AIAA-2000-1191

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American Institute of Aeronautics and Astronautics

APPLICATION OF HIGH POWER OUTPUT MULTIPLEXERS FOR COMMUNICATIONS SATELLITES

S. Lundquist, M. Mississian, Dr. M. Yu, D. Smith

COM DEV Cambridge, CANADA

ABSTRACT

This paper describes the development and qualification of high power dielectric multiplexers at C-band and L-band for commercial satellite applications. New packaging techniques and the employment of the single mode dielectric filter configuration has simplified fabrication and tuning while reducing mass and volume by more than 40% and has improved RF performance over existing TE111 dual mode Invar technology. It has been demonstrated that the use of dielectric technology in output multiplexers offers an alternative solution to reduce size and mass of such equipment.

INTRODUCTION

Low power single mode dielectric filters and multiplexers are widely used in communications satellite transponders. However, the development of high power dielectric filters has been slow due to the difficulty in configuring a high Q, thermally stable structure that can operate under high power conditions. High power dielectric filters require novel thermal and structural design approaches. Further, materials and assembly techniques required for high power applications, differ from those used in low power dielectric filters currently employed in input multiplexers.

This paper describes a new type of single mode dielectric filter, specifically designed for high power applications. Further, this paper demonstrates that this new filter is appropriate for use, and that it has been qualified for high reliability space applications.

PRIOR ART

Dielectrically loaded filters at low power have virtually replaced conventional air-filled waveguide filters for communication satellites at a fraction of the size and mass. Several dielectric filter configurations have been used as described in [1], [2], and [3]. Filters for high power applications have been considered as described in [4] which include modified planar designs, and quarter/half cut resonators. While these structures can be viable for power applications of up to 10 Watts per

channel, performance deteriorates quickly and limits their applications. Other designs described in [5], [6], and [7] have been demonstrated at moderate average power applications.

The filter configuration described in this paper is based on single mode planar design with novel thermal/structural construction. This filter is designed to handle high average and peak power input signals common in today’s communications satellites.

HIGH POWER MULTIPLEXER REQUIREMENTS

An output multiplexer must comply with stringent RF passband and stopband characteristics over a wide environmental temperature range while combining high power input signals that are typical of output multiplexers. Typically, output multiplexers must operate between environmental temperature extremes of -10 C and +90 C. Input power levels typically extend to 60 watts per channel for C-band systems. An output multiplexer assembly must be light weight while it must also be able to withstand the launch environment. To demonstrate launch survivability, vibration test requirements include random vibration to 25 Grms and shock vibration to 2000 G’s. The output multiplexer must also be compact with a small footprint and configured in a manner which can be efficiently combined with other output components, such as switches, cables, isolators and lowpass filters. A compact multiplexer with a small footprint reduces the requirements on the supporting structure resulting in further weight reductions from smaller panel size and potentially fewer structural panel brackets.

HIGH POWER DIELECTRIC FILTER CONFIGURATION

The basic building block of a dielectric channel filter is a cylindrical dielectric resonator mounted in a waveguide cavity operating below cutoff. The dielectric material used has a relatively high dielectric constant and a very low loss tangent. The resonator is mounted on a dielectric support having a relatively low dielectric constant and a very low loss tangent. The

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dielectric resonator is bonded to the support and the support is bonded to the pedestal which is then secured to the cavity. This configuration maximizes Q and minimizes changes in filter center frequency with variations in temperature.

Figure 1 illustrates a cross section view, with the lid removed, of a dielectric resonator assembly mounted in a filter cavity. Shown is the resonator assembly which consists of a dielectric resonator, pedestal, support and the bonding material. The resonator assembly is fastened to the cavity via the support and a small nut which is exterior to the cavity. The resonator assembly has thus been made modular by bonding the resonator assembly prior to installing it into the cavity. This ensures that the critical bond lines can be easily inspected and tested prior to final filter assembly.

Figure 1: Cross Section View of a Dielectric Resonator Assembly Mounted in a Cavity

Dielectric filters used in these applications are typically four, five or six pole filters with one pair of transmission zeroes. These filters are realized in a folded structure with appropriate inter-cavity couplings, primarily iris slots, to achieve the required filter function. A channel filter typically consists of four, five or six dielectrically loaded cavities operating in the dominant TE01δ propagating mode.

