KU- BAND TEMPERATURE COMPENSATED HIGH POWER MULTIPLEXERS S. Lundquist, M. Yu, D. Smith, W. Fitzpatrick COM DEV, Cambridge, CANADA

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1 AIAA KU- BAND TEMPERATURE COMPENSATED HIGH POWER MULTIPLEXERS S. Lundquist, M. Yu, D. Smith, W. Fitzpatrick COM DEV, Cambridge, CANADA 20 th AIAA International Communication Satellite Systems Conference & Exhibit May 2002 Montreal, Quebec, CANADA

2 AIAA KU- BAND TEMPERATURE COMPENSATED HIGH POWER MULTIPLEXERS S. Lundquist, M. Yu, D. Smith, W. Fitzpatrick COM DEV Cambridge, CANADA ABSTRACT COM DEV has developed a new generation of temperature compensated Ku-band output multiplexers offering significant size, mass, power handling and RF performance improvements over existing technologies. Changing the base material of the filters to Aluminum, from the more conventional Invar, greatly reduces mass, while improving the heat transfer from the filter to the spacecraft. COM DEV s unique temperature compensation technique ensures superior RF performance of the Aluminum filter by reducing the equivalent linear frequency drift, (ELFD), to less than one ppm/c. This paper presents an overview of the design and the results of the qualification testing carried out by COM DEV. disadvantage in that it has poor thermal conduction and high mass. Large aluminum brackets must be used between the filter and the panel to conduct the heat away from the filter. Another factor in the temperature stability is the differential expansion between the aluminum panel and the Invar filter. A large cross sectional area on the brackets will reduce the maximum temperature of the filter but will cause stress on the filter and degrade the electrical performance of the filter. Many tradeoffs must be made to maintain the electrical performance over temperature, which limit the maximum power handling ability of each channel filter of the multiplexer. INTRODUCTION Modern requirements for Ku-band Output Multiplexers include high average power levels, (>120 Watts/channel), narrow channel bandwidths, (<36 MHz) and very stable RF performance over wide operating temperature extremes while maintaining a configuration capable of being lightweight with a small footprint Conventional dual mode Invar channel filter solutions struggle to meet today s requirements due to the poor thermal conductivity and the high mass of Invar. To meet today s high power requirements a lightweight Aluminum temperature compensated channel filter is coupled with a traditional waveguide manifold resulting in a multiplexer assembly whose RF performance, mass properties, thermal conductivity and footprint size are superior to traditional Invar designs. PRIOR ART A prior art 4-2 elliptic microwave resonator filter is shown in Figure 1 typical of traditional Invar filters. [1] The filter includes two electrically conductive cylindrical cavities, which operate in the TE113 dual mode in the longitudinal cascade configuration. The frequency stability of the filter with temperature is determined mainly by the thermal expansion rate of the cavity material. Invar is typically chosen as it has the lowest expansion rate with temperature and results in an ELFD of approximately 1.2 ppm/c. This material will keep the electrical performance stable but has the Figure 1: Traditional Invar 4-2 Elliptic Filter HIGH POWER Ku-BAND MULTIPLEXER REQUIREMENTS An output multiplexer must comply with stringent RF passband and stopband characteristics over a wide temperature environmental temperature range while combining power input signals that are typical of output multiplexers. Typically output multiplexers must operate between environmental temperature extremes of 10 and +120 C. Input power levels typically extend to 250 watts per channel for Ku-band systems. An output multiplexer assembly must be lightweight while it must be able to withstand the launch environment. To demonstrate launch survivability, vibration test 2

3 requirements include random vibration to 25 Grms and shock vibration to G s. The output multiplexer must also be compact with a small footprint and configured in a manner that can be efficiently combined with other output components such as switches, waveguides, isolators and lowpass filters. A compact multiplexer with a small footprint reduces the requirements on the supporting structure resulting in further weight reductions due to smaller baseplate sizes. CTE than Invar the channel filter relies on an external temperature compensation scheme to maintain a low ELFD. The configuration of the Ku-band temperature compensated channel filter is shown in Figure 1. HIGH POWER KU-BAND TEMPERATURE COMPENSATED FILTER CONFIGURATION Channel filters configured for use in Ku-band output multiplexers have been traditionally designed as silver plated Invar TE113 dual mode structures with cascaded resonant cavities coupled via the circular end walls using iris slots. This traditional design approach has a number of limitations when used for satellite high power output applications. Firstly, cascading cavities in an end to end fashion tends to result in a larger footprint when compared to a dual mode structure which utilizes sidewall coupling allowing the cavities to be oriented side by side with the circular cavity ends positioned towards the mounting plane. The larger footprint of the conventional design then requires a baseplate of a greater surface area to accommodate the multiplexer resulting in increased assembly mass. Secondly, traditional filters have relied on Invar primarily for thermal stability. While this has worked well, this material has impacted the channel filter mass and thermal properties. Invar is a relatively heavy material as compared to other commonly available materials. Also the relatively poor thermal conductivity of Invar makes it a less than ideal choice for use in structures that inherently dissipate, (considerable), heat during normal operation and even more heat during fault conditions. While Invar provides for the low equivalent linear frequency drift required in Ku-band output multiplexers it adversely affects the mass properties and the thermal performance of the multiplexer assembly. Com Dev s temperature compensated Ku-band filter structure addresses the above limitations primarily by utilizing sidewall couplings and eliminating Invar from the channel filter design. The temperature compensated filter presented in this paper is fabricated from Aluminum as a lightweight highly thermally conductive alternative to Invar. As Aluminum has a much higher Figure 2: Temperature Compensated Ku-Band Channel Filter The temperature compensated channel filter as shown in Figure 2 is a 4-2 elliptic TE113 filter [2], [3], [4]. Two dual mode cavities are oriented with the end walls towards the mounting plane. The housing is primarily composed of two sections, a top half and a bottom half, each machined from a solid piece of Aluminum. The top and bottom halves are fastened together using a number of screws. Compensating bridges are positioned across each cavity end wall, top and bottom. causing the end walls to move inwards as the temperature increases in order to compensate the filter center frequency. Input / output coupling is provided through a single iris slot on each cavity. Inter-cavity couplings are provided using coupling slots located on the cavity sidewalls common to each of the cavities. Tuning and coupling screws are provided for each resonant mode, entering the cavity through each of the side walls. The mounting brackets are machined into the housing bottom section. 3

