By Markus Helfenstein; George S Moschytz
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If this leakage is via radiation coupling into the antenna, then this may vary with the local environment. Finally, non-linearities in the mixer may cause signals to be generated at DC from other interferers. The latter mechanisms can be time varying, and any offset cancellation needs to be able to respond to time variation. It is only when special precautions are taken to cancel DC offsets that direct-conversion architectures can be used. More details of the direct-conversion issues are given in .
Since the frequency spectrum in modern wireless communication systems must be used as efficiently as possible, channels are placed together very closely. The signal level of the desired receive channel can be very small, whereas adjacent channels can have very large power levels. Therefore the phase noise specifications for the LO signal are very hard to meet. Meanwhile, mobile communication means low power consumption, low cost and low weight. This implies that a completely integrated synthesiser is desirable, where “integrated” means a standard CMOS technology without any external components or processing steps.
The absolute frequency is determined by the reference oscillator and the frequency offset/division block. These need to be carefully planned. Offsetting the frequency requires down-mixing, and it is important to filter unwanted products, as for the superheterodyne receiver. The frequency planning will however ensure that the VCOs should not be susceptible to pulling, and the main VCO is not susceptible as it is phase coherent with the PA. It is not clear, however, whether for a given frequency requirement, this architecture can produce a design with low-frequency external filter which is easily integrable.
Circuits and systems for wireless communications by Markus Helfenstein; George S Moschytz