Design amd implementation of a VHF/UHF front-end using tunable dual band filters

Alaca, Fatih
For the new generation wireless communication systems, there is an increasing demand for devices that covers more than one frequency band. This results in a need for wide-band tunable front-ends. The main objective of this study is to use dual band filters in the design of a multi-band front-end. A wide-band low noise amplifier is also required. To accomplish this project, a fixed frequency bandstop filter, a tunable dual-band filter and a wide-band LNA are designed and implemented successfully. The predefined specifications of this front-end include gain, gain flatness, spurious signal rejection, frequency tuning range, noise figure and linearity. Total power dissipation and number of elements are also taken into consideration. Test results of the manufactured front-end are compared with the results of existing single band front-ends. In order to design a good tunable wide-band filter, just tuning its center frequency will not be enough. The noise figure of this dual-band filter will be proportional to its insertion loss if it will be used as a pre-selection filter in front of a LNA. Hence its insertion loss will affect the overall noise figure of the system. If it will be used after the LNA, its linearity will be more important. When a bandpass filter is tuned over wide range of frequencies, its bandwidth varies significantly which leads to a degradation in rejection of the spurious signals. Therefore, there must be a simultaneous control of center frequency, bandwidth and insertion loss while providing enough linearity. In order to accomplish this mission, a filter that has two passbands is designed and implemented. The first passband is tunable between 136MHz and 174MHz while the second one is tunable between 380MHz and 470MHz. Also, the low noise amplifier works properly between 136MHz and 470MHz. As a result, a front-end that covers two bands is obtained.