Band Reject Filter

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An ideal band reject filter completely attenuates frequencies above its lower cut-off frequency and below its upper cut-off frequency. Conversely, for frequencies greater than the upper cut-off and frequencies less than the lower cut-off the signal is allowed to pass. This region is referred to as the passband. In practice, one finds that the attenuation has a finite slope. Thus, either a shape factor or stopband width must be specified. In addition, due to spurious modes both the high side and low side of the passband will eventually degenerate. This necessitates the inclusion of an outer pass bandwidth. Other names for a band reject filter are band elimination filter, band reject filter, band suppression filter, and band reject filter.

Filtronetics, Inc. supplies band reject filters that utilize a wide array of technologies. The most common is LC band reject filter, or lumped element, which is comprised of inductors and capacitors. We also offer crystal band reject filters. For higher frequency applications, microwave band reject filter structures are utilized. Both LC and crystal band reject filters can be made with elliptic or monotonic designs.

The center frequency and stop bandwidth determines the type of technology employed in constructing a band reject filter. A quartz crystal band reject filter is used when a narrow band filter is required. This is due to the crystal’s high, unloaded Q-factor, which is in the range of 10,000 to 100,000. Crystal filters with low insertion loss in the passband are achievable for stopband widths from 0.005% to 0.1% of the center frequency. Moreover, center frequencies are available from 5 KHz to 600 MHz. Passbands are restricted to no more than 25% of the center frequency for wideband crystal band reject filter designs. LC passbands can be much wider.

Microwave band reject filters can be designed as low as 50 MHZ. Conversely; LC band reject structures start at DC and end at 1 GHz. The stopband width is mathematically limited to twice the center frequency. At this upper bandwidth, it becomes a highpass filter. The lower limit is determined by insertion loss limitations and stability. In extreme cases, filters can be as narrow as 0.5% of the center frequency. Filtronetics, Inc. offers custom band reject filters from DC to over 2 GHz.

Our band reject filters have been utilized in systems that are in space, military, undersea, airborne, testing, and commercial applications. We can produce from one to over a million band reject filters for your application. Band reject filters are available in Surface mount (SMD), Through-hole, and connectorized (BNC, TNC, SMA, or N filter) packages.