A low power high spectral purity frequency translational loop for wireless applications

PLLs with a mixer in the loop can perform the up-conversion function in communication systems which use constant envelope modulation techniques. These loops, usually named Frequency Translational Loops (FTL), perform the up-conversion of the modulated signal from an intermediate frequency to the transmitter frequency. Frequency translational loops used in portable wireless communications applications, such as cellular telephony, are required to achieve low phase noise and spurious levels. This paper presents the design of a monolithic FTL which operates in the IF frequency range from 100 MHz to 450 MHz and the frequency range from 900 MHz to 1.9 GHz. The output phase noise level is -120 dBc/Hz at 400 kHz offset and 165 dBc/Hz at 20 MHz offset from a 900 MHz carrier and the spurious levels are lower than 60 dB below the carrier. These characteristics make the FTL suitable for use in cellular telephony applications such as GSM/DCS.