A five-pole varactor-tuned filter with a 5% passband operating at 1.9 GHz is proposed. The tunability is realized by reverse-biased GaAs diodes loading the resonators, which are formed by a section of multicoupled microstrip lines. A measured center frequency shift of 200 MHz (10% tunability) was made at 22 V DC bias. The insertion loss varies in the range of 6 -4.5 dB, while matching is better than 12 dB. The output IP3 range is -1.6 dBm at the lowest bias, increasing to 24 dBm at the highest bias voltage, and the temperature stability is about 1.8% (from -50° to 125°C) at 1 V and 0.5% at 10 V.
Index Terms -GaAs Varactor, Microstrip, Tunable filters
I. INTRODUCTIONTunable filters, with applications in transmitter and receiver systems, have been realized in multiple ways, whereby the tuning tool may be selected from non-linear components, such as ferroelectrics [1], semiconductor based ones [2, 3] and MEMS (microelectromechanical systems) [3 -5]. A 60% tuned varactor-diode filter with low insertion loss (< 3 dB) has been proposed [2]. Some materials, components, or both, are more suitable in transmitter systems, with typical power levels around 5W [6], such as mechanically [3] and ferroelectrically tuned filters [7,8], i.e. bulk ferroelectrics. The main considerations for other types of materials, including thin-film ferroelectrics, are the higher frequency harmonics generation and intermodulation phenomena, although IP3 levels at 34 dBm for thin film ferroelectric based filters have been reported [9], and 40 dBm for MEMS [4]. Another issue is the temperature sensitivity; this is qualitatively very different for the abovementioned types of tuning components. The permittivity of ferroelectrics and perovskites [10] may have very strong and complex temperature dependency, whereas diodes (a semiconductor based component) comprise the freeze out/complete ionization state of donors, and have a constant charge carrier concentration, in the typical temperature range 170 -450K before the intrinsic carrier density dominates [11,12]. That is the reason for temperature insensitivity of capacitance of typical varactor diodes in this temperature range.In this work, a compact and small-size five-pole tuneable filter with a 5% pass-band is demonstrated including simulations, fabrication and measurements of S-parameters and non-linear response. The filter is measured to have average insertion losses around 5dB and the IP3 (third order intercept point) ranges from -1.6 dBm to 24 dBm increasing with bias voltage. A qualitative temperature sensitivity analysis is also presented. The temperature stability is
