to shorten a convergence time of DC offset variations during gain control and the cut-off frequency is reduced at the completion of the gain control. 
     However, in the case where cut-off frequencies are switched while a signal is received, a signal level at the time of switching causes a DC offset which slowly converges from that moment according to the response characteristics of an HPF having a low cut-off frequency. 
     Thus, unfortunately, a time for gain control may increase or the DC offset may be left at a time when DC offset variations should converge, leading to degradation of receiving performance. 
     The following embodiment will propose a semiconductor integrated circuit and a radio receiver that can achieve higher receiving performance. 
     The embodiment will be described below with reference to the accompanying drawings. For simple explanation, an LPF for realizing an HPF having a high cut-off frequency will be called a fast LPF and an LPF for realizing an HPF having a low cut-off frequency will be called a slow LPF. The LPFs are switched to switch HPF cut-off frequencies. 
     The cut-off frequency depends on a loop gain. Thus, when the gain of a variable gain amplifier circuit is changed, the gain of the high-speed LPF is also changed so as to prevent a change of the loop gain. 
     First Embodiment 
       FIG. 1  is a block diagram illustrating an example of the configuration of a radio receiver  1000  according to a first embodiment. 
     As illustrated in  FIG. 1 , the radio receiver  1000  includes a semiconductor integrated circuit  100 , an antenna  1001 , a low-noise amplifier circuit (LNA)  1002 , a local oscillating circuit  1003 , a mixer circuit  1004 , a low-pass filter  1005 , and an analog/digital converter circuit  1006 . 
     The antenna  1001  receives an RF signal. 
     The low-noise amplifier circuit  1002  amplifies the received RF signal and outputs the signal. 
     The local oscillating circuit  1003  generates a local oscillation signal and outputs the signal. 
     The mixer circuit  1004  outputs a mixed signal of the signal outputted from the low-noise amplifier circuit  1003  and the local oscillation signal. 
     The low-pass filter  1005  filters the signal outputted from the mixer circuit  1004  and outputs the signal. 
     The semiconductor integrated circuit  100  amplifies the signal outputted from the low-pass filter  1005  and outputs the signal. The semiconductor integrated circuit  100  has a controllable gain. 
     The analog/digital converter circuit  1006  converts, to a digital signal, the signal outputted from the semiconductor integrated circuit  100 . 
       FIG. 2  is a block diagram illustrating an example of the configuration of the semiconductor integrated circuit  100  illustrated in  FIG. 1 . 
     As illustrated in  FIG. 2 , the semiconductor integrated circuit  100  includes an input terminal Tin, an output terminal Tout, a first adder A 1 , a second adder A 2 , a variable gain amplifier circuit VGA, a first low-pass filter (fast LPF) F 1 , a second low-pass filter (slow LPF) F 2 , a third low-pass filter (fast LPF) F 3 , a first control circuit CON 1 , a second control circuit CON 2 , a first switch element SW 1 , a second switch element SW 2 , and an inverter INV. 
     The first adder A 1  outputs a first addition signal SA 1  that is obtained by adding an input signal Sin inputted to the input terminal Tin and a first inverted signal obtained by inverting a first feedback signal outputted from a first terminal X. 
     The variable gain amplifier circuit VGA outputs, to the output terminal Tout, an output signal Sout obtained by amplifying the first addition signal SA 1 . The variable gain amplifier circuit VGA has a variable gain. 
     The first low-pass filter LPF 1  is fed with the output signal Sout and outputs a signal SF 1  obtained by filtering the output signal Sout. The first low-pass filter LPF 1  has a variable cut-off frequency. 
     The second low-pass filter F 2  is fed with the output signal Sout and outputs a signal SF 2  obtained by filtering the output signal Sout. The cut-off frequency of the second low-pass filter SF 2  is set lower than that of the first low-pass filter LPF 1 . 
     The second adder A 2  outputs a second addition signal SA 2  that is obtained by adding the input signal Sin and a second inverted signal obtained by inverting a second feedback signal inputted to a second terminal Y. 
