Find the Right Channels With Switch Filter Banks
Learn everything you need to know about switch filter banks!
The Quantic Corry team is excited to announce that we have developed another free highly-informative white paper. Our new comprehensive guide contains everything you need to know about switch filter banks that our fellow engineers will find both interesting and helpful.
Our new white paper: Find The Right Channels With Switch Filter Banks is full of great information, including:
- How switch filter banks can save size, weight, and power when miniaturizing a system
- How switch filter banks can be developed with different types of switch technologies
- Electrical, mechanical, and environmental requirements switch filter banks
Switch Filter Banks
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Abstract
Switches and filters are electronic components used in many RF/microwave systems, such as transmitters and receivers. Each provides functionality to aid high-frequency systems, but the functions can also be combined into switch filter banks to save size, weight, and power when miniaturizing a system. For the best results when using switch filter banks, system designers and integrators need to understand how a switch filter bank differs from the separate components. They also need to be familiar with the different parameters to consider when specifying a switch filter bank for different applications. This white paper will take a closer look at switch filter banks and some of the benefits they can provide when properly fitted into a high-frequency system.
This simple block diagram is an example of a signal path through a basic switch filter bank.
Introduction
Switch filter banks join two key component functions—switches and filters—under one roof. In doing so, they save size, weight, and power (SWaP) compared to individual components with their own housings and connectors. Close impedance matching of the switches and filters can provide electrical performance benefits in both wired and wireless systems for applications in commercial, industrial, military, and space markets and can reduce the tuning of individual components required to achieve the same results at RF through microwave and millimeter-wave frequencies.
How different is specifying a switch filter bank than the separate components that might do the same thing? The performance of a switch filter bank is described by the capabilities of its components, such as switching speed and filter passband loss and out-of -band rejection. But, a switch filter bank is a subsystem that processes signals from an input port to an output port, hopefully without discontinuities in the signal path, making it possible to specify multiple channels with closely matched amplitude and phase characteristics over a desired frequency range and even over a set of environmental conditions, such as temperature and humidity.
A switch filter bank can be formed with a bank of filters to provide the required channel responses and a pair of multithrow switches at the inputs and outputs of the filters to select a signal path (Fig. 1). The filter bank may be as simple as several bandpass filters (BPFs) with equal bandwidths at different center frequencies across a frequency range of interest to divide the frequency span into channels. Each filter preserves the signal located at its center frequency while rejecting signals outside of its channel bandwidth. Channels in a switch filter bank can also be formed with other types of filters, such as band-reject or notch filters, combinations of filters, and without filters, as bypass channels.
At the very least, a switch filter bank can be made smaller and lighter than the separate components by eliminating the packages and interconnections of the separate components. It can also achieve higher integration by adding companion components, such as switch drivers, power-supply circuitry, control logic, and even RF/microwave attenuators and amplifiers to adjust or balance signal power levels. As a miniaturized, integrated unit, a switch filter bank can be assembled and tested for special requirements, such as amplitude- and phase-matched channels with tolerances beyond the range of the separate components. The reduction in materials compared to separate components also enables switch filter banks (depending upon engineering expenses) to be produced at a lower cost than packing separate switch and filter components into an enclosure.
As an integrated assembly, a switch filter bank can be produced without the variations common when interconnecting signal lines, power lines, and control lines for discrete switches and filters in a system. Often, even slight variations in mechanical assembly, such as differences in cable lengths and connector torque between components, can result in channel-to-channel amplitude and phase variations, which can be minimized or avoided by effective manufacturing and production testing practices.
Switches and Filters
Many of the separate key component-level performance parameters for switches¹ and filters² can be found in the white papers available for free download from the Quantic Corry, LLC, website at www.cormic.com. For example, switch filter banks can be developed with different types of switch technologies, including high-isolation electromechanical switches, faster electronic switches, and even novel hybrid micro-electromechanical-systems (MEMS) switches with characteristics of both mechanical and electronic switches. Similarly, many different types of filter types and technologies can be incorporated into a switch filter bank, such as bandpass, band-reject, lowpass, and highpass filters. They can also be incorporated into other technologies, such as cavity, combline, LC, stripline, bulk-acoustic-wave (BAW), and surface-acoustic-wave (SAW) filters. Depending upon signal frequencies and power levels to be processed, filters can be quite different in size and shape. They can range from tiny surface-mount-technology (SMT) filters to much larger waveguide filters.
Each of the separate components contribute to the overall performance of a switch filter bank, with both switches and filters. For example, they may exhibit their own VSWR and insertion loss over a frequency range. Each can contribute to the overall performance limits of a switch filter bank since switches and filters each have their own usable frequency ranges and power levels. Different types of switches, like mechanical versus electronic switches, offer different switching speeds, power-handling capabilities, and operating lifetimes, which will play a role in the performance and operating lifetime possible for a switch filter bank.
