Frequency Agile Diplexer

A miniaturized ultra-wideband (UWB) frequency agile diplexer filter (bandpass/lowpass) is based on conventional microstrip line, lumped components and a composite dual right- and left-handed resonator in a distributed-tapped D-CRLH microstrip structure. In the right-handed mode it is weakly coupled, while in the left-handed mode it is tightly coupled, achieving quasi 0 dB coupling. Its insertion loss is less than 5 dB, and port-to-port isolation is greater than 20 dB.

A reconfigurable software defined cognitive radio (SDCR) system represents a potential solution for interference, cost, size and power consumption problems encountered in wireless communications systems. A reconfigurable radio system must be frequency agile in order to dynamically select frequency bands or channels, and one of its enabling components is the radio frequency (RF) transceiver diplexer.

The diplexer is generally composed of two bandpass filters tuned to different frequency bands that appear electrically open with respect to each other over their passbands. Frequency agile RF front-end components that have been extensively studied for this application include RF filters,^1,2 couplers³ and demodulators.⁴ It is challenging, however, to design a diplexer with a tuning range wide enough to cover the operating frequencies needed for practical applications. This article describes a varactor-tuned diplexer based on two frequency agile dual-mode bandpass filters for IEEE 801.16 standard system applications over 2.3 to 2.69 GHz and 3.3 to 3.8 GHz.⁴

BACKGROUND

The performance of tunable diplexers and filters may be specified using the same criteria as for fixed-frequency filters with additional requirements for tuning range, tuning speed and tuning linearity. Not all may be satisfied simultaneously, so priorities should be established.

Currently, these requirements are best met with mechanical tuning. As an example, Rohde & Schwarz FU221 and FD221 filters are each made up of two coaxial resonators, fixed coupled to form a compact two-section filter plug- in. Following a frequency change input from the radio, the gear is driven by a microprocessor-controlled stepping motor.⁵ A robust and mechanically stiff layout using temperature-stable invar (iron-nickel alloy) for filter bodies, spindles and coupling loops, in combination with silver plating, guarantees its specifications over a wide temperature range and under high power, 100 percent duty-cycle operation.

If system requirements call for multioctave tuning, a yittrium iron garnet (YIG) filter is a likely choice.⁶ It has a relatively spurious-free response and low insertion loss due to the high quality factor (Q) of the resonator; however, its structure is not planar. Also, magnetic hysteresis limits its tuning speed.^6,7 As with any mechanical high Q structure, microphonic noise can also be a problem.⁸

Newer component technologies such as micro-electromechanical systems (MEMS) and barium strontium titanate (BST) varactors have been applied to tunable filters. MEMS varactors are small, have a wide tuning range and provide fast tuning speed with low insertion loss; but they also have a poor capacitance ratio resulting in low frequency resolution.⁹ BST varactors are small and can be easily implemented in integrated circuits. They may be suitable for high power applications, but have a relatively low Q.¹⁰

There is no tunable filter that will satisfy all requirements simultaneously. Most research activities have focused on the methodology for changing the center frequency of the filter, concentrating less on its frequency response.^2,6

CRLH TRANSMISSION LINES

Metamaterial transmission lines, also known as composite right/left–handed (CRLH) transmission lines or negative refractive index (NRI) transmission lines, have some unique characteristics such as backward wave propagation, simultaneous negative permittivity and permeability, zeroth order resonance and tight coupling.¹¹

A widely used microstrip metamaterial transmission line consists of a series of interdigital capacitors and shunted transmission line inductors. These are practical at high frequencies, but because the structures become large, lumped components are used at lower frequencies.¹² At the center, or transit, frequency there is no phase shift. It is right-handed in the region above the transit frequency and left-handed in the region below it.

A counterpart of the CRLH transmission line, the dual CRLH (D-CRLH) transmission line,¹¹ has a stop band between the right-handed and left-handed regions. It is right-handed below the stop band and is left-handed above it, i.e., the inverse of a CRLH transmission line.

D-CRLH Transmission Line

CRLH and D-CRLH transmission line equivalent circuits are shown in Figure 1. The D-CRLH transmission line model has parallel LC resonators in series branches and series LC resonators in parallel branches, while the CRLH transmission line model has series LC resonators in series branches and parallel LC resonators in parallel branches.

For the D-CRLH transmission line, LC circuits are¹

The resonant frequencies of the series and shunt resonators are

The resonant frequencies of the left/right handed circuits are

To minimize the band gap between the left/right handed regions, the following condition should be satisfied

which is identical to the balanced case of the CRLH transmission line. From image impedance analysis, the characteristic impedance ZC of the D-CRLH transmission line in the balanced case is

The characteristic impedances of a D-CRLH transmission line in the balanced and unbalanced cases are shown in Figure 2.

