As a modulation method of a digital signal, orthogonal frequency division multiplexing (OFDM) is known. The OFDM is adopted in digital television broadcasting, a mobile phone, and a wireless LAN.
Meanwhile, a portable terminal such as a smart phone and a tablet PC can be exemplified as an electronic apparatus receiving an OFDM-modulated signal (hereinafter, referred to as the OFDM signal). Because there is a limitation in a battery capacity of the portable terminal, low consumption power is demanded when the OFDM signal is received. In an LSI mounted to the portable terminal and having a function of receiving the OFDM signal to realize the demand, an SOC solution in which an analog circuit and a digital circuit are mounted on the same silicon is frequently adopted.
Meanwhile, as miniaturization of a semiconductor such as the LSI advances, a digital circuit in the LSI can realize a small size and low consumption power. However, in an analog circuit in the LSI, because consumption power increases or a circuit scale increases to suppress an influence such as reduction of a power-supply voltage, a benefit of the miniaturization may not be used. For this reason, in the LSI in which the SOC solution is adopted, a small size and low consumption power in the analog circuit are demanded.
FIG. 1 illustrates an example of a configuration of a receiving device according to the related art that is mounted as the LSI to the portable terminal and receives an OFDM signal.
A receiving device 10 includes a variable gain amplifier 11, frequency converting units 12I and 12Q, BB filters 13I and 13Q, variable step gain BB amplifiers 14I and 14Q, AD converting units (ADC) 15I and 15Q, a local oscillating unit (LO Gen.) 16, an AGC control unit (AGC CTRL LOGIC) 17, and an OFDM demodulating unit (DMD) 18. The variable gain amplifier 11 continuously amplifies an OFDM signal to be an RF signal, according to RF gain notified from the AGC control unit 17, and outputs the OFDM signal to the frequency converting units 12I and 12Q. The frequency converting unit 12I mixes the amplified OFDM signal with a signal for frequency conversion from the local oscillating unit 16, converts the RF signal into an IF signal, and outputs the IF signal to the BB filter 13I. The frequency converting unit 12Q mixes the amplified OFDM signal with a signal for frequency conversion from the local oscillating unit 16, converts the RF signal into an IF signal, and outputs the IF signal to the BB filter 13Q.
The BB filter 13I extracts a base band signal of an I component of the OFDM signal converted into the IF signal and outputs the base band signal to the variable step gain BB amplifier 14I. The BB filter 13Q extracts a base band signal of a Q component of the OFDM signal converted into the IF signal and outputs the base band signal to the variable step gain BB amplifier 14Q.
The variable step gain BB amplifier 14I gradually amplifies the base band signal of the I component of the OFDM signal, according to COMP. gain notified from the AGC control unit 17, and outputs the base band signal to the AD converting unit 15I. The variable step gain BB amplifier 14Q gradually amplifies the base band signal of the Q component of the OFDM signal, according to COMP. gain notified from the AGC control unit 17, and outputs the base band signal to the AD converting unit 15Q.
The AD converting unit 15I converts the base band signal of the I component of the OFDM signal into a digital signal and outputs the digital signal to the AGC control unit 17 and the OFDM demodulating unit 18. The AD converting unit 15Q converts the base band signal of the Q component of the OFDM signal into a digital signal and outputs the digital signal to the AGC control unit 17 and the OFDM demodulating unit 18.
The local oscillating unit 16 supplies a signal for frequency conversion to the frequency converting unit 12I. In addition, the local oscillating unit 16 shifts a phase of the signal for the frequency conversion supplied to the frequency converting unit 12I by 90 degrees and supplies the signal for the frequency conversion to the frequency converting unit 12Q.
The AGC control unit 17 determines the RF gain and the COMP. gain on the basis of outputs of the AD converting unit 15I and 15Q and notifies the variable gain amplifier 11 and the variable step gain BB amplifiers 14I and 14Q of the RF gain and the COMP. gain, respectively. The OFDM demodulating unit 18 demodulates a transmitted symbol on the basis of the I component and the Q component of the digitized OFDM signal and outputs the symbol to a rear step.
In the receiving device 10, the OFDM signal input as the RF signal is amplified by the variable gain amplifier 11 according to the RF gain and is converted into the IF signal by the frequency converting units 12I and 12Q and the base band signals of the I component and the Q component are extracted by the BB filters 13I and 13Q. In addition, the I component and the Q component of the OFDM signal are amplified by the variable step gain BB amplifiers 14I and 14Q according to the COMP. gain, are digitized by the AD converting units 15I and 15Q, and are demodulated into the symbol by the OFDM demodulating unit 18.
As illustrated in FIG. 1, in the receiving device 10 mounted to the portable terminal, a discrete control type amplifier in which an analog circuit is reduced and a small size and low consumption power are realized is adopted in partial components (the variable step gain BB amplifiers 14I and 14Q). However, in the case in which the discrete control type amplifier is adopted, if a steep gain variation is generated, a characteristic of the OFDM demodulation may be notably deteriorated in general.
Therefore, a method of additionally mounting a continuous control amplifier to suppress the steep gain variation from being generated is known (for example, refer to NPL 1). However, in this method, a circuit scale and consumption power may be increased.
Therefore, as illustrated in FIG. 2, a method of performing digital correction on an ADC output is suggested as another method of suppressing the steep gain variation from being generated (for example, refer to NPL 2).
FIG. 2 illustrates an example of a configuration of a receiving device according to the related art that includes a digital correcting unit to perform the digital correction on the ADC output, is mounted as an LSI to a portable terminal, and receives an OFDM signal.
A receiving device 20 includes a variable step gain amplifier 21, frequency converting units 12I and 12Q, BB filters 13I and 13Q, variable step gain BB amplifiers 14I and 14Q, AD converting units (ADC) 15I and 15Q, digital correcting units 22I and 22Q, a local oscillating unit (LO Gen.) 16, an AGC control unit (AGC CTRL LOGIC) 23, and an OFDM demodulating unit (DMD) 18.
In the receiving device 20, the variable gain amplifier 11 and the AGC control unit 17 of the receiving device 10 of FIG. 1 are replaced by a variable step gain amplifier 21 and an AGC control unit 23, respectively, a digital correcting unit 22I is provided between the AD converting unit 15I and the OFDM demodulating unit 18, and a digital correcting unit 22Q is provided between the AD converting unit 15Q and the OFDM demodulating unit 18.
The variable step gain amplifier 21 gradually amplifies an OFDM signal to be an RF signal, according to the RF gain notified from the AGC control unit 17, and outputs the OFDM signal to the frequency converting units 12I and 12Q.
The digital correcting unit 22I is operated to correct a gain variation generated in an output of the AD converting unit 15I by a step response (gradual amplification) in an analog circuit, according to correction gain notified from the AGC control unit 23. The digital correcting unit 22Q is operated to correct a gain variation generated in an output of the AD converting unit 15Q by a step response in an analog circuit, according to correction gain notified from the AGC control unit 23.
The AGC control unit 23 determines the RF gain, the COMP. gain, and the correction gain, on the basis of outputs of the digital correcting units 22I and 22Q, and notifies the variable step gain amplifier 21, the variable step gain BB amplifiers 14I and 14Q, and the digital correcting units 22I and 22Q of the RF gain, the COMP. gain, and the correction gain.
Because components common to the components of the receiving device 10 of FIG. 1 are denoted with the same reference numerals, explanation thereof is omitted.