Filtering Key Needs for Millimeter-Wave Satcom

Satellites are increasingly important components in high-data-rate networks and millimeter-wave frequency bands provide the extended bandwidth needed for wirelessly moving large amounts of data quickly. Data will be generated increasingly at higher frequencies, including from Internet of Things (IoTs) sensors transferring data via millimeter-wave signals and needing the support of high-speed data communications networks aided by satellites. Counting on satellites for reliable, long-term service requires durable, high-performance electronic components, such as bandpass filters. To achieve high performance at millimeter-wave frequencies, especially with the reduced size, weight, and power (SWaP) and dense integration driving payloads in newer satellite communications (satcom) systems, millimeter-wave bandpass filters must be optimally designed and manufactured. This can be done through careful selection of circuit designs and materials to meet the long-term needs of satellite-to-satellite links in growing numbers of low-Earth-orbit-satellite (LEOS) constellations as they are used as part of the Internet cloud.

Millimeter-wave signals extend from 30 to 300 GHz at wavelengths of 10 mm (30 GHz) to 1 mm (300 GHz) in what the International Telecommunication Union (ITU) refers to as the extremely high frequency (EHF) band. They propagate by direct line of sight (LOS) rather than by reflections from the ionosphere or ground waves in the manner of lower-frequency signals within their longer wavelengths.

Millimeter-wave frequency bands contain wide bandwidth in support of moving large amounts of data at high data rates, such as between LEOS. Because of the tiny wavelengths of millimeter-wave electromagnetic (EM) signals compared to lower frequencies, they may lack the propagation distances and capabilities of traveling through buildings, but millimeter-wave signals are well suited for shorter-distance point-to-point communications, such as between LEOS. Bandpass filters are instrumental in organizing the effective transfer of desired millimeter-wave signals between satellites while rejecting unwanted signals above and below a designated millimeter-wave frequency band of interest.

While LEOS are being developed for microwave frequencies between terrestrial platforms and users, high-capacity links are also required for inter-satellite communications of large amounts of data between satellites. The International Telecommunications Union (ITU) has approved segments of Q-band (33 to 50 GHz) and V-band frequencies (40 to 75 GHz) for use in LEOS constellations as part of space-to-space and ground-to-space communications, although atmospheric attenuation increases with frequency and millimeter-wave bands are better suited for secure, short-distance satellite-to-satellite links.

Bandpass filters for satcom links at millimeter-wave frequencies are commonly within the EHF range. They transfer signals with the passband with low loss while rejecting signals with high attenuation above and below the passband. Millimeter-wave frequencies within the EHF span include Ka-band (27 to 40 GHz), Q-band (33 to 50 GHz), V-band (40 to 75 GHz), E-band (60 to 90 GHz), W-band (75 to 110 GHz), and D-band (110 to 170 GHz) signals. LEOS orbiting 200 to 2000 km above the Earth’s surface can make use of one or more of those millimeter-wave bands for inter-satellite communications. With the wide bandwidths available at millimeter-wave frequencies, wireless links can quickly transfer large amounts of data between LEOS. To ensure reliable data communications, the links must maintain excellent signal integrity with minimal interference from adjacent signals and EM energy within or outside of the frequency band of interest. Miniature, high-performance bandpass filters can contribute to high-quality inter-satellite datalinks at millimeter-wave frequencies.

Perusing Parameters

Critical performance parameters for passive bandpass filters operating at millimeter-wave frequencies include the filter’s percentage bandwidth at the operating frequency, the passband insertion loss and return loss, rejection of signals and noise outside the passband, maximum signal input power, operating temperature range and, when processing high-speed data, the latency or delay added by signals passing through a filter. Filter latency can also affect signals in the analog realm, in applications that measure signal timing and phase, such as radars. Variations in latency or group delay from filter to filter can degrade the performance of millimeter-wave applications using multiple filters, such as phased-array antennas and radar systems.

