Lowpass Filters

Pseudo-elliptic designs developed by our engineers reduce the number of poles required to meet specified rejection responses, resulting in smaller filters with lower insertion loss (0.1 dB) and reduced cost. Similar techniques can also be applied to provide passband group delay equalization or to increase rejection across a specified stopband region.

Spectrum Control Low Loss (0.1 dB) Bandpass Filters, known worldwide as innovators in the RF filter marketplace, offer innovative cavity filter topologies when performance cannot be compromised. Spectrum Control cavity filter advantages stem from their high-Q designs and creative strategies to reduce size and increase power-handling robustness.

Spectrum Control Low Loss (0.1 dB) Bandpass Filters, known worldwide as innovators in the RF filter marketplace, offer innovative cavity filter topologies when performance cannot be compromised. Spectrum Control cavity filter advantages stem from their high-Q designs and creative strategies to reduce size and increase power-handling robustness.

Spectrum Control bandpass cavity filters offer ultra-low insertion loss (0.1 dB) because they have very low internal resistance, due in part to plating that is often silver. This means Spectrum Control cavity filter designs are more efficient and better able to handle radiated signal power. Spectrum Control cavity filters can handle thousands of watts; their larger physical surface area compared with lumped element filters, along with the use of air dielectrics, helps prevent arcing that could destroy a traditional lumped element filter.

Spectrum Control cavity filters, with their high unloaded-Q designs, can achieve incredibly narrow fractional bandwidths with very steep skirts. This allows Spectrum Control cavity filters to pass a narrow band while heavily attenuating tones just a few MHz away from the band edge. Spectrum Control bandpass filters in a cavity topology are often designed from Invar, which offers a very low coefficient of thermal expansion. This helps ensure Spectrum’s frequency response doesn’t drift as ambient temperatures change, unlike some other manufacturers’ designs.

With a Spectrum Control bandpass filter in a cavity topology, the solid metal housing acts like a Faraday cage, preventing EMI from leaking out and external noise from another channel from leaking in. Known for unparalleled attenuation levels, Spectrum Control cavity filters can provide massive attenuation (100+ dB), which is normally difficult to achieve due to parasitic coupling. Unlike some other filter manufacturers, Spectrum Control offers unique resonator designs, like the one pictured here, to reduce overall size and increase peak power handling.

Spectrum Control bandpass filters use unique design approaches to reduce unit size, one of which is iris coupling. Iris coupling transfers electromagnetic energy between adjacent cavities through precisely shaped apertures in the cavity walls. These apertures behave as reactive shunt inductances or capacitances, increasing coupling between resonators and shaping the overall filter response.

Spectrum’s pseudo-elliptic designs incorporate cross-coupling to create transmission zeros, resulting in enhanced rejection performance. Unlike Chebyshev or Butterworth designs, where rejection increases gradually as you move farther from the center frequency, Spectrum Control pseudo-elliptic filters use cross-coupling to force the signal to zero at specific nearby frequencies, creating a steeper skirt between the passband and the stopband.

Spectrum Control bandpass filters can incorporate a unique low-dielectric-constant stabilizing structure to reduce overall sensitivity to shock and vibration. Even microscopic shifts in resonator positions can induce microphonics or frequency modulation. If a support structure like the one shown to the left had a higher dielectric constant, even a small movement caused by heating could produce a significant shift in center frequency. These innovative structures from Spectrum Control mitigate shifting effects through three primary mechanisms: they reduce electromagnetic disruptions, add mechanical damping and stiffness, and use low-dielectric materials to reduce mass and weight, making them less likely to shift.

Spectrum engineers routinely use an integrated cleanup low-pass filter to provide extended stopband performance. An integrated cleanup low-pass filter is a complementary filtering stage whose primary role is to suppress multiple harmonic resonances and spurious tones in resonant cavity structures. Without a cleanup low-pass stage, a transmitter could leak high-power harmonics that may interfere with out-of-band receivers.

Tightly controlled conductor spacing and surface finish yield very high power-handling capability. Spectrum Control bandpass filters utilize tightly controlled conductor spacing. Cavity filters are naturally high-Q filters; by design, they store significant electromagnetic energy within their resonators or sections. The space, or distance, between the resonator and the interior cavity walls is critical to optimizing power-handling capability.

Using silver plating on our resonators and cavity interiors reduces loss and provides higher Q values than the less expensive plating methods used by other filter manufacturers. Due to its atomic structure, silver has the highest electrical conductivity of any metal. In high-Q Spectrum Control cavity filters, Q is partly defined by the ratio of stored energy to dissipated energy. Using a more conductive finish such as silver directly reduces the amount of energy lost as heat, resulting in higher Q and lower insertion loss.

Spectrum Control bandpass filters use innovative cross-coupling techniques to achieve optimal rejection by introducing electromagnetic energy between non-adjacent resonators, creating signal paths that cancel at specific frequencies. Spectrum Control also incorporates bimetallic resonators to improve temperature stability, minimizing frequency drift caused by thermal changes that can alter cavity dimensions and shift the center frequency across the band.