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A Low Loss Dielectric Resonator Filter for Cellular and PCS Applications

A dielectric resonator filter capable of achieving very low passband insertion loss and high near-band rejection is featured on this month's cover

April 1, 2000
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A Low Loss Dielectric Resonator Filter for Cellular and PCS Applications

Trilithic
Indianapolis, IN

Because of the growing need for increased capacity in cellular and PCS networks, low insertion loss and high out-of-band rejection are critical parameters for service providers to deliver cost-effective coverage. The use of complex digital modulation schemes to increase capacity also requires that a transmit filter exhibit a wide dynamic range; that is, it must be capable of handling moderate to high power levels while simultaneously maintaining high near-band rejection. Dielectric resonator filters are attractive for their low passband insertion loss. Through careful design and fabrication, a dielectric resonator filter has been designed that is capable of achieving very low passband insertion loss and high near-band rejection.

At present, three types of bandpass filters are typically employed in cellular, PCS and satellite communications base stations: combline cavity/waveguide, high temperature superconducting (HTS) and dielectric resonator. Combline cavity filters are the most widely used in cellular base stations because they are relatively inexpensive and easy to manufacture. At cellular frequencies, these filters are capable of achieving unloaded quality factors (Q) between 2000 and 9000. The high unloaded Qs translate into highly selective frequency responses with low passband insertion loss and high rejection. Combline cavity filters, with fractional bandwidths of 0.5 to five percent (typ), exhibit typical insertion losses of 1 dB or less at cellular frequencies with out-of-band rejection of better than 60 dB and spurious performance of -60 dBc or better. Depending on the filter size and housing, it is not uncommon for a combline cavity filter to handle input power levels of 500 W (avg). Although a waveguide filter can be used for those applications requiring extremely low passband insertion loss, combline cavity filters can be housed in smaller enclosures (as increasingly required for PCS and microcellular applications).

HTS filters have been used for several years at both cellular and PCS frequencies. Fabricated with superconducting films on special substrates rather than with conventional metal conductors on ceramic or polytetrafluoroethylene (PTFE) substrates, these filters are capable of unloaded Qs exceeding 50,000. Originally developed by IBM for possible use in supercomputers, superconductors initially required extremely cold temperatures (typically the 4.2 K temperature of liquid helium) to achieve proper operation. Below a certain critical temperature (Tc ) the resistance of a superconductor approaches zero. However, above that temperature, the resistance of the superconductor tends to be even higher than that of conventional room-temperature conductors.

In recent years, improvements in HTS materials have made it possible for high performance HTS bandpass filters to operate at temperatures equal to that of liquid nitrogen (77 K) or higher using solid-state cryocoolers to maintain the critical temperature. Because of the limitations of HTS films for handling high current densities, the power-handling capabilities of HTS filters are generally confined to receiver applications. HTS bandpass filters are capable of impressive selectivity performance since their dielectric losses can be reduced to negligible levels by the use of low loss, single-crystal substrates.1 Packaging losses and spurious coupling must be controlled; nonetheless, HTS bandpass filters have been realized with out-of-band rejection of better than 40 dB at cellular frequencies, spurious performance approaching -100 dBc and passband insertion loss of less than 0.5 dB. For receiver applications, HTS bandpass filters offer the best electrical performance of the three filter types with unloaded Qs approaching 50,000. These filters can be designed with fractional bandwidths ranging from 0.1 to two percent, although the power-handling capability is typically limited to a maximum of 25 W average power.

Dielectric resonator filters are constructed of resonators comprising cylindrical rods or tubes of dielectric material usually mounted on a dielectric substrate in the proximity of supporting microstrip circuitry. The size of the dielectric structure relative to the wavelength of the signal of interest determines the frequency coverage. As the number of resonators (or poles) in the filter increases, the skirts of the filter’s frequency response become steeper and the out-of-band rejection increases. However, the passband insertion loss also increases with the number of resonators, especially in narrowband designs.

In a dielectric resonator, the EM field for a resonant mode is largely confined to the ceramic resonator material. The field strength outside the resonator falls off rapidly (approximately exponentially) at distances much shorter than the free-space wavelength of the resonator. The primary loss mechanism in a dielectric resonator is the friction loss of the electronic dipoles at each lattice of the dielectric material and is characterized by the dielectric resonator’s loss tangent. Nevertheless, dielectric materials are currently available with unloaded Qs of several thousand for resonator frequencies up to and exceeding 20 GHz.

The new low loss dielectric resonator filter developed for cellular and PCS applications employs careful materials selection and circuit matching to achieve electrical performance approaching that of HTS filters, but without their power-handling limitations. With unloaded Qs between 5000 and 50,000, these dielectric resonator filters can be fabricated with fractional bandwidths between 0.1 and two percent at cellular and PCS frequencies.

Fig. 1 The dielectric filter's frequency response The new filters are designed for typical operating temperatures of -55° to +80°C and can handle average power levels exceeding 1 kW at room temperature. The measured insertion loss at 1900 MHz is less than 0.8 dB, as shown in Figure 1 , with rejection of better than 60 dB at 40 MHz from the carrier and spurious levels exceeding -100 dBc. A filter fabricated with 1.5 kW average power-handling capability at PCS frequencies exhibits a 3-to-60-dB shape factor of 1.4:1 and a fractional bandwidth of 0.2 percent, making it ideal for both receiver and transmitter applications. Table 1 lists the dielectric resonator filter specifications.

Table 1
Dielectric Resonator Filter Specifications

Passband frequency (MHz)

2300

Bandwidth (%)

0.2

Passband insertion loss (dB)

0.8

Stopband rejection (dB)

60

Spurious levels (dBc)

-100

Power-handling capability (avg) (kW)

1.5

Operating temperature range (°C)

-55 to +80

Size (")

13 x 7 x 3

Weight (lb)

<10

These dielectric resonator filters offer the performance of HTS filters with enhanced power-handling capability at a fraction of the cost. They are comparable in size to combline cavity filters and considerably smaller than HTS filters with the low insertion loss and high near-band rejection required by modern communications systems based on complex digital modulation schemes such as wideband code division multiple access. Additional information is available at the company’s Web site at www.trilithic.com or via e-mail at sales@trilithic.com.

References
1. Theodore Van Duzer and Charles W. Turner, Principles of Superconductive Devices and Circuits , Second Edition, Prentice-Hall, Upper Saddle River, NJ, 1999, pp. 150-151.
2. Charles A. Harper, Passive Electronic Component Handbook , Second Edition, McGraw-Hill, New York, 1997, pp. 556-561.

Trilithic, Indianapolis, IN (800) 344-2412 or (317) 895-3600.

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