Mon. June 11, 2007
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.
