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Passive Components / Subsystems and Systems

Interference Mitigation Using Narrowband Filters

June 11, 2007
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With today’s increasingly crowded frequency spectrum, it is becoming imperative that one transmission source remain isolated from other nearby transmission sources. A key component in keeping everyone off of each others toes is an ultra-narrowband notch or bandpass filter.

In most RF and Microwave applications desired signals must be transmitted with minimum loss, while unwanted frequencies need to be strongly attenuated. For most applications, notch or band-pass filters provide adequate protection. While in other cases, a high-pass, or a low-pass filter may be utilized. Occasionally, however, there are interfering frequencies which are particularly close in and strong that need to be eliminated. In these instances, an extremely narrow band high performance notch or bandpass filter is required.

This paper will explore the advantages of selecting one of the four filter types mentioned above, and the benefits of using each choice.

Notch Filter

In its most basic form, a notch filter attenuates a single frequency or small band of frequencies while allowing signals below and above the attenuated band to transmit unobstructed. On a more complex level, a unit can be designed to attenuate several non-adjacent signal bands in one compact unit. A great example of this would be a single notch filter which attenuates the GPS L1 (1575.42 MHz) and GPS L2 (1227.6 MHz) frequencies while allowing relatively unobstructed transmission of frequencies below L1, between L1 and L2, and above L2.

Reactel offers a wide range of notch filters covering from a few MHz to 50 GHz. These include; LC, coaxial ceramics, coaxial cavities, dielectrically loaded cavities, and waveguide topologies. The type of notch filters required for a particular application depends on the frequency, attenuation requirements, loss, bandwidth, and power handling requirements.

Some typical characteristics of notch filters are as follows:

• Low Side Passband: extends all the way to DC

• High Side Passband: extends up to 3 times the center frequency

• VSWR: 1.5:1 typical

• Loss: ≤ 1.0 dB in the Passband.

• 3 dB Bandwidth: 0.05% and up

• Selectivity: Depends on the number of sections

Notch filters with the narrowest bandwidths are designed utilizing dielectric pucks with Q’s in excess of 10,000, and configured with proprietary techniques that we have developed. There are several techniques used to realize broader-band notch filters, and these will vary depending on a particular application.

Bandpass Filter

The exact opposite of a notch filter is a bandpass filter. This type of unit will allow a signal, or band of signals to pass with little attenuation while severely attenuating signals below and above it.

Using the GPS example from above, a unit can be manufactured to pass an extremely narrow band surrounding a GPS frequency, while severely attenuating adjacent signals. On a more complex level, a unit can be designed to pass several non-adjacent signal bands in one fairly compact unit. An example of this would be a dual passband filter which can pass a narrow band surrounding the GPS L1 (1575.42 MHz) and GPS L2 (1227.6 MHz) frequencies while severely attenuating frequencies below L1, between L1 and L2, and above L2.

Reactel offers a wide range of bandpass filters covering from a few MHz to 50 GHz. These include; LC, coaxial ceramics, combline, tubular, coaxial cavities, dielectrically loaded cavities, and waveguide topologies. The type of bandpass filter required for a particular application depends on the frequency, attenuation requirements, loss, bandwidth, and power handling requirements.

Some typical characteristics of bandpass filters are as follows:

• Low Side Reject Band: extends all the way to DC

• High Side Reject Band: extends up to 3 times the center frequency

• VSWR: 1.5:1 typical

• Loss: ≤ 1.0 dB in the Passband.

• 3 dB Bandwidth: 0.05% and up

• Selectivity: Depends on the number of sections

Bandpass filters with the narrowest bandwidths are designed utilizing loaded resonators or configured with proprietary techniques that we have developed. There are several techniques used to realize broader-band bandpass filters, and these will vary depending on a particular application.

Lowpass and Highpass Filter

Lastly, simple highpass and low pass filters are available when attenuation is required on only one side of the desired passband. These units can be constructed using most any available filter topology and offer the same high level of performance demonstrated in the more complex notch and bandpass designs noted above.

Reactel offers a wide range of bandpass filters covering from a few MHz to 50 GHz. These include; LC, coaxial ceramics, combline, tubular, coaxial cavities, dielectrically loaded cavities, and waveguide topologies. The type of bandpass filter required for a particular application depends on the frequency, attenuation requirements, loss, bandwidth, and power handling requirements.

Some typical characteristics of lowpass and highpass filters are as follows:

• Low Side Reject Band: extends all the way to DC

• High Side Reject Band: extends up to 10 times the center frequency

• VSWR: 1.5:1 typical

• Loss: ≤ 1.0 dB in the Passband.

• Selectivity: Depends on the number of sections

So as you can see, there are a variety of methods which can be mixed and matched to provide an optimal solution for your co-location interference problems. The key to an effective solution is a thorough evaluation of the frequency environment so that the filter manufacturer can propose the correct filter for your need.

Reactel, Incorporated was founded in 1979, Reactel is a leader in the design and manufacturing of application specific RF and Microwave filters, multiplexers, switched filter banks and sub-assemblies.

The company operates on a build to order program, however, all units, even first articles, can be turned within 2-3 weeks. There is no “typical order” as order sizes range from one unit for a sophisticated space application to thousands for volume commercial work. The company is housed in a 15,000 square foot, state-of-the-art building located in the Baltimore – Washington High Technology Corridor. The company is ISO 9001:2000 certified, and serves customers world-wide in the defense, commercial, industrial, and medical industries.

Reactel specializes in high reliability, difficult to design filters delivered quickly and on-spec. We welcome the opportunity to review your filter requirement and offer advice on configuring your system.

Reactel, Incorporated
8031 Cessna Avenue
Gaithersburg, Maryland 20879
tel: 301-519-3660
fax: 301-519-2447

e-mail: reactel@reactel.com

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