Two miniaturized tunable filters with two zeros to result in sharp skirts were developed at 1.4-2.0 GHz on epsivr=6.15 substrate. The filters were built using single and back-to-back varactor diodes and compared for linearity characteristics. The single-diode filter has a 1-dB bandwidth of 5plusmn0.5% and an insertion loss of 2.5-1.8 dB. The back-to-back diode filter has a 1-dB bandwidth of 4.9 plusmn0.5% and an insertion loss of 2.9-1.25 dB (resonator Q of 56-125). The back-to-back diode filter was designed for improved linearity characteristics using two uniformly doped varactor diode and biased with a high impedance circuit. A detailed Volterra series analysis is done on the back-to-back diode including the effect of the bias network and diode mismatches. The measured IIP3 for the back-to-back diode tunable filter is 22-41 dBm depending on the bias voltage and is 13-15 dB better than the single-diode design. The power handling capabilities of both designs is explored using large-signal S 21 measurements. To our knowledge, these planar tunable filters represent state-of-the-art insertion loss and linearity characteristics performance with varactor diodes as the tuning elements.

A Two-Pole Two-Zero Tunable Filter With Improved Linearity

A new methodology for the synthesis of tunable patch filters is presented. The methodology helps the designer to perform a theoretical analysis of the filter through a coupling matrix that includes the effect of the tuning elements used to tune the filter. This general methodology accounts for any tuning parameter desired and was applied to the design of a tunable dual-mode patch filter with independent control of center frequency and bandwidth (BW). The bandpass filter uses a single triangular resonator with two etched slots that split the fundamental degenerate modes and form the filter passband. Varactor diodes assembled across the slots are used to vary the frequency of each degenerate fundamental mode independently, which is feasible due to the nature of the coupling scheme of the filter. The varactor diode model used in simulations, their assembling, the dc bias configuration, and measured results are presented. The theory results are compared to the simulations and to measurements showing a very good agreement and validating the proposed methodology. The fabricated filter presents an elliptic response with 20% of center frequency tuning range around 3.2 GHz and a fractional BW variation from 4% to 12% with low insertion loss and high power handling with a 1-dB compression point higher than .

Synthesis Methodology Applied to a Tunable Patch Filter With Independent Frequency and Bandwidth Control

With advances in telecommunications, an increasing number of services rely on high data rate spectrum access. These critical services include banking, telemedicine, and exchange of technical information. As a result, spectrum resources are in ever-greater demand and the radio spectrum has become overly crowded. For efficient usage of spectrum, smart or cognitive radios are sought after. However, current wireless phones can only select a few specific bands. In this paper, we discuss the advantages of reconfigurable radios in not only increasing the efficiency of spectrum usage but also in potentially reducing the cost of wireless handsets and the barriers for new wireless service providers to enter the market. We review available technologies that make the implementation of reconfigurable radios possible and discuss technical challenges that need to be overcome before multistandard reconfigurable radios are put into practice. We also evaluate the ability of reconfigurable radios in reducing entry costs for new competitors in wireless service.

Reconfigurable Radios: A Possible Solution to Reduce Entry Costs in Wireless Phones

This paper reports on tunable 2- and 3-pole bandpass filters with a wide frequency tuning range (tuning ratio >3) and a constant bandwidth using switchable varactor-tuned resonators. The wide center frequency tuning range is obtained using p-i-n diodes to switch in and out quarter-wavelength ( $\lambda $ /4) or half-wavelength ( $\lambda $ /2) resonators for low-band or high-band modes, without increasing the tuning capacitance range of the varactors. A combination of electric and magnetic coupling is utilized to realize a near constant absolute bandwidth across the tuning range. A switchable feed line with a fixed matching capacitance is used to realize the external coupling. Two filters are designed and fabricated on a Duroid substrate with $\varepsilon _{r} = 2.2$ and $h =0.787$ mm. For the 2-pole filter, the center frequency is tuned from 550 to 1900 MHz while maintaining a 3-dB bandwidth of 92 ± 6 MHz, insertion loss of 3.2~4.4 dB, and return loss of better than 15 dB. For the 3-pole filter, the center frequency is tuned from 540 to 1800 MHz while maintaining a 3-dB bandwidth of 89 ± 4 MHz, insertion loss of 4~5.4 dB, and return loss of better than 12 dB. For both filter types, the third-order intercept point and 1-dB compression point ( $P_{\mathrm {1~dB}})$ are 11 and 7 dBm, respectively. The rejection level at 200-MHz offset frequency from the passband center frequency is better than 25 and 41 dB for 2- and 3-pole filters, respectively, across the entire tuning range. To the best of our knowledge, this planar bandpass filter exhibits the widest tuning range with a near-constant bandwidth.

