Quadrature mixer circuits and mobile terminal using the same
Because there are different voltages at two current output terminals of a current divider, the voltages at the current input terminals of two current switch circuits are not affected mutually even with a large amplitude of local signals. Accordingly, the performance of a quadrature mixer can be enhanced by increasing the amplitude of the local signals. Bias currents are supplied to the two current switch circuits through the current divider from a common DC current source which essentially supplies a bias current to a V/I converter and, therefore, power consumption is reduced.
1. A mobile terminal comprising:
a quadrature mixer circuit comprising:
an input terminal;
a divider which divides a signal from said input terminal into substantially equal two parts, a first output signal and a second output signal;
a first voltage-current converter to which a bias current is supplied from a first DC source and which converts the voltage of the first output signal from said divider into signal current;
a second voltage-current converter to which a bias current is supplied from a second DC source and which converts the voltage of the second output signal from said divider into signal current;
a local signal oscillator;
a 90° phase shifter which outputs a local signal whose phase is substantially 90 degrees ahead or behind the phase of a local signal from said local signal oscillator;
a first current switch circuit which switches on/off current output from said first voltage-current converter at timing of the local signal from said local signal oscillator;
a first current-voltage converter which converts current output from said first current switch circuit into a voltage signal;
a second current switch circuit which switches on/off current output from said second voltage-current converter at timing of the local signal output from said 90° phase shifter; and
a second current-voltage converter which converts current output from said second current switch circuit into a voltage signal,
wherein said quadrature mixer circuit includes an attenuator which attenuates signal current or voltage between a current output terminal of said first voltage-current converter and a current output terminal of said second voltage-current converter.
2. The mobile terminal as recited in claim 1 , wherein:
the bias current to be supplied to said first voltage-current converter is equal to or more than a bias current to be supplied to said first current switch circuit, and
the bias current to be supplied to said second voltage-current converter is equal to or more than a bias current to be supplied to said second current switch circuit.
3. The quadrature mixer circuit as recited in claim 1 , wherein:
said attenuator includes a resistor.
4. A semiconductor integrated circuit for RF communication in which a Gilbert cell type quadrature mixer circuit is built, said quadrature mixer circuit comprising:
a first differential circuit which receives an RF received signal voltage or an IF received signal voltage converted from the RF received signal voltage and converts the signal voltage into first and second RF received signal currents with 180 degree phase difference or first and second IF received signal currents with 180 degree phase difference;
a sixth differential circuit which has structure identical to the structure of the first differential circuit, receives said RF received signal voltage or said IF received signal voltage, and converts the signal voltage into third and fourth RF received signal currents with 180 degree phase difference or third and fourth IF received signal currents with 180 degree phase difference;
a local signal oscillator;
a 90° phase shifter which outputs a local signal whose phase is 90 degrees ahead or behind the phase of a local signal from the local signal oscillator;
a second differential circuit which has a first current input terminal through which current is input, receives the local signal from said local signal oscillator, switches on/off the current input through said first current input terminal at timing of said local signal oscillator, and converts the input current into first and second I output signal currents with 180 degree phase difference;
a third differential circuit which has a second current input terminal through which current is input, receives the local signal from said local signal oscillator, switches on/off the current input through said second current input terminal at timing of 180 degree phase difference from said local signal oscillator, and converts the input current into third and fourth I output signal currents with 180 degree phase difference;
a fourth differential circuit which has a third current input terminal through which current is input, receives the local signal output from said 90° phase shifter, switches on/off the current input through said third current input terminal at timing of the local signal output from said 90° phase shifter, and converts the input current into first and second Q output signal currents with 180 degree phase difference;
a fifth differential circuit which has a fourth current input terminal through which current is input, receives the local signal output from said 90° phase shifter, switches on/off the current input through said fourth current input terminal at timing of 180 degree phase difference from the local signal output from said 90° phase shifter, and converts the input current into third and fourth Q output signal currents with 180 degree phase difference;
a first I signal current addition and coupling point at which said first I output signal current and said third I output signal current are added and coupled and a resultant fifth I signal current is output;
a second I signal current addition and coupling point at which said second I output signal current and said fourth I output signal current are added and coupled and a resultant sixth I signal current is output;
a first Q signal current addition and coupling point at which said first Q output signal current and said third Q output signal current are added and coupled and a resultant fifth Q signal current is output; and
a second Q signal current addition and coupling point at which said second Q output signal current and said fourth Q output signal current are added and coupled and a resultant sixth Q signal current is output;
wherein:
the first RF received signal current or first IF received signal current from said first differential circuit is routed to said first current input terminal;
the second RF received signal current or second IF received signal current from said first differential circuit is routed to said second current input terminal;
the third RF received signal current or third IF received signal current from said sixth differential circuit is routed to said third current input terminal;
the fourth RF received signal current or fourth IF received signal current from said sixth differential circuit is routed to said fourth current input terminal;
said first current input terminal is connected via a first voltage dropping element to said third current input terminal;
said second current input terminal is connected via a second voltage dropping element having equal impedance to the impedance of said first voltage dropping element to said fourth current input terminal; and
said Gilbert cell type quadrature mixer-circuit uses bipolar transistors.