Abstract
Passive polyphase filter and first order active polyphase filter building blocks have been commonly used in the image reject downconverter. However, to operate in a wideband system and to achieve sufficient image rejection, the filter requires a cascade of multiple building blocks, which leads to high signal loss, noise and power consumption. This paper describes a second order polyphase filter building block and its implementation in the image reject downconverter. The transfer function of the proposed filter is obtained by applying a frequency translation, converting a second order low pass filter to an asymmetrical bandpass filter. This bandpass characteristic eliminates the need for low pass filter when it is implemented in the image reject downconverter. Unlike the passive polyphase filter, the proposed circuit may provide gain to the signal. Furthermore, it requires fewer stages of building blocks to operate in the same bandwidth and the same image reject ratio compared to existing polyphase filter building block. This paper reports the performance of an image reject downconverter using a fifth order polyphase filter circuit built from the combination of a first order filter and two second order filters. The circuit, implemented in 0.18mum CMOS technology, is designed to downconvert a 60MHz bandwidth RF signal centred around 1.8GHz to IF signal centred at 70MHz while rejecting the mirror image signal appearing around 1.66GHz. The polyphase filter gives more than 60dB image rejection while consuming 8.272mA from a 2.5 power supply.