The types of switches and filters best suited for integration in a switch filter bank should be determined by the needs of an application and the signals to be processed. Switch filter banks can be designed with a bank of same-type filters, such as bandpass filters, which isolate separate channels in a communications system. They can also be assembled with a mixture of filter responses, such as bandpass filters, to preserve communications channels and band-reject or notch filters to reject unwanted signal interference within a frequency range of interest.
Switch filter banks can be constructed as tightly integrated units in small enclosures with coaxial connectors for ease of installation in a system.
A modular approach allows channels to be added and subtracted to a switch filter bank as the needs of an application change.
Combining Components
While the performance levels of individual available switch and filter components provide reference points for the performance that might be expected from a switch filter bank, such as 50 ns switching speed and 60-dB filter rejection, the switch filter bank’s performance is as a subsystem, with other components, such as switch drivers and control lines, contributing to the overall performance of the switch filter bank. Switch filter banks can be equipped with different control interfaces, such as TTL, RS-232, and USB ports. And, how that control is implemented within the system will play a hand in the control and performance of the switch filter bank within the system.
The type of signals to be handled will also test the capabilities of a switch filter bank since any infidelity within the switch and filter elements of the unit can cause distortion of high-speed pulsed signals with fast rise times and high-frequency signals with complex forms of modulation. The switch performance and filter bandwidths must be suitable to minimize signal distortion under normal operating conditions. Depending upon the application, the performance of a switch filter bank will also be related to its environmental conditions, such as operating temperature range, shock, vibration, and humidity.
Performance levels for individual switch and filter components provide a starting point for what can be expected in terms of performance for a switch filter bank, such as in-band loss, out-of-band rejection, and channel switching speed. However, a switch filter bank is much more than interconnected switches and filters. A specifier may establish a set of performance levels based on known switches and filters. However, those performance levels are subject to numerous variables, such as temperature, shock, vibration, and humidity.
A bandpass filter’s insertion loss, for example, depends upon frequency and typically increases with increasing frequency. The amount of rejection of signals in a filter’s stopbands will also depend on frequency. Both performance parameters may also depend upon the temperature and the filter’s thermal stability. So, the performance parameters of a switch filter bank must be considered under the electrical, mechanical, and environmental specifications required for an application. For example, industrial-grade applications require wider operating temperature ranges than commercial applications but not as wide as military-grade applications.
Switch and filter performance can differ when comparing components between the RF/microwave (typically DC to 18 GHz) and millimeter-wave (30 to 300 GHz) frequency ranges due to the differences in wavelength sizes and power levels. The performance levels of switches, filters, and switch filter banks are very much frequency-dependent and a function of electrical, mechanical, and environmental requirements. For example, the larger wavelengths of RF/microwave signals make it difficult to design and fabricate a switch filter bank with smaller dimensions than a switch filter bank for millimeter-wave frequencies, especially when it may be required to handle about ten times as much signal power at the lower frequencies.
Choosing Channels
The specifications of a switch filter bank are part of a subsystem or subset of a larger system used in an application. Determination of those requirements should be guided by the application and the needs of the system for the switch filter bank. A simple starting point when specifying a switch filter bank is to determine the number of channels required in a system application and what will be done with those channels. Signals intended for one application may be interference for one or more other applications. Whether for wired or wireless signals, switch filter banks provide the means to keep different high-frequency signals separate but equal within the same operating environment.
The electrical, mechanical, and environmental requirements of a switch filter bank can be determined by understanding the spectral requirements of an application. Automotive radar systems, for example, operate within different portions of the millimeter-wave frequency range and provide optimal performance when interference is minimized. Although little interference would be expected in the millimeter-wave frequency range, automotive radars can generate their own interference if not properly filtered. The third harmonic of 24-GHz radar pulse falls close to the range of 77-GH automotive radars. Different types of pulsed and modulated waveforms are used in these radars, resulting in creating a complex, mixed-signal environment. Even for the low power levels at millimeter-wave frequencies, signals must be isolated to avoid interference. In this case, defining the signal environment helps define the requirements of a switch filter bank for ADAS radar applications.
The types of filter responses employed in a switch filter bank will be determined by the types of signals to beprocessed by each channel. For example, bandpass filters are designed to transfer signals around a specific center frequency, while rejecting signals lower and higher in frequency. Band-reject, or notch filters, are aimed at known interference, rejecting a typically narrow bandwidth around a center frequency. At times, filtering may not be required and low-loss bypass channels can be included, raising the total channel count of a switch filter bank. The response of each channel in a switch filter bank should be defined by the spectral requirements of the application.
