Design and development of a novel architecture for multistage rf down-conversion with improved image rejection and non-linearity corrections for 1–10 ghz range

The modern day wireless device requires sub systems for frequency translation. The frequency translation devices of today suffer from the primary drawback in that the non-removal of the image frequency can potentially render the actual IF signal to be undetectable. The second drawback is an enhanced mixer noise due to the usage of active devices can impair the performance of the overall RF receiver. This work takes a critical look at future evolution of mixers with inherent image rejection for wideband wireless communication, limitations of present technology of mixers in terms of noise figure and image rejection, and an examination of current mixer technology.

The present work addresses system level issues including design aspects and flexibility of mixers, design optimization and simulation of a two stage image reject mixer targeted at a bandwidth between 1 GHz and 10 GHz. Novel mixer architecture has been designed and simulated to work up to 10GHz. Due to the demanding requirements on RF receivers, mixers need to meet stringent criteria with regard to noise figure, conversion gain, port to port isolation, image rejection and IIP3. The problem of image fold over has been addressed by the design of a dual stage Hartley image reject mixer with an image rejection ratio of 80 dB. Balancing is achieved with the usage of a diode double balanced mixer in the first stage and a double balanced Gilbert Cell mixer in the second stage. All simulations are performed on the ADS tools.

In the proposed mixer architecture the first stage of the designed down-converter consisting of the diode double balanced mixer down converts the incoming RF at 1 GHz-10 GHz to an IF of 250 MHz. The second stage consists of double balanced Gilbert Cell mixer downconverts the incoming 250 MHz signal to an IF of 50 MHz. The developed stages are cascaded to form the two stage downconversion system and further cascaded with a downstream SAW filter/Composite filter for channel selection. The combined system achieves overall NF of 7.5 dB, conversion gain of 22–26 dB and an image rejection of 80 dB for a target frequency of 1 GHz-10 GHz. Finally a novel architecture is proposed for mixer non linearity correction.