Abstract: A loop-back self-test circuit (10) that has particular application for a cellular telephone base station, where the components of the self-test circuit (10) are integrated on a common integrated circuit chip (20). The transmit signal and an LO signal are applied to a mixer (28) to convert the frequency of the transmit signal to the frequency of a receive signal. A VCO (56) generates the LO signal and a PLL (26) synchronizes the phase of the VCO signal to the phase of a reference signal. A divided reference signal and a divided VCO signal are applied to a phase comparator (52) that generates an error signal indicative of the phase difference between the reference signal and the VCO signal. The error signal is applied to a charge pump (60) that generates a current signal to tune a tank circuit (66). A voltage signal from a loop filter (62) is applied to the VCO (56) to tune it to the LO frequency.

Abstract: A loop-back self-test circuit (10) that has particular application for a cellular telephone base station, where the components of the self-test circuit (10) are integrated on a common integrated circuit chip (20). The transmit signal and an LO signal are applied to a mixer (28) to convert the frequency of the transmit signal to the frequency of a receive signal. A VCO (56) generates the LO signal and a PLL (26) synchronizes the phase of the VCO signal to the phase of a reference signal. A divided reference signal and a divided VCO signal are applied to a phase comparator (52) that generates an error signal indicative of the phase difference between the reference signal and the VCO signal. The error signal is applied to a charge pump (60) that generates a current signal to tune a tank circuit (66). A voltage signal from a loop filter (62) is applied to the VCO (56) to tune it to the LO frequency.