     The third low-pass filter LPF 3  is fed with the second addition signal SA 2  and outputs a signal SF 3  as the second feedback signal to the second terminal Y. The signal SF 3  is obtained by filtering the second addition signal SA 2 . 
     The gain of the third low-pass filter LPF 3  is set equal to that of the first low-pass filter LPF 1  when the gain of the variable gain amplifier circuit VGA is set at a minimum. 
     Moreover, a transfer function f 3  of the third low-pass filter LPF 3  is set equal to the product of a transfer function fv of the variable gain amplifier circuit VGA and a transfer function f 1  of the first low-pass filter LPF 1 . 
     The first control circuit CON 1  controls the gain of the variable gain amplifier circuit VGA and the cut-off frequency of the first low-pass filter LPF 1  so as to keep constant a transfer function fin 1  from the input terminal Tin to the first terminal X through the first low-pass filter LPF 1  (i.e., to keep constant the product of the transfer function fv of the variable gain amplifier circuit VGA and the transfer function f 1  of the first low-pass filter LPF 1 ). 
     The first control circuit CON 1  notifies the second control circuit CON 2  that the gain control of the variable gain amplifier circuit VGA has completed. 
     The first switch element SW 1  is connected between the first terminal X and the output of the first low-pass filter LPF 1 . The first switch element SW 1  is turned on/off by the second control circuit CON 2 . 
     In the example of  FIG. 2 , the first switch element SW 1  is controlled by a control signal CS 1  outputted from the second control circuit CON 2 . 
     The second switch element SW 2  is connected between the first terminal X and the output of the second low-pass filter LPF 2 . The second switch element SW 2  is turned on/off by the second control circuit CON 2 . 
     In the example of  FIG. 2 , the second switch element SW 2  is controlled by a control signal CS 2  obtained by inverting, by means of the inverter INV, the control signal CS 1  outputted from the second control circuit CON 2 . 
     In other words, the first switch element SW 1  and the second switch element SW 2  are controlled by the second control circuit CON 2  so as to be complementarily turned on/off. 
     The second control circuit CON 2  compares the level of the second feedback signal and a preset threshold value Vth after the completion of the gain control of the variable gain amplifier circuit VGA. 
     The second control circuit CON 2  turns on the first switch element SW 1  and turns off the second switch element SW 2  as long as the level of the second feedback signal is not lower than the threshold value Vth during the gain control of the variable gain amplifier circuit VGA and after the completion of the gain control of the variable gain amplifier circuit VGA. 
     In other words, the second control circuit CON 2  inputs the signal SF 1 , which is outputted from the first low-pass filter LPF 1 , as the first feedback signal to the first terminal X in a period during which the level of the second feedback signal is not lower than the threshold value Vth during the gain control of the variable gain amplifier circuit VGA and after the completion of the gain control of the variable gain amplifier circuit VGA. 
     The second control circuit CON 2  turns off the first switch element SW 1  and turns on the second switch element SW 2  when the level of the second feedback signal falls below the threshold value Vth after the completion of the gain control of the variable gain amplifier circuit VGA. After that, this state is maintained even if the level of the second feedback signal exceeds the threshold value Vth. 
     In other words, the second control circuit CON 2  keeps inputting the signal SF 2 , which is outputted from the second low-pass filter LPF 2 , as the first feedback signal to the first terminal X in the case where the level of the second feedback signal falls below the threshold value Vth once after the completion of the gain control of the variable gain amplifier circuit VGA. 
       FIG. 3  is a circuit diagram illustrating an example of the configuration of the variable gain amplifier circuit VGA illustrated in  FIG. 2 . 
     As illustrated in  FIG. 3 , the variable gain amplifier circuit VGA includes, for example, a first variable resistor R 1 , a second variable resistor R 2 , and an amplifier circuit AMP. 
     The first variable resistor R 1  has one end connected to the output (terminal T) of the first adder A 1 . 
     The amplifier circuit AMP has an inverting Input terminal connected to the other end of the first variable resistor R 1 , a non-inverting input terminal connected to ground, and an output connected to the output terminal Tout. 
     The second variable resistor R 2  has one end connected to the inverting input terminal of the amplifier circuit AMP and the other end connected to the output of the amplifier circuit AMP. 
