1. Field of the Invention
The present invention relates to mixer circuit and its application to radio system, in particular to mixer performing two-step frequency translation and radio system employing such mixer.
2. Description of Related Art
In a radio receiver, a receiving mixer receives two input signals, i.e. a RF (radio frequency) signal and a LO (local oscillator) signal, and generates an output signal, i.e. an IF (intermediate frequency) signal, whose frequency is a difference between a frequency of the RF signal and a frequency of the LO signal. Conversely, in a radio transmitter, a transmitting mixer receives two input signals, i.e. an IF signal and a LO signal, and generates an output signal, i.e. a RF signal, whose frequency is a sum of a frequency of the IF signal and a frequency of the LO signal. In either case, a mixer performs a frequency translation but at the same time also introduces noise and distortion to the desired output signal (the IF signal in a receiving mixer or the RF signal in a transmitting signal). Two types of mixers are commonly used in a radio system: active mixer and passive mixer. An active mixer, which needs a constant bias, can provide a gain to the desired output signal but usually is noisy and has poor linearity. On the other hand, a passive mixer, which does not need a constant bias, causes a loss to the desired output signal but usually is less noisy and has good linearity.
A tuner is an apparatus used to select a particular channel of a narrow-band RF signal amongst a wide-band RF signal comprising a plurality of channels. A tuner needs to utilize a combination of various amplifiers, filters, and mixers to perform the desired channel selection. Rotzoll disclosed a highly integrated TV tuner in U.S. Pat. No. 5,737,035, as illustrated in FIG. 1. In tuner 100 of FIG. 1, a wide-band RF signal is received from an antenna, then filtered by a pre-filter 110, then amplified by a LNA (low-noise amplifier) 120, then converted to a first IF signal by a first mixer 130 using a first LO signal LO1, then converted to a second IF signal by a second mixer 140 using a second LO signal LO2, and then filtered by a BPF (band-pass filter) 150, resulting in a narrow band signal to be further processed by a subsequent demodulator. The performance of tuner 100 is highly limited, due to using two mixers 130 and 140, which either introduces significant noise and distortion (for using an active mixer) or loss (for using a passive mixer) to the desired narrow-band output signal.
What is needed is a method for reducing the noise and distortion or loss caused by using two mixers while still achieving the desired frequency translation.