Introduction

In a wireless receiver, the desired signal, which can be very weak, can be accompanied by strong out-of-band blockers. To amplify the weak desired signal and overcome the noise of the following stages, the incoming signal must pass through a low noise amplifier. Also, for a very weak desired signal, the gain of the LNA is maximized. If they are not attenuated before experiencing the LNA gain, the strong out-of-band blockers compress the LNA. In practice, external SAW filters are used in front of the LNA to attenuate the out-of-band blockers. Such external SAW filters are not only expensive and bulky, they increase the receiver noise figure due to their inevitable insertion loss. Additionally, such SAW filters require typically external matching components to convert the impedance seen from the RFIC to the 50Ω they need for proper operation.

In multiband receivers, because each radio requires its own off-chip SAW filter and matching components, the RF sections (low noise amplifiers and mixers) cannot easily be shared. Not only is there no hardware sharing for the RF blocks, the number of RF pads increases with the number of received bands. These factors complicate the design at all levels, including packaging and board design. To have a single RF input for the RFIC, these external SAW filters must be replaced with a single external SAW filter that is capable of having its center frequency and bandwidth tuned. Tunable SAW filters with high quality factors and low insertion losses are still the subject of research and have not yet been implemented.