Continuously Tunable 0.55–1.9-GHz Bandpass Filter With a Constant Bandwidth Using Switchable Varactor-Tuned Resonators

There has been recent progress on CMOS non-magnetic circulators based on switch-based spatio-temporal conductivity modulation, but these initial demonstrations remain limited in transmitter power handling, linearity, and ability to combat antenna variations. This paper describes a non-magnetic circulator in 180nm SOI CMOS that uses no external components, and employs various linearity enhancement techniques such as device stacking, optimal switch biasing, and localized ESD design to achieve >1W TX-ANT PldB and >+50dBm TX-ANT IIP3 at 1GHz. A new loss-free and inductor-free antenna balancing approach enables high isolation for as high as 1.85 ANT VSWR and beyond. The circulator also exhibits low insertion losses of 2.1dB/2.9dB in the TX-ANT and ANT-RX paths, and ANT-RX NF of 3.1dB. These results represent a 10–100×enhancement in linearity/power handling over prior CMOS non-reciprocal circulators, and are shown to lower the power consumption of a communication link when compared with state-of-the-art electrical balance duplexers in scenarios where dynamic range is limited by P1dB, NF.

Fully-Integrated Non-Magnetic 180nm SOI Circulator with > 1W P1dB, >+50dBm IIP3 and High Isolation Across 1.85 VSWR

This paper introduces hybrid structures with unit cells that are a combination of lattice and ladder topologies. These unit cells dramatically increase the quality factor, miniaturization, and maximum frequency of synthetic transmission lines. Also, it is shown that lattice topologies in general are less vulnerable to the effect of parasitics, such as self resonance of loading capacitors or mutual inductance between different cells. The underlying theory is studied, and simple formulas are given for predicting the behavior of these lines. A few examples are presented that, based on full-wave simulations, show the large gain in miniaturization, quality factor, or bandwidth compared to the simple ladder or lattice structures. Measured results for a fabricated synthetic line are presented that show a miniaturization factor of n=?{?eff?eff}=16.5 for Z 0=50 ? and a very small dispersive behavior from dc to 2.5 GHz.

Miniaturized Transmission Lines Based on Hybrid Lattice-Ladder Topology

A basic wideband tunable filter design based on combline topology is presented. At the presence of the parasitic effects, the structure of the filter must be modified by introducing additional degrees of freedom to the geometry of the coupled line segment. A design procedure involving iterative steps will be described. This procedure is used to design two bandpass filters with more than one octave tuning range in the UHF band. The experimental results are presented for the filter prototypes implemented using printed circuit boards and PIN diodes.

Tunable Filters With Nonuniform Microstrip Coupled Lines

A switchable filter topology is proposed in the form of a self-scalable comb-line structure. This topology is capable of implementing a tunable frequency response with consistent performance over very large tuning ranges. A two-pole prototype has been fabricated using 10 discrete PIN diodes and is capable of covering the frequency range of 290 to 600 MHz in four steps (2-bit tuning). The in-band loss of the filter is better than 2 dB for all modes when a DC voltage of 5 V is used for biasing the PIN diodes. The filter has a IIP3 of greater than 32 DBR, and its RF structure occupies an area of 9 times 21 mm2.

Switchable Bandpass Filter for 0.3-0.6 GHz

A switchable bandpass filter includes a coupled line segment (comb) including a plurality of coupled transmission lines of substantially equal lengths that are each connected or otherwise coupled to the common RF ground at their first end, a plurality of adjustable capacitors each coupled proximate a second end of respective ones of the transmission lines, and a plurality of shunt switches coupled to points along a length of each of the transmission lines. Shunt switches may be implemented by various device technologies including MEMS and FET switches and PIN diodes. The adjustable capacitors may be implemented as an array or tree of switched capacitors using suitable switching components (e.g., as previously enumerated) or by other suitable electrically controllable devices such as varactors or varactor arrays. A differential switchable filter may be formed by the symmetric repetition of individual bandpass filter modules.

Compact switchable filter for software-defined radio

A multi-scale filter topology is proposed in the form of a switched comb-line structure loaded by varactors. This topology is capable of implementing a tunable frequency response with consistent performance over very large tuning ranges. Two prototype filters of orders two and four have been fabricated in a coplanar geometry. The filters are tunable in a 6.5:1 frequency range.

Masoud Koochakzadeh

Papers

Miniaturized Transmission Lines Based on Hybrid Lattice-Ladder Topology

Tunable Filters With Nonuniform Microstrip Coupled Lines

Switchable Bandpass Filter for 0.3-0.6 GHz

Compact switchable filter for software-defined radio

Multi-scale tunable filter covering a frequency range of 6.5:1