The equivalent linear frequency drift, (ELFD), of a filter is attributed mainly to the temperature coefficients of the dielectric resonator material, the thermal expansion coefficients of the tuning screw, the support material, and the housing material. By proper selection of the materials, the ELFD of the channel filter can be controlled.

A dielectric filter housing is typically constructed from thin wall aluminum for low mass. Tight tolerances on

the surface flatness and finish are applied to all mating joints to minimize RF leakage.

Channel filters may utilize either coaxial or waveguide interfaces. Coaxial interfaces are provided through the use of a coaxial probe which couples into the E-field of the first or last resonator. A waveguide interface can be provided through the use of an iris coupling energy to the first or last resonator. For C-band applications the input is typically TNCF and the output is WR229 ½ height waveguide while for lower frequency applications, a coaxial output interface is used with a coaxial manifold instead of a waveguide manifold.

In this paper, two examples of dielectric output multiplexers are presented. The first is a six channel C-band output multiplexer and the second is a three channel L-band output multiplexer. Both of these designs have been recently qualified for use in high reliability space applications.

DIELECTRIC C-BAND OUTPUT MULTIPLEXER

Dielectric C-band Output Multiplexer Configuration

The qualification model dielectric resonator C-band output multiplexer, (DRCO), consists of six 4-pole dielectric filters multiplexed using a ½ height WR 229 waveguide manifold. The filters are configured with the highest frequency channel closest to the short. The filters are coupled off of the broadwall of the manifold in a herringbone configuration. The C-band output multiplexer assembly frequency plan is shown in Table 1. The channel filter usable bandwidths are 36 MHz. The upper four channels and lower two channels have center frequencies spaced by 40 MHz.

Table 1: Dielectric C-band Output Multiplexer Frequency Plan

Channel Number Center Frequency, (MHz)

1 4180

2 4140

3 4100

4 4060

5 3644

6 3604 The mechanical layout of the DRCO is shown in Figure 2.

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Each channel filter has bolt on brackets which can be customized for a given mounting requirement. Each filter housing is machined from aluminum and contains four cavities which form the four pole elliptic filter. The cavities are electrically connected by irises which are slots machined directly into the cavity walls. Since all major components are of the same material as a typical spacecraft panel, thermal stress has been eliminated.

Figure 2: Dielectric C-band Output Multiplexer Mechanical Configuration

Dielectric C-Band Output Multiplexer Qualification Summary

A six channel dielectric C-band output multiplexer assembly was designed, fabricated and tested as part of an internal product platform qualification program. The qualification test program included an extensive sequence of testing as summarized in Table 2.

Table 2: C-band Output Multiplexer Qualification Test Summary

Test Phase Condition Initial Ambient RF baseline RF performance at lab

ambient conditions Non-Operational Thermal Cycling

six cycles from -40 to +115C

Post Thermal Cycling RF check at lab ambient conditions

Vibration 25 Grms random and 2000 G shock

Post Vibration RF check at lab ambient conditions

T-Vac Small signal RF check at plateaus between -10 C and +90 C in 10 C increments

Band Center Thermal Balance

Four channels operated at 60 WCW per channel at band center in a +90 C environment

Band Edge Average Power Handling

Four channels operated with 60 WCW per channel, three channels at band center and one channel at band edge in the hot , +90 C, environment

Band Edge Peak Power (Multipaction)

Three channels operated with 60 WCW per channel input at band center and one channel operated with 240 Wpeak, 25 % duty cycle at band edge in the hot, +90 C environment.

Band Edge Power Shock

A single channel operated with 60 WCW in the cold, -10 C, environment. The power was cycled 3 minutes on and 10 minutes off for 20 cycles.

Final Ambient Testing

Final RF performance at lab ambient conditions, included radiated emissions

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Table 3 summarizes the dielectric C-band output multiplexer measured electrical and mechanical performance. Figures 4 through 7 illustrate the measured common port return loss, input return loss, insertion loss, group delay variation and close to band rejection.

