Varactor-Tuned Dual-Band Quadrature Hybrid Coupler

A tunable dual-band quadrature hybrid coupler for mobile GSM bands (900 and 1800MHz) is proposed and characterized. The structure is composed of four quarter-wavelength sections at a frequency located between the two operating frequencies and two pairs of open-ended stubs. Frequency tuning is achieved by modifying the electrical length of the open-ended stubs with varactor-diode capacitors. A dual-band tunability of 33% and 10% is obtained for the first and the second bands, respectively     

Multilayer Dual-Mode Dual-Bandpass Filter

A dual-mode dual-bandpass filter for the U-NII bands is proposed and demonstrated. Its effective size reduction is achieved by using a multilayer configuration. The first and the third layers incorporate microstrip dual-mode bandpass filters with operating center-frequencies of f₁ =5.2 GHz and f₂ =5.8 GHz, respectively. The second layer is used as a common ground plane for both filters, which also serves as a decoupling interface. Capacitive coupling transition is used to connect both filters to I/O coplanar waveguide (CPW) ports. Single and dual-band passband filter prototypes are designed, fabricated, and measured in this work, thus validating the design principle. Designed topologies of single passband filters with center frequencies of 5.2 and 5.8 GHz exhibit an out-of-band rejection better than 40 dB with a 3 dB bandwidth of 5.8% and 6%, respectively. The proposed multilayer dual-passband response with center frequencies of 5.2 and 5.8 GHz provide band-to-band isolation better than 30 dB. Measured insertion losses are lower than 2.76 dB, with 3 dB bandwidth lower than 5%.     

Varactor-tuned quarter-wavelength dual-bandpass filter

A tunable dual-band bandpass microstrip filter is proposed and presented. It is composed of two pairs of quarter-wavelength transmission line resonators and two bandstop structures made of open stubs. Each operation bandpass frequency, denoted by f/sub 1/and f/sub 2/, is generated by a pair of coupled resonators. The first coupled resonator is fed via a tap-coupled structure, while the second is fed through a coupling structure. The tunability of each bandpass and harmonic suppression is achieved by modifying the electrical length of the resonators and the electrical length of the open stubs by using varactor-diode capacitors. Finally, a 2.5 and 4.25 GHz dual band bandpass filter is designed, fabricated and measured as one example and a tunability of 22.2% and 19.6%, respectively, is obtained for the first and second bands. These excellent experimental results validate the proposed structure and design technique for dual-band bandpass filters.     

Frequency-Agile Dual-Band Direct Conversion Receiver for Cognitive Radio Systems

A frequency-agile dual-band direct conversion receiver is proposed and experimentally validated for cognitive radio system applications. Two types of tunable receiver architectures are presented and analyzed in this work; one structure relying on a tunable bandpass filter, and a second system based on a varactor—based tunable six-port demodulator, with the latter showing better sensitivity and dynamic range. In order to demodulate phase-shift-keying modulated signals at multiple operating frequencies at a high bit rate of 40 Mb/s, the receiver is designed using a wideband power detector in connection with a high-speed quad comparator. An experimental prototype and a complete measurement test bench have been realized in order to obtain the bit error rate performance of the receiver in a more realistic application environment. In summary, the proposed electronically tunable system allows for an operation in multiband frequency ranges including global system for mobile communications cellular networks (1900 MHz) and IEEE802.11 a/b/g applications (2.4 and 5.8 GHz).