1. Field of the Invention
The present invention relates to a broadcast receiver for use in a mobile communication terminal.
2. Description of the Related Art
In recent years, mobile communication terminals, such as cell phones, for example, with which the user can receive mobile broadcasts and view and listen to video, audio, and other media of the mobile broadcasts have been widely used. In Japan, digital terrestrial broadcasting using one segment (one-seg) targeted for mobile communication terminals began in 2005 and one-seg-capable cell phones have rapidly appeared.
One-seg-capable cell phones include a receiving antenna for receiving broadcast signals and an antenna tuning circuit connected to the receiving antenna. A typical reception system is a tuning-type system. Specifically, a tuning-type system is a system in which, by applying a proper tuned voltage, an antenna tuning circuit defines a resonant circuit that resonates at frequencies equal to channel frequencies being received and only a required frequency band is received by an antenna. A quarter-wavelength whip antenna is suitable for this antenna in view of the characteristics of cell phones.
FIG. 4 shows a configuration of a broadcast receiver for use in a mobile communication terminal according to the related art. In FIG. 4, reference numeral 1 represents a receiving antenna, reference numeral 2 represents an antenna tuning circuit, reference numeral 4 represents a tuned voltage application circuit, AMP1 represents an amplifier, MIX1 represents a frequency mixer, and reference numeral 10 represents a local oscillator. The antenna tuning circuit 2 includes a tuning coil L10 and a varactor D10, which define a resonant circuit.
When a voltage output from the tuned voltage application circuit 4 is applied to the varactor D10, the receiving antenna 1 receives broadcast signals tuned to the resonant frequency of the resonant circuit including the varactor D10 and tuning coil L10. The received signals are input into the frequency mixer MIX1 via the amplifier AMP1. Simultaneously, signals output from the local oscillator 10 are input into the frequency mixer MIX1. The frequency mixer MIX1 mixes these signals to generate intermediate-frequency signals, which are then output to an intermediate-frequency signal output terminal 12. Adjustment of a voltage output from the tuned voltage application circuit 4 can change broadcast signals to be received by the antenna 1, thereby receiving desired broadcast signals.
Japanese Unexamined Patent Application Publication No. 2003-289259 discloses an example of a broadcast receiver according to the related art.
When the receiving circuit described in FIG. 4 is applied to a one-seg-capable cell phone, the following problem occurs.
A cell phone, which is receiving broadcast signals, may transmit transmission signals to a base station via a transmission antenna (not shown) in order to transmit the location of the cell phone. In this case, the transmission signals transmitted to the base station may induce a high-frequency voltage in the receiving antenna 1. Due to such a high-frequency voltage, the resonant frequency of the antenna tuning circuit 2, which is set to match a receiving frequency corresponding to a desired channel, deviates from the receiving frequency. This results in a degradation of the reception sensitivity and the reception quality in the desired channel.