Table 3: Dielectric C-band Output Multiplexer

Assembly Performance Summary

Parameter Performance

Temperature Range, (°C)

Operating

Non-operating

-10 to +90

-35 to +125

Channel Power, (W)

Average per channel

Peak per channel

60

240

Channel Filter Q @ +90°C > 10,800

Worst case ELFD, ppm/C -1.3

Mass / Channel, (g) 262

Random Vibration, (Grms) 25

Shock Vibration, (G) 2000

Gamma Radiation, (Mrad) 400

Figure 3: C-band Output Multiplexer, Common Port Return Loss

Figure 4: C-band Output Multiplexer, Input Port Return Loss

Figure 5: C-band Output Multiplexer, Insertion Loss

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Figure 6: C-band Output Multiplexer, Group Delay Variation

Figure 7: C-band Output Multiplexer, Close-to-band Rejection

DIELECTRIC L-BAND OUTPUT MULTIPLEXER

Dielectric L-band Output Multiplexer Configuration

An L-band 3-channel dielectric output multiplexer (Triplexer) has been developed and built for commercial applications. The Triplexer consists of three dielectric resonator filters coupled onto a coaxial airline manifold. The coaxial manifold is formed by a central one piece silver wire within a machined housing which is silver plated to reduce insertion loss. Accurate in-house software determined the manifold arm positions and lengths. The mechanical layout of the Triplexer is shown in Figure 8.

The three channels of the triplexer are a 4, 5 and 6 pole elliptic and pseudo elliptic dielectric filters. Each filter is secured to the manifold via a TNC connector. Each filter housing is machined from aluminum and contains 4, 5 and 6 cavities which form the respective filters. The cavities are electrically connected by irises which are machined directly into the cavity walls. Filter construction is similar to that described for the C-band application.

Figure 8: L-band Triplexer Mechanical Configuration

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Dielectric L-band Output Multiplexer Performance Summary

The Triplexer frequency plan is shown in Table 4.

Table 4: Triplexer Frequency Plan

Channel Number

Center Frequency, (MHz)

Bandwidth, (MHz)

1 1676 6 2 1685.7 4.74 3 1692.7 0.2

Table 5 summaries the testing performed on the L-band Triplexer, detailing the test levels and conditions. Table 6 summarizes the dielectric L-band Triplexer measured electrical and mechanical performance. Figure 9 illustrates the measured return loss and isolation of the Triplexer at ambient temperature. Figures 10 through 14 illustrate the measured performance of the L-Band Triplexer at lab ambient and 80°C.

Table 5: Elegant Bread Board Model L-band Output Multiplexer Test Summary

Test Phase Condition Initial Ambient RF Baseline RF

performance at lab ambient conditions

Non-Operational Thermal Cycling

Six cycles from -37°C to +90°C

Operational Thermal Cycling

One cycle from -1°C to +80°C

Post Operational Thermal Cycling

RF Check at lab ambient conditions

Average Power Handling

One channel tested at 35 Watts CW at a baseplate temperature of +54°C for a duration of 2 hours

Peak Power Handling One channel tested at 70 Watts peak with a duty cycle of 50% at a baseplate temperature of +54°C for a duration of 1 hour

Final Ambient RF Final Ambient RF performance compared to Initial Ambient RF performance

Figure 9: Measured Common Port Return Loss and

Isolation at Ambient Temperature

Figure 10: Measured Input Return Loss, Channel 1 at Lab Ambient and 80°C

L-Band Triplexer

Thermal Hot

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Figure 11: Measured Output Return Loss, Channel 1, at Lab Ambient and 80°C

Figure 12: Measured Close to Band Rejection,

Channel 1, at Lab Ambient and 80°C

Figure 13: Measured Insertion Loss, Channel 1, at Lab Ambient and 80°C

Figure 14: Measured Group Delay Variation,

Channel 1, at Lab Ambient and 80°C

L-Band Triplexer

L-Band Triplexer

L-Band Triplexer

L-Band Triplexer

Thermal Hot

Thermal Hot

Thermal Hot

Thermal Hot

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Table 6: Dielectric L-band Triplexer Performance Summary

Parameter Performance

Temperature Range, (°C)

