Switched Filter Banks Provide Unique Filtering Solution for Defense Applications
By Narda Microwave-East

The complex signal processing requirements of broadband intelligence gathering and other electronic warfare systems have long employed switched banks of filters before the receiver front end to remove out-of-band spectral components from the signals captured by the antenna before they are presented to the receiver. The simplicity of the name “switched filter banks” belies the complex nature of these subsystems, which must combine multiple types of filters and topologies as well as high-performance switches and other components to achieve the desired result in each channel. Narda Microwave-East has developed a wide range of switched filter banks that range from comparatively simple units to those with up to ten different signal paths and both active and passive components. They cover frequencies as broad as 10 MHz to 26 GHz, are extremely rugged, provide high-speed channel selection with low insertion loss, and excellent rejection characteristics in a very compact package. Most are screened to MIL-STD-883.

Narda has manufactured switched filter banks with up to ten channels. Their switching speed is less than 1 µs, and faster speeds can be specified. The eight-channel type shown in Figure 1 operates to 6 GHz, employs cavity and lumped-element filters, and has typical insertion loss of 6 dB. The 6-to-18 GHz models have insertion loss less than 7 dB. Detailed specifications are shown in Table 1.

Simple in Name Only
As its name implies, a switched filter bank routes an input signal to one of several transmission paths or channels using a solid-state switch. Each channel employs a filter (typically a bandpass type) that provides a low-loss passband and high rejection to out-of-band signals. The filtered channels are designed to separate a broad frequency range (from 1 to 18 GHz, for example), into narrower bands to allow the use of narrowband circuitry in order to improve the noise figure and spur free dynamic range of the receiver.

The goal in each case is to allow the receiver to accept an input signal that contains only those signa

ls of interest with out-of-band signals significantly attenuated, which makes the task of signal processing after downconversion to an intermediate frequency considerably easier and avoids overloading the receiver. This is especially important in ESM and ELINT systems in which a weak signal must be extracted from a dense signal environment.

On first inspection, the job of a switched filter bank might seem to be relatively simple, but the desired results can be very difficult to achieve. Each filtering path may require a different type of filter, such as cavity or lumped element, with a specific response type, such as Chebyshev or Butterworth. As a result, a switched filter bank may combine discrete, microstrip, and mechanical (in the case of a cavity filter) fabrication techniques, all within an enclosure that must be extremely rugged, highly stable over temperature and other environmental conditions, and as small and lightweight as possible.

Consequently, switched filter banks very often contain more than just filters and the switches and drivers required to route the signal through various paths. To compensate for signal loss along the path, amplification may be needed to maintain signal levels. Gain equalization circuits are also commonly employed to maintain gain levels through the path, as well as RF limiters to protect the amplifiers from overload (high power levels). Temperature compensation circuits ensure gain stays independent of temperature changes. It is also often desirable to sample the signal somewhere in the path for testing or monitoring purposes, which requires the addition of a coupler.

A “Typical” Example
Although nearly all switched filter banks are custom designs, the example of a Narda switched filter bank shown in Figure 3 illustrates major functions. In this case, a limiting circuit is provided at the input to keep the RF input signals experienced by the low-noise amplifiers later in the circuit at a safe level by attenuating signals above a specific threshold. Without this circuit, receiver performance could be degraded or sensitive components destroyed. The signal from the limiter is routed to the various signal paths with a PIN-diode switch. Narda typically uses this type of switch rather those based on GaAs FETs because it can operate at higher power, has lower loss, higher IP3 and off-isolation, and has a faster response time. Isolation is critical; the input and output PIN switches must have greater off-isolation than the filter rejection.

The types of filters employed are highly dependent on the mission of each subsystem, since all types of filters have strengths and weaknesses. For example, cavity filters are usable from 1 or 2 GHz to 30 GHz and sometimes higher and are well suited for bandpass configurations. They can handle high power levels, have low insertion loss and high isolation, high Q, and very sharp rejection. A coupled resonator structure is employed over narrow bandwidths, a combline structure over moderate bandwidths, and an interdigital configuration at the widest bandwidths. However, cavity filters are considerably larger than any other type of filter, are more expensive to fabricate, and are less accommodating to the confines of a switched filter bank. The Narda switched filter bank in Figure 2 employs cavity filters to cover 6 to 18 GHz.

In contrast, lumped-element filters, perhaps the most widely-used filters in RF and microwave design, can be made as lowpass, bandpass, bandstop, or highpass configurations, but have a maximum usable frequency of about 3 GHz because of fabrication issues, component resonances and tolerances. They are typically used at lower frequencies because their size remains comparatively small, and they can accommodate filter responses ranging from Chebyshev, to elliptical, Bessel, Butterworth, constant-impedance, and constant group delay. Steep transitions can be achieved from passband to the rejection points. However, LC filters cannot achieve extremely narrow bandwidths because of the coupling between elements and their achievable Q is limited. Microstrip and printed filters can accommodate a very broad array of filter types and responses, which makes it arguably the most versatile. They are small and economical, but typically have lower Q and higher loss.

Perhaps the most difficult challenge in optimizing performance is matching the impedance of the filter to the rest of the circuit, since filter response is directly dependent on how well the filter is matched. This requires considerable attention to the overall circuit design, component choice, and fabrication.

After the filter bank, the signal is passed to a MMIC amplifier that provides gain to compensate for insertion loss incurred in the previous stage, and then to an equalizer circuit. This circuit is essentially an attenuator whose frequency response characteristic is such that it provides a gain slope inverse to frequency to compensate for the gain roll-off of other components, ensuring that gain remains flat throughout the signal path. Another amplifier follows the equalization circuit, which once again increases signal amplitude, and this is followed by a temperature compensation circuit that maintains the gain independent of temperature over the operating temperature range. In the case of Narda switched filter banks this is generally -55o to +85o C because the systems are often deployed in airborne environments in which broad and rapidly changing temperature extremes are encountered.

This circuit is followed by another attenuator to boost gain once again. Finally, a directional coupler can be placed in the subsystem to act as a sampling port. Other components can also be added, including power dividers to split the signal to even greater numbers of filter banks or to provide a means of signal sampling before the input to the filter bank. When placed at the output, they allow the signal to be sent to multiple receivers. Additional options include a “bypass” mode, in which the signal passes straight through the filter bank without filtering, which is desirable in some cases, and a digital attenuator that allows levels to be adjusted at very high speeds and high resolution.

Narda switched filer banks are generally controlled by TTL lines, but other communications options such as RS-232 can be specified as well. Narda builds all of the required components of their switched filter banks to most efficiently meet customer electrical and physical requirements.

As should be obvious at this point, the switched filter bank can be far more than its simple name implies, and can be made with a large number of channels, accommodate a wide variety of ancillary components, and be configured to suit most every application. Like all custom components, Narda’s switched filter banks are designed and manufactured to meet specific customer requirements, and standard models can be modified in their microwave and control and specifications, physical configuration, and incorporation of additional components such as couplers, power dividers, and attenuators. More information is available at our website.

NARDA MICROWAVE-EAST
www.nardamicrowave.com/east
Email this article to a friend!