Cavity filters and dielectric resonator (DR) filters are widely used in communication systems. However, the need for more efficient filters is even greater with the advent of new 4G networks overlaying the existing 3G infrastructure, particularly in areas of limited spectrum and base station resources. Requirements for high attenuation with low insertion loss and small size are key issues in filter technology for communication systems. Many other companies have endeavored to develop dual mode and triple mode filters to fulfill these requirements.

An alternative solution has been proposed by utilizing superconductor technology. But the disadvantages of size, cost, weight and maintenance, not to mention the power required to cool it make superconductor filters impractical. KMW has recently introduced the “Black Hole Filter (BHF),” using the first commercialized “Triple-Mode” technology, which is capable of providing high stop band attenuation with low pass band insertion loss in a relatively small package.

Black Hole Filter’s Triple Mode Resonance
“Triple-Mode” technology enables three distinctive resonances with only one resonant cavity, utilizing a dielectric resonator with high Q value inside the single pocket. Compared to conventional DR filters, the triple mode “Black Hole Filter” offers very low insertion loss, extremely high attenuation, with a significant size reduction. Inside the pocket, waves can travel without bending and distortion, achieving the highest possible Q and steepest band-edge skirts ever in a single pocket filter.

Figure 1 Single mode vs. triple mode filter.

Figure 2 Single mode filter with eight resonators and triple mode filter with two resonators.

The conventional single mode filter shown in Figure 1 provides only a single resonance mode in the single pocket. The triple mode filter generates three distinctive resonances using the TE01δ mode as the same as the single mode filter. It also enables two additional TE01δ modes allowing the single cavity to function as three cavities, which improves the performance and lowers the manufacturing cost. Figure 2 compares the size of the single mode and triple mode filter. The single mode filter with eight resonators can be replaced by the triple mode filter comprised of only two resonators while achieving the same performance, occupying less than a third of the size of the conventional cavity filter.

With the deployment of micro base stations including Remote Radio Head (RRH) systems, and the overlay of multiple frequency bands for multiple standards on a single cellular site, the space available for the RF hardware is decreasing. Since RF filters typically occupy a significant fraction of the base station volume, base station manufacturers are looking for filter technologies that offer size and cost reduction, while still meeting the stringent base station RF specifications of insertion loss, rejection and power handling. The multi-mode filter is now an attractive alternative featuring improvements in all these categories.

The differences of the KMW triple mode filter over previous others include:

Previous Triple Mode Filters

  • Use DR structure for coupling (limit of structure)
  • Difficult to achieve TE01δ mode in applications (limit of function)
  • Tuning screw required for tuning (limit of production)
  • Difficult to achieve notch filter response (limit of application)

KMW’s Triple Mode Filter

  • Uses coupling network for coupling (no structural restriction)
  • Simple dielectric structure (low cost)
  • No tuning screw required for tuning (easy production)
  • Notches can be implemented (free application)
  • Simple inside structure (simple structure)

Figure 3 Single pocket performance of the Black Hole Filter in the 800 MHz band.

Figure 3 shows the single pocket performance achieved by the triple mode BHF in the 800 MHz band for Receive Channel blocking to prevent high power signals from adjacent channel. 20 dB rejection at 1.5 MHz offset from the band edge can be provided by the single pocket BHF. The cascaded BHF pockets can provide more rejection should the application require it.

KMW’s BHF technology enables a size reduction of about 1/3, and much higher Q value compared to other conventional filters (more than 9 times the Q of cavity filters and 1.6 times that of superconductor filters). Comparative Q values featuring these three filter technologies are shown in Table 1, based on an estimate of a reference 90 × 90 × 90 mm cavity in the 860 MHz band. These advantages are demonstrated by the triple mode resonance and triple notches generated in this single pocket.

Key Applications for the Black Hole Filter
Some examples of key applications that may be addressed by the BHF are in wireless communication systems where Tx spurious suppression is necessary to meet emission mask requirements, Rx blocking in an interference environment to protect receivers, and co-siting solutions to combine different services in same RF line for reduced CAPEX and OPEX.

Figure 4 Revised 700 MHz band plan.

700 MHz/Upper D Block and Public Safety Broadband
The FCC revised the 700 MHz frequency band plan and service rules to promote the creation of a nationwide interoperable broadband network for public safety and to increase the availability of new and innovative wireless broadband services for consumers. The Commission designated the lower half of the 700 MHz Public Safety Band (763 to 768/793 to 798 MHz) for broadband communications. The FCC also consolidated existing narrowband allocations to the upper half of the 700 MHz Public Safety block (769 to 775/799 to 805 MHz). To minimize interference between broadband and narrowband operations, the Commission adopted a one megahertz guard band (768 to 769/798 to 799 MHz) between the public safety broadband and narrowband segments (see Figure 4). The FCC requires a strict emission mask of 76+10log(P) at 1 MHz offset from the Public Safety Broadband channel band edge to protect Public Safety Narrow Band channel integrity.

Figure 5 700 MHz duplexer for upper D and Public Safety Broadband.

Table 2 tabulates estimated signal and attenuation levels necessary to meet the 76+10log(P) FCC requirements. According to these estimates, with a single tone at the band edge of 46.5 dBm, the filter should provide more than 40 dB attenuation at 1 MHz offset from that band edge. Figure 5 presents the simulation results of a duplexer of the 700 MHz upper D and Public Safety Broadband satisfying the FCC regulation by applying a BHF with 0.6 dB max insertion loss. It shows -48 dB at the Public Safety Narrow Band down link starting point of 769 MHz and -51 dB at 799 MHz at same point on the up link side.

Figure 6 Lower 700 MHz spectrum.

Lower 700 MHz Rx Blocking filter to protect from the DTV and MediaFlo
Another useful application is Rx blocking to prevent high power signal leakage from DTV channel 51 and MediaFlo on the lower 700 MHz band, as shown in Figure 6. New LTE systems will be deployed in lower 700 MHz band A, B and C Block. The high power signal from adjacent broadcasting systems will impact the LTE receiver and significantly degrade system performance. To avoid receiver performance degradation, the system needs to employ a filter with improved out of band rejection. The wall type filter shown in Figure 7 produces more than 40 dB attenuation at 500 kHz offset. The BHF filter appears to be the best solution for Rx blocking in this high-interference environment.

Figure 7 700 MHz Wall Type filter to protect from DTV and MediaFlo.

Co-siting Solutions
2G, 3G and 4G LTE/M-WiMAX technologies are expected to co-exist, installing in same sites and sharing antennas. This implies that co-siting solutions will be required for CAPEX and OPEX. Figure 8 shows a typical example of an in-band combiner to support that of three carriers, CDMA and the new 5 MHz LTE that must be combined over 10 MHz of bandwidth with a narrow guard band. Conventional combiners require more than 5 MHz for guard band, but the Constant Impedance BPF (CIB) implemented by the BHF only needs 625 kHz guard band to combine two different services. The simulation results are presented in Figure 9.

Figure 8 Co-siting example of CDMA plus LTE.

KMW has been bringing industry leading filter technology and RF products to market since the early days of CDMA in 1995. KMW enables a new world of filter technology “Black Hole Filter” by the “Triple Mode” resonance and offers solutions that are aimed at maximizing spectrum utilization and reducing size by market demands.

Figure 9 Filter response of co-siting combiner (CIB).

Fullerton, CA
(714) 515-1100
RS No. 301