For potential millimeter-wave filter parameters, a bandpass filter with a center frequency of 30 GHz and 10% bandwidth would have about a 3 GHz passband. To conserve signal power, the filter would be designed for low insertion loss and return loss across its passband, such as 28.5 to 31.5 GHz. A filter’s passband is usually defined according to an acceptable amount of insertion loss for the passband, such as a 3-dB passband. Since environmental conditions such as temperature and humidity impact a filter’s performance, filters used in harsh environments must be designed and characterized for good stability with humidity and temperature, so that the filter’s center frequency, passband, and loss change little with changes in temperature and humidity.

In contrast to geosynchronous orbiting satellites (GEOS), remaining in fixed positions about 22,236 miles above the Earth’s equator, LEOS are only 311 to 1243 miles above the Earth, moving at high speeds in orbit relative to the Earth’s surface. LEOS are designed with or without inter-satellite links (ISLs), with ISLs for delay-tolerant applications and without ISLs for delay-intolerant applications. LEOS are smaller than GEOS and constructed with rugged but lightweight materials, such as aluminum honeycomb structures combined with glass fibers, capable of protecting their electronic payloads. LEOS are designed with solar arrays serving as power supplies, backed by onboard rechargeable batteries. Thermal stability is achieved by on-board thermal control subsystems, including temperature sensors, heaters, and thermal blankets.

LEOS constellations may employ space-based and/or ground-based signal routing. With space-based routing, the signal will travel from a terrestrial user’s transmitter to the nearest satellite above the user. Depending upon the destination of the signal and the location of the receiver, the satellite will transfer the signal to a neighboring LEOS and from that satellite to the next until reaching the satellite above the receiver. With ground-based signal routing, the satellite above a transmitting user will transfer the signal to a ground station which then determines where to switch the signal, although with greater delays than for space-based signal routing. In either case, depending upon the number of users, bandwidth must be used wisely, and millimeter-wave bands generously provide available bandwidth.

Serving Satellites

For LEOS and other satellites employing millimeter-wave frequencies, Benchmark Lark Technology has developed its mmW-STL and mmW-FH Series millimeter-wave bandpass filters. Both are manufactured in miniature surface-mount-technology (SMT) package styles to meet the reduced SWaP requirements of satellite payloads. The filters serve passbands from 5 to 40 GHz for operating temperatures from -40 to +85°C. Filters in the mmW-STL series are available with bandwidths from 10% to 25% while filters in the mmW-FH line can isolate bandwidths from 2% to 10% from 5 to 40 GHz, or percentage bandwidths as wide as 4 GHz.

An mmW-STL Series bandpass filter developed with 15.2% bandwidth at a center frequency of 40.2 GHz (Fig. 1), a 3-dB passband of 6124 MHz, adds only 0.275 × 0.080 × 0.025 in. to a satellite payload. Even with such a small size, it achieves more than 40 dB rejection of unwanted, out-of-band signals above and below the passband while suffering only 3.58 dB insertion loss at the passband center frequency.

Fig.1 Compact mmW-STL bandpass filters are available for 10% to 25% passbands at center frequencies from 5 to 40 GHz.

An mmW-FH Series filter with 9.1% bandwidth at a center frequency of 30.9 GHz has a 3-dB passband of 2824 MHz (Fig. 2). Passband loss is low, with a typical insertion loss of 1.8 dB and return loss of more than 10 dB at the center frequency. The filter features a combination of advanced circuit materials and substrate-integrated-waveguide (SIW) circuit technology to achieve excellent performance for an SMT package measuring just 0.360 × 0.120 × 0.070 in.

Fig. 2 Fitting miniature SMT housings, mmW-FH series bandpass filters are available for 2% to 10% passbands from 5 to 40 GHz. 

Filters in the mmW-STL series, which are based on stripline circuit technology, and filters in the SIW-based mmW-FH series, are customizable to meet specialized requirements at millimeter-wave frequencies. They can be quickly designed and developed for higher-power applications, such as in radar systems, and for extremely low-loss use, such as in unmanned aerial vehicles (UAVs) with onboard video surveillance systems. Both filter lines meet the most challenging SWaP requirements while providing essential filtering needs at Ka- and Ku-band frequencies, whether for space-based or terrestrial satcom infrastructure applications.