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Notch Filters
Low insertion loss notch filters (~0.1 dB) are essential for modern defense and wireless systems requiring high spectral fidelity. Spectrum Control’s designs enable receivers to detect low-power targets and distinguish signals in interference-heavy environments. These filters help identify weak communications and extract valuable intelligence in crowded spectra.
Visit our Notch Filter Customization Tool today to optimize your demanding filter requirements.
Our database of low-loss notch filter designs (0.1 dB) helps eliminate the need for additional gain when Size, Weight, and Power (SWaP) initiatives are in place. Low-loss band-reject filters from Spectrum Control also suppress harmonics at the receiver front end when complex pulse or chirp signals are cascaded in the receive-side chain. A flat amplitude response is critical to prevent distortion and unwanted spurious modulation.Spectrum Control’s line of low-loss band-reject filter designs delivers insertion loss as low as 0.1 dB while optimizing pole-placement strategies to achieve maximum rejection in a compact design.
These low-loss notch filters (0.1 dB) also help maintain receiver sensitivity in crowded wireless spectra. Excessive insertion loss can lead to unwanted heat, requiring additional cooling strategies or heavier metal packaging to prevent thermal runaway. Low-loss notch filters from Spectrum Control improve signal-to-noise ratio
(SNR) performance by providing a lower-loss path for weak signals. They maintain spectral purity without introducing harmful bit-error-rate (BER) degradation. Spectrum Control also offers Rapid Filter options, providing band-reject filters in both Chebyshev and Elliptic responses to meet critical demand requirements.
Notch filter frequency response
Lumped Element Notch/Band-Reject Filters
Spectrum Control's superior low-loss (0.1 dB passband) lumped element notch filters are ideal for applications where size and weight are critical. Our filter engineers are experts in lumped element design techniques and employ a range of innovative methods to meet today's demanding rejection specifications.
- Lightweight surface-mount packages for airborne applications
- Silver plating to reduce passband insertion loss
- Integral shielding for improved isolation
- Strategically placed poles for maximum notch-band rejection
- Multiple topologies integrated within a single package for broad frequency coverage
Lumped Element Filter and Frequency Response
Built from discrete inductors and capacitors, these notch filters cover frequencies from HF through portions of S-band (approximately 10 MHz to 3 GHz), rejecting a narrow band of interfering signals while passing frequencies above and below it with minimal loss. Because discrete components are much smaller than the operating wavelength, lumped element designs achieve a more compact footprint than cavity filters. As distributed-element structures, cavity filters must scale with wavelength and can exhibit spurious passbands or re-entrant responses that limit out-of-band rejection.
Spectrum Control's lumped element notch filters avoid these re-entrant modes, delivering a cleaner, deeper rejection null and wide tuning flexibility across frequency — performance that's difficult to replicate with narrowband cavity-style topologies. Easily tuned using air-spaced inductive coils and built from standard off-the-shelf components, they provide a lower-cost, highly customizable alternative to cavity filter designs for suppressing spurious signals, co-site interference, or a single problem frequency in a crowded spectrum.
Lumped element notch filters are relatively easy to tune by adjusting poles using air-spaced inductive coils to precisely set the rejection frequency. Because these designs typically use standard, off-the-shelf discrete elements and don't require complex specialized machining (unlike cavity filters), they generally offer lower cost and high customizability.
Spectrum Control’s lumped element filters are designed using discrete inductors and capacitors, covering HF, VHF, UHF, L-band, and portions of S-band (approximately 10 MHz to 3000 MHz). Below 1000 MHz, wavelengths are relatively long; distributed-element filters like cavity filters must be at least a fraction of the operating wavelength, which can make cavities physically larger than other topologies such as lumped elements.
Because discrete capacitors and inductors are much smaller than the operating wavelength, lumped element filters can achieve a smaller footprint than cavity designs.
Distributed-element filters, such as cavity filters, can exhibit spurious passbands, harmonic passbands, or a re-entrant response because their performance depends on the physical dimensions of the structure relative to the wavelength.
By contrast, Spectrum Control’s lumped element filters are smaller than the operating wavelength and therefore do not exhibit re-entrant modes or undesirable harmonic passbands, resulting in a cleaner response. They can also achieve very wide fractional bandwidths (roughly 10% to 90%), which can be difficult to replicate with narrow-band cavity-style topologies.