     In this configuration, the first control circuit CON 1  controls the transfer function fv of the variable gain amplifier circuit VGA by controlling the resistance value of the first variable resistor R 1  and the resistance value of the second variable resistor R 2 . 
       FIG. 4  is a circuit diagram illustrating an example of the configuration of the first low-pass filter LPF 1  illustrated in  FIG. 2 . 
     As illustrated in  FIG. 4 , the first low-pass filter LPF 1  includes, for example, a first resistor Ra and a variable capacitor Ca. 
     The first resistor Ra has one end connected to a terminal Tin 1  fed with the output signal Sout and has the other end connected to a terminal Tout 1  for outputting the signal SF 1  obtained by filtering the output signal Sout. The first resistor Ra may further include a buffer connected between the terminal Tin 1  and the one end of the first resistor Ra. 
     The variable capacitor Ca is connected between the other end of the first resistor Ra and the ground. 
     In this configuration, the first control circuit CON 1  controls the cut-off frequency (transfer function f 1 ) of the low-pass filter LPF 1  by controlling the capacitance value of the variable capacitor Ca. 
       FIG. 5  is a circuit diagram illustrating an example of the configuration of the second low-pass filter LPF 2  illustrated in  FIG. 2 . 
     As illustrated in  FIG. 5 , the second low-pass filter LPF 2  includes, for example, a second resistor Rb and a first capacitor Cb. 
     The second resistor Rb has one end connected to a terminal Tin 2  fed with the output signal Sout and has the other end connected to a terminal Tout 2  for outputting the signal SF 2  obtained by filtering the output signal Sout. The second resistor Rb may further include a buffer connected between the terminal Tin 2  and the one end of the second resistor Rb. 
     The first capacitor Cb is connected between the other end of the second resistor Rb and the ground. 
       FIG. 6  is a circuit diagram illustrating an example of the configuration of the third low-pass filter LPF 3  illustrated in  FIG. 2 . 
     As illustrated in  FIG. 6 , the third low-pass filter LPF 3  includes, for example, a third resistor Rc and a second capacitor Cc. 
     The third resistor Rc has one end connected to a terminal Tin 3  fed with the second addition signal SA 2  and the other end connected to a terminal Tout 3  for outputting the signal SF 3  obtained by filtering the output signal Sout. 
     The second capacitor Cc is connected between the other end of the third resistor Rc and the ground. 
     As described above, the gain of the third low-pass filter LPF 3  is set equal to that of the first low-pass filter LPF 1  when the gain of the variable gain amplifier circuit VGA is set at the minimum. 
     In this case, the cut-off frequency of the third low-pass filter LPF 3  can be set higher. Specifically, the product of the capacitance value of the second capacitor Cc and the resistance value of the third resistor Rc can be reduced, allowing the semiconductor integrated circuit  100  to have a smaller circuit area. 
     In the semiconductor integrated circuit  100  configured thus, the transfer function from the input to the output serves as an HPF characteristic and a cut-off frequency is determined by a loop gain and the cut-off frequency of an LPF when the overall circuit is viewed as an HPF. The value of the cut-off frequency is changed to obtain HPF characteristics at a high cut-off frequency and HPF characteristics at a low cut-off frequency. 
     The following will discuss characteristics for switching the cut-off frequencies of the semiconductor integrated circuit  100  having the foregoing configuration and functions. 
     As described above, in order to improve receiving performance, a DC offset caused by the input signal Sin needs to be suppressed when the cut-off frequencies are switched. 
     A larger DC offset occurs if the cut-off frequencies are switched when the first terminal X has a higher signal level. 
     Thus, in the present embodiment, the cut-off frequencies are switched when the first terminal X has a low signal level. 
     In this case, the signal of the first terminal X also contains DC components for correcting the DC offset of the variable gain amplifier circuit VGA. Hence, for comparison of the level of the input signal Sin, the number of DC components needs to be recognized. 
     In the present embodiment, as described above, the third low-pass filter LPF 3  and the second control circuit CON 2  ( FIG. 2 ) are used to detect the level of the signal SF 1  at the first terminal X when the effect of a DC offset in the circuit is eliminated. 
