Military electronic systems rely on rugged filters to separate desired and undesired signals. As systems and components shrink to meet reduced size, weight and power (SWaP) requirements, filters must also shrink as they extend into Ku-/Ka-Band and mmWave frequency ranges. With short wavelengths, mmWave filters are smaller than their lower frequency counterparts.
Filters for electronic warfare (EW) and radar applications present design and fabrication challenges at mmWave frequencies. Benchmark Lark Technology’s mmWave bandpass filters demonstrate that careful selection of circuit materials and manufacturing processes can reduce mmWave SWaP. With low loss passbands through 40 GHz and high out-of-band rejection, the filters fit in small SMT packages. Though small, the filters meet demanding EW and radar performance requirements in harsh operating environments with wide temperature ranges.
High frequency filters are essential for spectrum management in EW and radar systems. The need for filters grows as spectrum usage extends into mmWave frequencies. Because of shorter mmWave wavelengths, critical components like bandpass filters must be small and often require more fabrication time. Close spacing of filter transmission lines can create unwanted electromagnetic coupling and pose challenges for manufacturing processes requiring tight component tolerances.
Printed circuit board (PCB)-based bandpass filters, like SMT filters, enable miniaturization in single-layer and multilayer configurations. They support reduced SWaP trends in defense systems and equipment. EW and radar systems often need narrow passbands with high selectivity. Increasing system frequencies limit signal power levels and passband loss becomes an essential performance parameter. In addition, more than 30 dB rejection of unwanted signals outside the passband frequency is required in those systems. Radar applications may also require extended power-handling capabilities for high-power pulsed signals and thermal stability to maintain consistent electrical performance at these power levels.
Careful transmission line layout contributes to tightly controlled electrical characteristics despite the shrinking size of mmWave circuits. Transmission lines include microstrip, stripline and substrate-integrated-waveguide (SIW) technologies. High frequency PCB designers are familiar with microstrip and stripline transmission lines, but perhaps less so for mmWave frequencies. SIW circuits are well suited for closely-spaced transmission lines typical of mmWave circuits. SIW filters fabricated on suitable circuit materials can consistently perform in hostile environments, like those with varying operating temperatures and humidity.
Miniature SIW bandpass filters manufactured with PCB processes can produce consistent and repeatable unit-to-unit performance. Producing SIW filters on low loss substrate materials can reduce non-recurring engineering costs and the time required to transform a prototype into a production unit. Microstrip, stripline and coplanar-waveguide technologies also support mmWave filter responses and circuit miniaturization. These technologies must be fabricated on high-quality dielectric materials with high conductivity plating to achieve the required filter responses at mmWave frequencies.
EW AND RADAR FILTERS
High frequency EW and radar bandpass filter requirements include well-defined, often narrow, passbands with low passband insertion and return loss. High selectivity and minimal center frequency loss ensure the filter will not obscure passband signals of interest. Filter rejection levels, characterized by the lower and upper stopbands, characterize unwanted signal suppression in the filter’s operating frequency range. Power-handling capabilities determine the maximum input power level before signal distortion. The maximum input power rating will be CW or pulsed depending upon the application.
Filter circuit topology and circuit material choices complicate bandpass and other filter responses at mmWave frequencies. Smaller mmWave circuit dimensions can stress manufacturing process capabilities, lowering yields and achieving tight dimensional tolerances to enhance production yields raises the manufacturing cost per filter. Circuit material choices also impact filter manufacturing yield, especially at mmWave frequencies. Material parameters like dielectric constant or dissipation factor consistency across the material impact filter consistency, repeatability and performance.
EXPLORING EXAMPLES
Figure 1 mmW-STL filters feature temperature-stable stripline circuits.
Through careful design and thoughtful choice of circuit materials, Benchmark Lark Technology has developed several mmWave filter families that provide the performance needed for EW and radar systems and are small enough to enable system miniaturization. The mmW-STL and mmW-FH mmWave bandpass filters are available in SMT formats for PCB mounting. Both have customizable passbands from 5 to 40 GHz and operating temperatures from -40°C to +85°C.
The mmW-STL bandpass filters in Figure 1 have impressive capabilities in SMT housings. Their performance and size rely on stripline circuits fabricated on advanced substrate materials with a proprietary blend of soft thermoplastic substrates and rigid thermoset materials. This combination of materials provides the electrical and mechanical characteristics needed for rugged, high performance mmWave SMT filters. Because the stripline filter circuits can be fabricated with standard PCB manufacturing processes, mmW-STL bandpass filters can be quickly transformed from design to production. They can be modified for 10 to 25 percent custom passbands from 5 to 40 GHz.
For example, an mmW-STL bandpass filter with a 40.2 GHz center frequency and 6124 MHz (15.2 percent bandwidth) 3 dB passband was designed for a 0.275 × 0.080 × 0.025 in. package. Figure 2 shows the transmission and reflection measurements. The loss is 3.58 dB at the center frequency and the passband return loss is greater than 10 dB. Unwanted, out-of-band signals are rejected by more than 40 dB.
Figure 2 Transmission and reflection plots for a 40 GHz mmW-STL filter.
An mmW-FH SIW filter with soft thermoplastic substrates and rigid thermoset materials was fabricated with a 2824 MHz 3 dB passband centered at 30.9 GHz (9.1 percent bandwidth). Figure 3 shows this device in a 0.360 × 0.120 × 0.070 in. package. Loss at center frequency is 1.8 dB, while return loss is better than 10 dB. Figure 4 shows transmission and reflection measurements of the mmW-FH SIW filter.
Figure 3 mmW-FH SIW filter.
Figure 4 Transmission and reflection responses of a mmW-FH filter.
Both mmWave filter series are customizable to electrical and mechanical requirements. Well-equipped test capabilities for design and production support the manufacturing processes that enable quick prototype-to-production transition. When SWaP is a concern for mmWave frequencies, filters can be quickly developed for the most demanding defense-related applications, including EW and radar systems.
Benchmark Lark Technology
Tempe, Ariz.
www.bench.com/lark