Lumped element filters are relatively easy to tune by adjusting poles using air-spaced inductive coils. Because these designs typically use standard, off-the-shelf discrete elements and don’t require complex specialized machining (unlike cavity filters), they generally offer lower cost and high customizability.
Cavity Notch/Band-Reject Filters
Spectrum Control's cavity notch filter designs deliver industry-leading rejection depth in the stopband while holding passband insertion loss as low as 0.1 dB, combined with high power-handling capability of up to 400 watts. Through careful process control and component selection, our engineers have developed specialized techniques, including intermodulation suppression, to meet the most demanding co-site interference and spectrum-clearing requirements.
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Lightweight aluminum alloy construction reduces overall unit weight | |
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NADCAP-controlled gold and silver plating processes | |
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Proprietary temperature-drift control, holding stability to under 1 ppm/°C | |
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Cross-coupled pseudo-elliptic topology sharpens rejection skirts on both sides of the notch | |
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Custom resonator geometry boosts peak power-handling capacity | |
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Silver-plated resonators and cavity interiors achieve higher Q than standard plating options | |
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Shock- and vibration-resistant stabilizing structures using low-dielectric-constant materials |
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Spectrum Control's low-loss cavity notch filters are recognized worldwide for their innovative topologies when a specific interfering frequency must be suppressed without disturbing the surrounding band. High-Q designs and creative engineering deliver a deep, narrow rejection notch in a compact footprint. |
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Spectrum Control's low-loss (as low as 0.1 dB) cavity lowpass filters are recognized worldwide for their innovative topologies when performance cannot be compromised. They combine high-Q designs with creative engineering techniques to reduce size while maximizing power-handling capability. |
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High-Q resonator technology, combined with space-efficient design, delivers a steep, deep rejection null while maintaining excellent power handling — ideal wherever a single problem frequency needs to be cleared from a crowded spectrum. |
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Spectrum Control cavity notch filters achieve ultra-low passband insertion loss because they have very low internal resistance, due in part to silver plating. This means the filters are efficient and able to handle radiated signal power even while heavily attenuating the target frequency; larger surface area and air dielectrics help prevent arcing that could destroy a lower-power design. |
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Spectrum Control cavity notch filters, with their high unloaded-Q designs, achieve incredibly narrow rejection bandwidths with very steep skirts — heavily attenuating an interfering tone just a few MHz wide while leaving adjacent frequencies essentially untouched. These designs are often built from Invar, whose low coefficient of thermal expansion keeps the notch frequency from drifting as ambient temperature changes. |
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The solid metal housing acts like a Faraday cage, preventing EMI from leaking out and external channel noise from leaking in. Combined with unique resonator designs, this allows Spectrum Control notch filters to achieve massive out-of-band attenuation (100+ dB) — difficult to reach with designs limited by parasitic coupling. |
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Iris coupling transfers electromagnetic energy between adjacent cavities through precisely shaped apertures in the cavity walls, acting as reactive shunt elements that shape the notch response and sharpen the rejection skirt. |
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Spectrum's pseudo-elliptic designs use cross-coupling to create transmission zeros precisely at the target interference frequency — driving rejection to a deep, narrow null rather than letting attenuation build gradually, which is critical when the interferer sits close to a frequency you still need to pass. |
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A unique low-dielectric-constant stabilizing structure reduces sensitivity to shock and vibration. Because even microscopic shifts in resonator position can shift the notch frequency off-target, this structure reduces electromagnetic disruption, adds mechanical damping, and minimizes mass — keeping the rejection null locked on frequency. |
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Spectrum engineers routinely pair the notch stage with an integrated cleanup filter to extend stopband performance, suppressing harmonic resonances and spurious tones the primary notch structure doesn't fully address. |
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Tightly controlled conductor spacing and surface finish yield high power-handling capability even at deep rejection levels — critical since cavity notch filters store significant electromagnetic energy in their resonators, and spacing to the cavity walls governs how much power the structure can handle. |
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Silver plating on resonators and cavity interiors reduces loss and delivers higher Q than lower-cost plating methods. Since silver has the highest electrical conductivity of any metal, less energy is lost as heat — resulting in a sharper, deeper notch. |
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Cross-coupling introduces electromagnetic energy between non-adjacent resonators, creating signal paths that cancel at the target frequency. Bimetallic resonators improve temperature stability, minimizing frequency drift so the rejection null stays locked on the interfering signal as ambient temperature changes. |
Ceramic Notch/Band-Reject Filters
Spectrum Control designers are experts in the application of multiple filter topologies, including creative mixed-topology approaches that can be integrated into a single design. High-complexity ceramic filters, such as a six-pole, 2100 MHz design with a 45/0.5 dB shape factor of less than 3:1, demonstrate exceptional performance while delivering strong value.
