Fixed Notch Filters
By CMT

The continuous expansion of the cellular phone and wireless infrastructure has resulted in the need for increasingly higher performance filters. As more and more service providers collocate on limited tower space, they can and do interfere with each other. Usually, the last provider on the tower will have to add filtering to his system or to an incumbent’s system to eliminate any interference before he can operate. The quickest way to eliminate this interference is to place a notch filter between the transceiver and the antenna. Modern notch filters use very high Q dielectric resonators to achieve high levels of rejection with minimal insertion loss over the desired frequency range.

As the number of service providers using the 800 MHz frequency spectrum has increased, collocation interference problems have also increased. This happens as a large number of subscribers using cell phones saturate a cell site, causing spectral regrowth. The power that spills into the adjacent frequency band interferes with neighboring service providers. One way to effectively eliminate this interference is to insert a band reject filter (also referred to as a “notch” filter) between the transmitter and/or receiver and the antenna. The notch filter provides a specific amount of attenuation over a specified band of frequencies, reducing the unwanted interference to a tolerable level while passing the desired frequency range with minimal attenuation.

Band reject filters can be made by placing series tuned circuits (resonators) in shunt along a transmission line at quarter wavelength intervals and tuning all of them to resonate at a desired frequency. (Figure 1)

This method is called synchronous tuning. (See Figure 2.) While effective, this approach is not very efficient. That is, one needs a large number of resonators to achieve a desired level of attenuation over a specific frequency range. A more efficient approach is to stagger the resonant frequencies like pickets in a fence and vary the electrical length of the transmission lines between the resonators. This method produces a frequency response more commonly known as elliptic. (Figure 3)

The elliptic function response is the most efficient of all common filter functions. It provides for an equal ripple response in the passband and equal minima in the stopband with the least number of resonators. The least number of resonators solution achieves the lowest amount of passband attenuation for a given notch attenuation level, thus preserving transmitter power and system noise figure.

There are situations when the passband frequency and reject frequency are very close to each other and the response needs to favor one side of the notch. That is, either the low side or high side of the notch needs to have a steeper slope. This is accomplished by altering the line lengths between the resonators. Very steep rejection rates between the passband and stopband can be achieved using this approach. (Figures 4 and 5)

It is critical that very high Q resonators are used when designing notch filters when the passband and high rejection frequencies are very close to each other. High Q resonators are necessary to achieve the lowest possible insertion loss in the passband while providing a sharp transition to the required rejection level in the stopband. For this reason, most high performance band reject filters are designed with dielectric resonators which can achieve Q values of 35000 and higher. (Figure 6)

Figure 7 illustrates how the resonator Q affects the shape of the filter response.
It is important to point out that any filter, especially elliptic filters, must be tuned properly to obtain the best possible performance. Precise tuning achieves very low VSWR in the passband and a uniform attenuation level over the notch frequency range. The examples shown are notch filters that have been designed and manufactured at Commercial Microwave Technology along with data to show actual performance.

Conclusion
A notch filter is an effective way to reduce or eliminate unwanted signals from radiating from a transmitter or interfering with a receiver. A well designed notch filter will provide high attenuation in the stopband with minimum insertion loss in the passband. The shape of the notch can be designed to achieve a steep slope on either high side or low side. A notch filter needs precision tuning to achieve the best performance.

CMT

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