NASHUA, N.H. — A radar, communications receiver or electronic warfare system can encounter radio-frequency energy across a wide range of frequencies. But the system usually needs only part of it.

A microwave filter controls which frequencies pass through an RF signal path and which are attenuated. Depending on its design, the filter can remove frequencies above or below a certain point or suppress a certain portion of the spectrum.

Filters appear throughout aerospace and defense RF systems, with requirements that vary by operating frequency and the equipment around them.

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How does a microwave filter select frequencies?

An RF signal can contain multiple frequencies at once. The frequencies a filter allows through make up its passband, while those it attenuates make up its stopband.

A filter does not create a perfectly abrupt boundary between the two, though. Its response changes over a transition region. How quickly the filter moves from passing a signal to strongly attenuating it is part of its selectivity.

What are the main types of RF and microwave filters?

Engineers use several filter configurations depending on which frequencies need to remain in the signal path. A low-pass filter passes frequencies below a specified cutoff while attenuating higher frequencies. A high-pass filter does the exact opposite, passing frequencies above its cutoff while attenuating lower ones.

A bandpass filter passes a defined range between lower and upper frequency limits while attenuating frequencies outside that range. Bandpass filters are common in RF receivers because they can isolate the portion of the spectrum the receiver is designed to process.

A band-stop filter, also called a band-reject or notch filter depending on its design, attenuates a selected range while allowing frequencies on either side to continue through the system. Frequency-selective filters can also work together in larger networks. Diplexers and multiplexers, for example, route different frequency bands along separate signal paths.

Beyond the basic filter type, engineers also consider characteristics such as bandwidth and Q. Bandwidth describes the width of a filter's passband. A narrowband filter covers a relatively small span of frequencies, while a wideband filter covers a larger span.

Q, or quality factor, describes how efficiently a resonator stores energy compared with how much it loses. Many microwave filters use resonators to help select frequencies, and higher-Q resonators can support filters with lower loss and greater frequency selectivity.

What is insertion loss?

A filter can reduce some of the power of a desired signal as it passes through. Engineers call this reduction insertion loss. Keeping insertion loss low helps preserve the desired signal within the passband.

This can be particularly important near the front end of a receiver. A weak signal arriving at an antenna may already have limited power, and losses before amplification can affect receiver performance.

Rejection describes the other side of the filter's job. While insertion loss concerns what happens to wanted frequencies, stopband attenuation indicates how strongly the filter suppresses unwanted frequencies. Filter design therefore involves controlling both what gets through and what does not.

Why aren't all microwave filters built the same way?

At lower frequencies, filters can be made from discrete inductors and capacitors. As frequencies increase and wavelengths become shorter, the filter's physical dimensions and electromagnetic behavior become increasingly important.

Microwave filters can use transmission lines, microstrip structures, ceramic or dielectric resonators, cavities, waveguides and other technologies. Each approach comes with different design tradeoffs.

For instance, a waveguide filter can provide low-loss filtering and handle substantial RF power, but its physical size may be difficult to accommodate in equipment with tight space constraints. Planar technologies can provide smaller structures that integrate more readily with other circuitry.

Aerospace filter designs can face constraints involving size, weight, power, temperature, vibration and available mounting space.

Where do microwave filters fit into aerospace and defense systems?

A single RF system can contain several filters at different points in its signal chain. In a receiver, filtering near the front end can limit the frequencies entering later stages. Other filters can separate channels or remove unwanted responses as the signal moves through the receiver.

Transmitters also use filters. Amplifiers and other RF components can produce energy outside the intended operating band, including harmonics. Filtering can suppress unwanted frequency content before the signal reaches the antenna.

Microwave filters appear in radar, satellite communications, electronic warfare, navigation and data-link systems, often at multiple points in their RF signal chains.