- Gold-plated surface-mount packages improve solderability and corrosion resistance
- Alternative coupling structures offer superior performance
- Capacitive coupling arrays provide enhanced reliability and repeatability
- Ceramic resonator designs as small as 2 mm reduce overall filter footprint
- Lead-free solders are used to comply with strict RoHS standards
- Select designs are laser-sealed using Spectrum Control’s in-house sealing methodologies
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3D Glass Notch/Band-Reject Filters
Spectrum Control continues to advance filter technology with the release of its new ultra-miniature glass filters. These high-Q filters, operating at frequencies up to 10 GHz, offer a range of outstanding features, including:
- Low insertion loss below 2 dB in many designs
- Rejection levels up to 70 dB
- Group delay of 1 ns across temperature
- Input power handling up to 1 watt
- Customization to meet demanding requirements
Suspended Substrate Filters
Spectrum’s low-loss suspended substrate filters enable complex transfer functions by integrating multiple filter topologies within a single package. Spectrum Control’s expert engineers optimize low-loss suspended substrate designs to meet demanding performance requirements. Suspended substrate features include:
- Expertise in combining lumped and distributed elements within a single suspended substrate design, providing enhanced unloaded Q and exceptionally low insertion loss
- Gold vias for superior isolation
- Cauer pole–placed transfer functions that yield low insertion loss performance
- Integrated cleanup low-pass filters for improved broadband performance
Technical Questions
Should a bandpass filter be placed before or after the LNA in an RF front end?
If the LNA has a low input power (damage) threshold, place the filter before it to protect against saturation from out-of-band interference. However, the filter’s insertion loss will increase the system noise figure. If the incoming signal is very weak, placing the LNA first allows its gain to mask the filter’s noise contribution. The downside is that any interferers will also be amplified before filtering.
Filter → LNA: Improved linearity, higher system noise figure, protects the LNA from out-of-band signals
LNA → Filter: Lower system noise figure, increased risk of LNA saturation
How does amplitude distortion in bandpass filters impact bit error rate (BER), especially in 64-QAM or 128-QAM schemes?
Filter responses such as Butterworth provide a smooth, flat amplitude characteristic, while Chebyshev and Cauer (elliptic) functions exhibit ripple and uneven weighting. This amplitude variation can distort the signal in the time domain, causing pulse spreading where one symbol overlaps into the next. The resulting intersymbol interference (ISI) can increase bit error rate.
How does simulating the entire board layout, not just the filter, prevent integration issues?
Full-layout simulation using tools like HFSS can identify unintended coupling (crosstalk), where input and output traces interact and potentially bypass the bandpass filter. At higher frequencies, such as 5G and emerging 6G bands, copper landing pads can behave as capacitive elements, detuning the filter’s band edges. Including pad geometries and surrounding structures in the simulation helps account for parasitics and provides a more accurate representation of interface impedance.
What are some items I can include as quick fixes in a checklist?
Tighten all connectors (as shown in the image below) to the manufacturer’s specified torque. Under-tightening can create air gaps, leading to impedance mismatches, while over-tightening can damage the internal dielectric and permanently compromise the integrity of the connector.
Verify connector torque (for SMA/SMP types). As frequency increases beyond UHF, layout precision becomes critical. Excess solder on pads or traces can alter the local impedance, which is typically designed for 50 ohms. Solder buildup can introduce additional capacitance near the pads, changing impedance and creating small reactive elements before or after the filter termination.
What are the primary filtering challenges for FR3 and the new 6G spectrum?
The 6G spectrum is still evolving but is expected to span roughly 7–24 GHz. It is not a single band, but a broader spectrum strategy. Designers working in FR3 will face increasing trade-offs between insertion loss and selectivity in an increasingly crowded spectrum. With dense carrier environments and minimal guard bands, high selectivity becomes critical, often driving the use of elliptic (Cauer) filter responses to mitigate adjacent-channel interference.
At these frequencies, implementation details, such as connector launches, board transitions, interposers, and antenna effects, become just as important as the filter design itself. Even small imperfections can degrade overall system performance and negate filter improvements.
