The Technical Requirements For Duplexers Are More Stringent Now Than Ever.

Chris W. Rossiter, Sr. Applications Engineer, Trilithic, Inc.

A new series of dielectric resonator/combline duplexers from Trilithic, exceed the minimum performance criteria required to serve the North American PCS E-block frequencies they serve, while remaining competitive in price. The combination of a dielectric resonator filter for the transmit line and coaxial combline filter with a classic Chebyshev response for the receive line provides unique benefits that neither type could alone achieve.

The E-88 Series of duplexers have been a fundamental component for communications systems for nearly as long as they have existed. However, their use has become more and more widespread in direct proportion to the need for spectral purity dictated by increasing spectrum congestion. The lowest possible insertion loss and highest out-of-band rejection are essential ingredients for service in these networks. The technical requirements for duplexers are more stringent now than ever, especially in the PCS bands, where the digital modulation schemes employed demand filters with high near-band rejection.

In the new filters, a narrowband dielectric resonator filter is used on the transmit side because it provides both very low passband insertion loss and very high near-band rejection. A combline filter is used because the receive filter specification is very wideband.

A Competitive Environment
The duplexers compete with other bandpass filters designed in the three most viable technologies:
•Combline filters
•High-temperature superconducting (HTS) filters
•Dielectric resonator filters
All three types have positive and negative attributes that define their performance. Combline cavity filters are the most popular because they are comparatively inexpensive to manufacture and can develop unloaded quality factors (Qu) of 2000 to 9000. This produces highly selective resonances with low passband insertion loss and high rejection. Typical combline filter performance at PCS frequencies includes fractional bandwidths of typically 0.5 to 5.0 %, insertion loss of 1.5 dB or less, and out-of-band rejection of at least 60 dB, and the best spurious suppression of the three filter types. They can usually handle input power up to 1 kW depending upon customer requirements.

HTS filters have generated considerable publicity because of their "gee-whiz" technology and their potential to deliver Qu's well in excess of that deliverable by any other technology. However, Trilithic has matched most of these performance benchmarks, including Qu of greater than 50,000, without the use of superconductivity (see sidebar).

The E-88 duplexers combine the desirable characteristics of both dielectric resonator filters (for transmit) and combline cavity filters (for receive) to deliver performance that is equal to or better than the specifications it was designed to meet (Figure 1). The filters have an unloaded Q up to 25,000, and fractional bandwidth between 0.1 and 2.0 %. The duplexers operate over temperatures of – 55 to 80 ° C with average power levels exceeding 1 kW. The measured receive-side insertion loss in the receive passband of 1885 to 1890 MHz is less than 0.67 dB, with rejection of at least 102 dB. Transmit-side insertion loss is 0.81 dB, and rejection from 100 to 1892 MHz is 96 dB.

These duplexers are competitive in size and weight with designs using other techniques, and are considerably smaller than HTS filters, while delivering low insertion loss and high near-band rejection required for E-block PCS service.