Operating

Non-operating

-27 to +54

-37 to +64

Channel Power, (W)

Average per channel

Peak per channel

35

70

Channel Filter Q @ +90°C > 10000

Worst case ELFD, ppm/C 0.5

Mass / Channel, kg 3.88/3 = 1.29

Random Vibration (Grms) 17.88

Shock Vibration (G) 1081

Radiation, (Mrad Si) 400

COMPARISON OF DIELECTRIC TO

CONVENTIONAL TECHNOLOGY

One of the major advantages of using dielectric loaded filters to conventional technology is a reduction in mass and volume. Figures 15 and 16 illustrate C-band and L-band dielectric multiplexer assemblies superimposed on their conventional Invar TE111 dual mode counterparts, respectively. The mass and volume savings using dielectric technology is described in Table 7. The numbers are based on using equivalent performance dielectric filter technology and comparing it with thin-wall dual/single and triple mode Invar filters coupled to a waveguide manifold.

Another benefit of using dielectric filter is the reduction of ELFD, whilst maintaining comparable RF performance.

Figure 15: Size Comparison of Conventional and Dielectric C-band Output Multiplexers

Figure 16: Size Comparison of Conventional and Dielectric L-band Output Multiplexers

Table 7: Mass and Volume Savings with Dielectric Technology

Dielectric Conventional Savings

C-band (12 ch) -Mass (kg) -Volume (in3)

3.1 681

5.2

1854

40% 63%

L-band (3 ch) -Mass (kg) -Volume (in3)

3.7 909

10.0 4410

63% 79%

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CONCLUSIONS

Dielectric output multiplexers at C-band and L-band have been built and qualified for commercial satellite applications. The C-band multiplexer has been qualified to handle 60 watts average and 240 watts peak per channel. The L-band multiplexer power handling is 35 watts average and 70 watts peak per channel. Further more, the L-band multiplexer has bandwidths as low as 0.1% and that represents a significant advance in the realization of such narrowband filters using dielectric technology. The multiplexers have undergone extensive qualification testing to show compliance to stringent passband characteristics over a wide environmental temperature range while combining high power input signals. Results indicate that whilst RF performance is comparable with conventional technology, ELFD is improved with the dielectric option. It has also been demonstrated that using dielectric technology in output multiplexers offers an attractive solution to reduce size and mass of such equipment.

ACKNOWLEDGMENTS

The authors wish to thank Technology Ontario for their funding of the development and qualification of the C-band dielectric output multiplexer. Also, thanks to Martin Pointer for his inputs regarding the L-band output multiplexer. Finally, thanks to Dr. Chandra Kudsia for his constructive comments during the preparation of this paper.

REFERENCES

[1] Wai-Cheung Tang, et al, “Planar Dual-Mode Cavity Filters Including Dielectric Resonators”, U.S. Patent # 4,652,843, issued March 24, 1987

[2] S.J. Fiedziuszko, “Dual Mode Dielectric

Resonator Loaded Cavity Filters”, IEEE Transactions on Microwave Theory and Techniques, Vol. MTT-30, pp 1311-1316

[3] Kawther A. Zaki, et al., “Canonical and

Longitudinal Dualo-Mode Dielectric Resonator Filters without Iris”, IEEE Trans. MTT, pp. 1130-1135, Vol 35, No. 12, December, 1987

[4] R. Cameron, et al., “Advances in Dielectric

Loaded Filters and Multiplexers for Communications Satellites”, AIAA 13th Communications Satellite Systems Conference, Los Angeles, CA pp. 823-828

[5] Wai-Cheung Tang, et al., “Dielectric Resonator

Output Multiplexer for C-Band Satellite Applications”, 1985 MTT-S Symposium, Boston MA pp. 343-345

[6] S.J. Fiedziuszko, et al., “Satellite L-Band Output

Multiplexer Utilizing Single and Dual Mode Dielectric Resonators”, 1989 MTT-S Symposium, Long Beach, CA pp. 683-686

[7] S. C. Holme, et al. “A 4 GHz Dielectric

Contiguous Output Multiplexer for Satellite Applications”, 1993 IEtEE MTT-S Digest pp. 443-445