4 This configuration results in a smaller footprint than the traditional Invar filter described above. Also this design has eliminated Invar as the primary filter material allowing the performance not to be limited by the mass and thermal properties of Invar. HIGH POWER TEMPERATURE COMPENSATED KU-BAND OUTPUT MULTIPLEXER CONFIGURATION A six channel Engineering Qualification Model, EQM, was designed, manufactured tuned and tested to demonstrate the performance of the Ku-band temperature compensated output multiplexer. The configuration of the EQM is shown in Figure 3. The channel filters are arranged in a comb fashion coupled off of the broad wall of a standard WR 75 waveguide manifold. The multiplexer assembly was integrated onto an Aluminum baseplate. Figure 3: EQM Temperature Compensated Ku-Band Output Multiplexer HIGH POWER KU-BAND TEMPERATURE COMPENSATED MULTIPLEXER QUALIFICATION SUMMARY The six channel Ku-band output Multiplexer was exposed to the test program as outlined in Table 1. Table 1: Ku Band Output Multiplexer Qualification Test Summary Test Phase Preliminary Test Thermal Cycling Initial Ambient Test Band Centre Thermal Balance Band Edge Power Test Band Edge Power Test, in Air Passive Intermodulation Test Thermal Vacuum Test Random Vibration Test Shock Vibration Test Final Ambient Test Thermal Operational Conditions Baseline RF performance at lab ambient conditions Six cycles from 40 C to +140 C RF check at lab ambient conditions All channels powered at 250 Watts, in thermal vacuum of +90 C With all channels at full power, one contiguous channel taken to band edge at 250 Watts Above test repeated in lab ambient conditions Two channels powered and output monitored for PIM products, at lab ambient conditions Small signal testing measured over 10 plateaus, covering 10 C to +90 C 26 Grms random G RF check at lab ambient conditions Small signal testing, at lab ambient pressure A summary of the qualification test results is given in Table 2. Figures 4 to 8 illustrate the common port return loss, typical input return loss, typical insertion loss, typical group delay variation and typical near band rejection. 4

5 Table 2: Ku Band Output Multiplexer Assembly Performance Summary Parameter Performance Temperature Range ( C) Operating Non-operating Channel Power (Watts) Average per channel Peak per channel Channel Filter +90 C Worst Case ELFD, -10 to to > ppm/ C Mass per channel (grams) 131 Random Vibration (g rms) 26 Shock Vibration (g) Figure 5: Ku Band Output Multiplexer, Measured Input Port Return Loss Figure 4: Ku Band Output Multiplexer, Measured Output Return Loss Figure 6: Ku Band Output Multiplexer, Measured Insertion Loss 5

6 COMPARISON TO TRADITIONAL INVAR TECHNOLOGY The key performance parameters, mass, footprint, ELFD and electrical Q are compared between a multiplexer which contains conventional Invar channel filters and one that contains temperature compensated channel filters. Also contained in Table 3 is a prediction of the filter to baseplate thermal gradient for both a conventional Invar filter and a temperature compensated filter given that each filter has 0.5 db insertion loss and 200 watts input power, operated at band center. In both cases the channel filter functions are 4-2 elliptic. Table 3 demonstrates the improved mass, footprint size and ELFD for multiplexers with channel filters of equivalent Q. Table 3: Comparison to Traditional Invar Technology Figure 7: Ku Band Output Multiplexer, Measured Group Delay Variation Parameter Invar Channel Filter Temperature Compensated Channel Filter Mass, (g/channel) Footprint, (six 13.2 x x 4.5 channel Omux, length x width, inches) Baseplate Cavity Temperature Gradient, (C) ELFD, (ppm/c, >2.5 <0.8 at multiplexer level) Q Figure 8: Ku Band Output Multiplexer, Measured Near Band Rejection 6

7 CONCLUSIONS This paper has presented a high performance temperature compensated Ku-band ouptut multiplexer assembly. A six channel EQM assembly was designed, manufactured tuned and tested to demonstrate performance. The assembly has been compared to a traditional Invar multiplexer assembly. The temperature compensated assembly is lightweight due to the use of Aluminum as the primary channel filter material. The footprint is less than the Invar assembly due to the configuration of the channel filters that use sidewall coupling instead of the traditional end wall coupling. The measured worst case ELFD was less than that typically observed on traditional Invar channel filters through the use of an external compensating end wall bridge. Finally all comparisons where made considering 4-2 elliptic channel filters of equivalent Q. REFERENCES [1] A. Atia and A. William, A Four Cavity Elliptic Waveguide Filter, IEEE Trans. on MTT-18, No.12, Dec 1970, pp [2] R. V. Snyder, Multicavity Dual Mode Filter, US Patent , Sept [3] M. Yu, D. J. Smith, A. Sivadas, W. Fitzpatrick, Sidecoupled Microwave Filter with Circumferentiallyspaced Irises, US Patent Pending. [4] Ming Yu, David J. Smith, Apu Sivadas and William Fitzpatrick A Dual Mode Filter with Trifurcated Iris and Reduced Footprint, accepted for presentation at the IEEE International Symposium June