RF architecture for cellular dual-band telephones

An RF dual band phone circuit operating in the DCS 1800 and GSM frequency bands comprising:

• a modulator for producing an output comprising a pair of baseband signals (I and Q) modulating a first intermediate frequency (IF) signal for the GSM band or a second intermediate frequency (IF) signal for the DCS 1800 band;
• an intermediate frequency (IF) filter having an input and an output, the input of the filter being connected to the output of the modulator;
• a transmit phase lock loop connected to the output of the IF filter for translating the filtered first IF signal to a GSM band radio frequency signal or the filtered second IF signal to a DCS 1800 band radio frequency signal, depending on which one of a first and second local oscillator signal is supplied to it the GSM band signal or the DCS 1800 band signal being down converted and fed back, to the IF input of the transmit phase lock loop;
• an amplifier for amplifying the output of the transmit phase lock loop;
• a down converter for receiving one of a GSM band signal and a DCS 1800 band signal and for supplying a down converted output signal, said output signal being selected by high injection for the GSM band and low side injection for the DCS 1800 band;
• and
• a local oscillator (LO) phase lock loop for supplying, on a single output, either the first LO frequency signal or the second LO frequency signal to the down converter and to the transmit phase lock loop for effecting a down conversion in the transmit phase lock loop of the GSM band or DCS 1800 band radio frequency signal, whereby a common phase lock loop is used for down conversion in both the transmit and receive paths of the circuit.

A method for generating a dual band RF output in an RE dual band phone operating in the DCS 1800 and GSM frequency band, the steps of the method comprising:

• modulating a first intermediate frequency (IF) signal for the GSM band on a second intermediate frequency (IF) signal for the DCS 1800 band with a pair of baseband signals (I and Q);
• generating a high side injection local oscillator signal for the GSM band and a low side injection local oscillator signal for the DCS 1800 band;
• translating the first IF signal to a GSM band radio frequency signal using the high side injection local oscillator signal;
• translating the second IF signal to a DCS 1800 band radio frequency signal using the low side injection local oscillator signal;
• and
• down converting the GSM band signal or the DCS 1800 signal and feeding it back to the IF signal translating steps as part of a phase lock loop.