Filters are a staple of RF designers everywhere, and especially in applications where the integrity of the signal cannot be compromised. When you’re only concerned about a single frequency, it’s relatively easy to add in a fixed-tuned filter. However, it becomes slightly more difficult when you need to cover multiple frequencies and can’t just utilize a roofing filter. To avoid having to use a bunch of individual filters, basically creating a complex switched filter bank module, a tunable filter may be the solution.
One of the most common applications for a tunable filter is as a pre-selector in a receive path, where there is a need to filter out adjacent channel (cosite) interference. It’s very common, especially in modern communication systems, for the receiver to need to operate over a wide frequency range. To make things worse, oftentimes the interference comes from co-located transmitters at the same location. Due to the minimal antenna isolation, especially on smaller command/control vehicles, filtering is needed to protect the receiver front end.
A tunable filter can also be used in the transmit path, as a post-selector on the output of an exciter or low power transmitter. This is especially useful if the transmitter is not spectrally clean, which is not uncommon at all. These lower level transmitters are often used to drive higher power amplifiers, so filtering is needed before further amplification to clean up the signal in order to avoid sending out ‘splatter’ across the spectrum.
Figure 1. HiPerTuner RF Broadband Preselector
Figure 2. HiPerTuner Performance Plot
There are a variety of choices available for frequency-agile tunable filter modules, so it all comes down to the desired specifications for a given application. One example is the HiPerTuner (pictured above), that provides filtering across the 200 MHz to 2.5 GHz frequency range. With 1 MHz channel spacing, it’s like having 2,301 bandpass filters in a single ruggedized chassis.
NuWaves Engineering is a veteran-owned, premier supplier of RF and Microwave solutions for Department of Defense (DoD), government, and industrial customers. An RF engineering powerhouse, NuWaves offers a broad range of design and engineering services related to the development and sustainment of key communications, telemetry and electronic warfare systems, as well as a complete line of commercially available RF products. NuWaves’ products include wideband frequency converters, high-efficiency and miniature solid-state power amplifiers and bidirectional amplifiers, high intercept low noise amplifiers, miniature RF filters, and cavity filters.
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