transmitter in mobile communication
In mobile communication applications (cordless, wireless) today, research aims at a one-chip solution with a single technology.
Alternative architectures and introduction of new techniques such as delta sigma modulation are investigated in the transceiver both at circuit and system level to reduce price and power consumption. The transceiver or RF-front end is composed of the receiver and the transmitter.
Several receiver architectures using delta-sigma modulation were described in di erent papers , : The tendency is to get the digital stage close to the antenna either with a superheterodyne (two down-conversion stages) structure or a low or zero-IF structure. The separation between two components, I and Q of the complex signal is done in the digital domain, after a baseband or low frequency analog to digital conversion. Such conversion is performed with a bandpass delta-sigma analog to digital converter. Concerning the transmitter part, little work was published on architecture using delta-sigma modulation. Usually transmitter architectures are only found in a few forms. The main challenge in the transmitter is the design of the output power ampli er, as requirements on band selectivity and image rejection are not as difficult to handle as in the case of the receiver . The transmitter controls the power contained in the baseband signal to reduce some of the noise issues, but severe tradeo s remain among efficiency, linearity and supply voltage to design a good power ampli er. The oscillator should also be sufficiently isolated from the power amplifier to limit the disturbance caused by the output power-amplier.
As the isolation problem can be solved, with for example a circulator placed between the oscillator stage and the output power-ampli er, we chose to concentrate on the modulator / frequency synthesizer block and issues as delta-sigma modulation in the transmitter design.
In existing designs, frequency up-conversion in the transmitter is either performed directly or with a single mixer stage to produce a single-sideband signal . As we will present later, both structures su er from disadvantage when one desires either to change technology or to meet several standards within a single implementation.