In accordance with recent developments of electronics devices, demands for a low price receiver have been increased. “RF MICROELECTRONICS” by Behzad Razavi (published by Prentice Hall) describes in its sections 5.2.1 and 5.2.2 that a received signal is directly converted into a baseband signal with a receiving method called a direct conversion method (also called a zero IF method). Such a direct conversion method does not involve image response in principle, and accordingly does not require a filter which (i) has a precipitous filtering characteristic for eliminating images and (ii) is required in a heterodyne method. Accordingly, the receiving method with direct conversion allows an integrated circuit (IC) to have a function of a filter for channel selection. Therefore, the direct conversion method is attracting attention as a receiving method for meeting the demand for price reduction.
FIG. 10 is a block diagram illustrating a basic configuration of a tuner 400 which is included in a conventional direct conversion receiver. The tuner 400 includes a front-end circuit 410 and a demodulator circuit 9. The front-end circuit 410 includes a variable gain amplifier (RFVGA) 2, a multiplier (mixer) 3, a low-pass filter 4, a variable gain amplifier (IFVGA) 5, a local oscillator (VCO) 6, a local frequency control circuit 7, and a DC offset cancel circuit 8.
The RFVGA 2 amplifies an RF signal Frf received by an antenna 1. Thus amplified signal is subjected to a frequency conversion in which the amplified signal is mixed with an after-mentioned local oscillation signal (hereinafter, referred to as “local signal”) by the mixer 3, so as to be converted into a baseband signal. The low-pass filter 4 attenuates components outside of a desired signal band of the baseband signal, whereby the desired signal is selected. Then, the desired signal is amplified by the IFVGA 5, and thus amplified desired signal is sent, as an IF output, to the demodulator circuit 9. The demodulator circuit 9 demodulates the IF output into a video signal and an audio signal.
The VCO 6 generates the local signal. The local frequency control circuit 7 controls a frequency of the local signal in accordance with a frequency of the RF signal, so that the baseband signal sent from the mixer 3 has a predetermined frequency.
The following describes a problem unique to the direct conversion receiver. In a case where a local signal sent from the VCO 6 is leaked to an RF port of the mixer 3, the mixer 3 mixes the leaked local signal with a local signal sent from the VCO 6 (self-mixing). In this case, the local signal sent from the VCO 6 and the leaked local signal have the same frequency. This causes the mixer 3 to output a baseband signal containing a DC component (DC offset). The DC offset may cause saturation in a subsequent circuit.
In view of the problem, the DC offset cancel circuit 8 included in the front-end circuit 410 eliminates the DC offset contained in the output supplied from the IFVGA 5. For example, the following Patent Documents 1 and 2 disclose techniques for eliminating DC offsets.
Citation List
Patent Documents
Patent Document 1
Japanese Patent Application Publication, Tokukaihei, No. 10-93647 A (Publication Date: Apr. 10, 1998)
Patent Document 2
Japanese Patent Application Publication, Tokukai, No. 2003-273945 A (Publication Date: Sep. 26, 2003)