Signals to be processed by a switch filter bank may be in many different forms, from short pulses to frequency-modulated-CW (FMCW) to in-phase/quadrature (I/Q) digitally modulated signals. The signals to be processed will best define the filtering characteristics of a switch filter bank’s channels in terms of center frequency, bandwidth, and power-handling capabilities. A bandpass filter (BPF) with minimal group delay and outstanding time-domain response, for example, will be needed to preserve the characteristics of a pulsed signal with fast risetime, while a notch or band-reject filter with high rejection may be required to suppress in-band interference signals within range of a receiver’s passband. While the number of channels for a switch filter bank will change according to the application, starting with as few as two when considering the losses for switches and filters, a practical limit for channels in a single switch filter bank is 20. When more channels are needed, multiple switch filter banks can be combined for synchronized control.
Making Everything Fit
An application will also help define the mechanical specifications for a switch filter bank based on available physical space within the system and factors related to physical size, such as power-handling capability. Enclosure-based switch filter banks offer the luxury of a metal housing for environmental protection and electromagnetic-interference (EMI) shielding. Although, it does have the need to occupy a relatively large volume within a system. Smaller, board-level switch filter banks occupy less volume within the system, but they lack the EMI shielding and power-handling capability of an enclosure-based unit. Size is one parameter that is part of a series of tradeoffs that must be balanced when specifying a switch filter bank. It is related to weight and power-handling capability, temperature stability, power consumption, and even frequency when considering smaller-wavelength and higher-frequency signals.
Switch filter banks can be constructed in several forms, including as highly-integrated units with connectors (Fig. 2) and modular assemblies in which functionality can be added or subtracted (Fig. 3). The requirements of an application and how well the tradeoffs can be balanced may favor one format over another. Communications base stations working in different geographic regions with different signal levels, may have different requirements for switch filter banks with or without optional amplification and/or attenuation. A modular approach also eases the addition of functionality, such as voltage regulation and reverse voltage protection, that may not have been included in the initial design of an integrated board-level switch filter bank.
For a switch filter bank with coaxial connectors, frequency will determine the type of signal connectors for a modular design. Within the frequency range often associated with electronic warfare (EW) applications—DC to 18 GHz—SMA connectors provide reliable input and output signal access for a switch filter bank. However, when it comes to higher frequencies through the millimeter-wave range, coaxial connectors with smaller dimensions, such as 2.92- or 3.5-mm connectors, will be needed to handle the higher-frequency and smaller-wavelength signals with low loss.
Switch filter banks are often designed to specialized, custom requirements; although, standard models are also available for typically large-scale applications. One of these is the model CMIINV-5BT2-RM switch filter bank from Quantic Corry, LLC, for wireless cellular network-based stations. It selects uplink and downlink signals in cellular communications bands 1, 2, 5, 13, and 17. It provides 50 dB or better isolation between all uplink and downlink channels, such as Band 1 downlink signals from 2110 to 2170 and Band 1 uplink signals from 1920 to 1980 MHz. For those in need of processing higher frequencies, such as EW systems, Quantic Corry also has the standard model CMISFB-80180T4 switch filter bank that separates the total span of 8 to 18 GHz into channels of 8 to 10 GHz, 10 to 12 GHz, 12 to 15 GHz, and 15 to 18 GHz with 100 ns or less switching speed between channels and supplied in a compact integrated unit.
Specifying a switch filter bank is not a routine matter. However, a well-designed and manufactured unit can make a difference in an electronic system, whether for aerospace, commercial, industrial, medical, or military application. Engineers at Quantic Corry, LLC, have partnered closely with customers to select the best switch and filter technologies and blend of tradeoffs to achieve required performance levels, while also meeting difficult SWaP demands. They have developed integrated and modular design approaches that help with the tradeoffs without sacrificing performance or operating lifetime.
Author Bios
James Price received a BS degree in Electrical Engineering from West Virginia University in Morgantown, WV in 1983. From 1985 to 1995, he was an engineer specializing in communications with RCA which eventually became Lockheed-Martin in Camden NJ. From 1995 through 2004 he designed CATV products for Tollgrade Communications, Cheswick, PA.
He is currently VP of Engineering for Quantic Corry, Warrendale, PA. His areas of focus include RF and microwave filters, multiplexers, switches and antennas.
Jeff Gibala is Principal Engineer at Quantic Corry. He received his BSEE degree from the Pennsylvania State University and over his 25 year career has held design and management positions in the Broadcast, Telecommunications, Medical and Wireless Power industries.