Patent ID: 7181168

Claim:
A loop-back self-test circuit for testing a transmitter and/or a receiver in a communications system, said transmitter generating a transmit signal at a transmit frequency and said receiver receiving a receive signal at a receive frequency where there is an offset between the transmit frequency and the receive frequency, said self-test circuit comprising: a mixer responsive to the transmit signal and a local oscillator (LO) signal, said mixer mixing the transmit signal and the LO signal to generate a test signal at the frequency of the receive signal; and a phase lock loop (PLL) circuit for generating the LO signal, said PLL circuit including a voltage controlled oscillator (VCO) for generating the LO signal and a phase comparator circuit responsive to the LO signal and a reference signal, said phase comparator circuit generating an error signal indicative of the phase difference between the LO signal and the reference signal, said PLL circuit further including a charge pump circuit responsive to the error signal, said charge pump circuit generating a charge pump signal, said PLL circuit further including a filter and tuning circuit responsive to the charge signal, said filter and tuning circuit generating a voltage signal, said VCO being responsive to the voltage signal to set the frequency of the LO signal, wherein the mixer, the VCO, the phase comparator circuit and the charge pump circuit are all formed on a common integrated circuit chip.