The stopband of the D-CRLH transmission line is determined with the condition that ZC is real. In the balanced case, the cut-off frequencies are then obtained from Equation 5 as

The band gap between right-handed and left-handed regions is desirable  with various left/right handed frequencies.¹¹

A microstrip realization of a D-CRLH transmission line is shown in Figure 3. The parallel LC resonator is composed of a pad, chip capacitor (CL) and a lumped inductor (LR). The series LC resonator is composed of a pad, chip capacitor (CR) and a lumped inductor (LL), chip or otherwise. LC components are adjusted at both ends to optimize impedance match and bandwidth.^11,14

A single varactor diode and an antiparallel pair have been considered. BST varactors are suitable for use in a single diode configuration since they do not have a rectifying effect,10 but they are difficult to procure and are lossy. A similar characteristic may be achieved with an anti-parallel pair of semiconductor varactors, but this is also lossy. Despite possibly larger nonlinearities, one diode per tank is chosen.

DIPLEXER SIMULATION

A distributed coupled D-CRLH circuit is used for the purpose of forming a frequency agile passband for the UWB bandpass/lowpass (BPF/LPF) diplexer. This is realized with a new D-CRLH coupled line diplexer structure (see Figure 4). A balanced D-CRLH is adopted to achieve a multiple broadband characteristic with no discontinuity between cutoff frequencies of the bandpass and lowpass filters.

The design is carried out with Z₀ = 45 Ω, ωcL = 1.2 GHz and ωcR = 2.7 GHz. The parameters in Equation 1 are calculated with CR=1.7 pF to 17 pF, LR = 1.91 nH, CL=1.7 pF to 17 pF and LL = 0.817 nH. A frequency agile passband of the UWB BPF/LPF diplexer is assumed and the corresponding geometrical values are found through iterative electromagnetic (EM) simulations using Ansoft Designer EM. The simulated performance of the circuit over the frequency band is shown in Figure 5. To verify performance over the entire frequency band, both impedance and the dispersion are shown. Multiple spikes are due to metamaterial structure of the filters consisting of multiple variable lossy resonators.

Both filters can be independently set (see Figure 6). The BPF has a tunable center frequency with a constant bandwidth. The LPF has a tunable 3 dB cutoff frequency and is capable of passing DC, if necessary. The LPF is intended to be the transmit path, as the current consumption is much higher than in the receive path. The “T” is not a “lumped T” nor is it tunable, but is distributed by the coupler.

Due to large impedance mismatches over the passband and discontinuities between the circuit and EM structures, a matching circuit is added to the frequency agile D-CRLH diplexer increasing its total size to 0.12 × 0.1 λg. Stop band rejection may be improved by adding stopband filters,⁵ but in tunable diplexer these must be tunable as well.

The prototype D-CRLH transmission line contains eight unit elements. This provides sufficient out-of-band attenuation for most purposes. The directional coupler is distributed among them. Increasing the number of segments for higher out-of-band attenuation increases the size, which may adversely affect coupler operation.

The coupling coefficients differ. When the frequency is below the band gap, it works in the right-handed mode and weak coupling is obtained, as shown in Figure 7. When the frequency is above the band gap, it works in the left-handed region and strong coupling is achieved. The coupling difference between left-handed and right-handed modes can be higher than 20 dB. If coupling is nearly 0 dB in the left-handed mode, it may reach -20 dB in the right-handed mode. Thus, microwave energy at different frequencies is directed to different ports depending on the mode.

DIPLEXER PROTOTYPE

The prototype layout is shown in Figure 8 and a photograph of the circuit is shown in Figure 9. The substrate is Rogers RT/duroid 6010 with thickness of 0.635 mm and relative permittivity of 10.2. Input and transmitted ports on the tunable D-CRLH coupler are physically separated and the isolated ports are terminated in 50 Ohms in order to preserve port-to-port isolation.

Three high voltage DC Burr–Brown OPA548 amplifier varactor drivers are mounted at the center of the PCB. The output voltages are limited with Zener diodes to the maximum ratings of NXP’s BB135 silicon varactors (-5V to +35V). Their 10:1 capacity ratio supports UWB operation, allowing 20 percent (LPF) and 22 percent (BPF) center-frequency tunability in transmit and receive filters bands, respectively.

Hand wound air core and commercially available ferrite core (EPCOS B82422A3100K10013) versions were evaluated. Since there are no essential differences between their performance characteristics, and both support UWB operation, hand wound cores are used. This increases the required height, which in future versions will be reduced with a multilayer PCB.

EXPERIMENTAL RESULTS

The D-CRLH has a fractional bandwidth greater than 100 percent with an insertion loss of less than 5 dB over the entire tuning range (see Figure 10). High isolation is achieved over a wide frequency band from DC to 1.1 GHz.

CONCLUSION

A frequency agile UWB BPF/LPF diplexer is designed to have an improved stopband by cascading it with a compact tunable matching element. It has potential applications in various microwave and UHF systems. Its size can be further reduced using multilayer technology; in this case, unwanted coupling must be tightly controlled for simultaneous receive/transmit systems.

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