     The input signal Sin is inputted to the additional third low-pass filter LPF 3  as well as an ordinary signal path. As described above, the third low-pass filter LPF 3  has the same characteristics as the first low-pass filter LPF 1  that removes a DC offset of a signal path when the gain of the variable gain amplifier circuit VGA is, for example, equal to a minimum gain of 0 dB (1 time). 
     The transfer function fin 2  from the input terminal Tin to the second terminal Y is expressed by equation (1) below.
 
 fin 2 =f 3/(1 +f 3)  (1)
 
     The transfer function fin 2  is equal to the transfer function fin 1  from the input to the first terminal X. The transfer function fin 1  is expressed by equation (2) below.
 
 fin 1 =f 1 ×fv /(1 +f 1 ×fv )  (2)
 
     The gain of the first low-pass filter LPF 1  is controlled by the first control circuit CON 1  concurrently with the gain of the variable gain amplifier circuit VGA to prevent a change of the loop gain. As described above, for example, the gain of the third low-pass filter LPF 3  is equal to the gain of the first low-pass filter LPF 1  when the gain of the variable gain amplifier circuit VGA is 0 dB (1 time). 
       FIG. 7  shows waveform simulation results of the signal SF 1  at the first terminal X and the signal SF 3  at the second terminal Y when a modulating signal is inputted under ideal conditions in which the signal path does not have a DC offset and DC offset variations are not found in the time of gain variations. 
     As shown in  FIG. 7 , the waveform of the signal SF 1  at the first terminal X and the waveform of the signal SF 3  at the second terminal Y are identical to each other. 
     Thus, the signal SF 3  at the second terminal Y can be used for detecting the level of the signal SF 1 . The influence of a DC offset is negligible in the case where the variable gain amplifier circuit VGA dominantly acts as, for example, a source of a DC offset and an input node has a low DC offset. In the case where the influence of a DC offset is not negligible, DC components may be removed on the input of the third low-pass filter LPF 3  by an HPF with a low cut-off frequency. 
     This is because gain control does not cause DC offset variations and thus convergence over a long time is acceptable. If sufficient convergence is obtained at the switching of cut-off frequencies, no problems occur. 
     As described above, the second control circuit CON 2  monitors the level of the second feedback signal at the second terminal Y. In the case where the level of the second feedback signal is higher than the threshold value Vth, the second control circuit CON 2  turns on the first switch element SW 1  and turns off the second switch element SW 2 . 
     Specifically, in the case where the level of the second feedback signal is higher than the threshold value Vth, the second control circuit CON 2  inputs the signal SF 1  outputted from the first low-pass filter LPF 1 , as the first feedback signal to the first terminal X. 
     Then, the second control circuit CON 2  monitors the level of the second feedback signal at the second terminal Y. When the signal level falls below the threshold value Vth for the first time after the completion of the gain control of the variable gain amplifier circuit VGA, the second control circuit CON 2  outputs a signal CS 1  for switching cut-off frequencies. 
     In other words, in the case where the level of the second feedback signal is lower than the threshold value Vth, the second control circuit CON 2  inputs the signal SF 2  outputted from the second low-pass filter LPF 2 , as the first feedback signal to the first terminal X. 
       FIG. 8  is a waveform chart of a comparative example in which cut-off frequencies are switched at a high input signal level.  FIG. 9  is a waveform chart of the output signal of the semiconductor integrated circuit  100  according to the first embodiment. 
     As shown in  FIG. 8 , in the comparative example, the influence of an input signal causes large DC offset variations. 
     In the semiconductor integrated circuit  100  according to the first embodiment, as shown in  FIG. 9 , cut-off frequencies are switched at a low input signal level, so that DC offset variations do not occur. 
     In other words, the semiconductor integrated circuit  100  according to the present embodiment automatically adjusts timing for switching cut-off frequencies in response to an input signal, thereby preventing a received signal from causing a DC offset and achieving faster gain control. 
     As described above, the semiconductor integrated circuit according to the present embodiment can improve receiving performance. 
     While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel methods and systems described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the methods and systems described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the inventions.