Last edited 2010.02.17
External cavity filters are used to avoid transmitter broadband noise interference and receiver blocking.
A cavity filter is a resonant filter and can be tuned to obtain different frequency response curves (band pass, pass reject, low/high pass, and notch) to either pass or reject a frequency or band of frequencies. The tuning is dependent on the actual configuration and the need for filtering.
Noise interference and receiver blocking is first of all relevant on co-located sites. Minimum RX-TX antenna separation to avoid filters is approx. 500 metres.
If the antenna separation is less than 500 metres, the need for filtering is dependent on the antenna separation [metres] and channel separation [kHz]. To be able to perform a detailed filter calculation these parameters are essential.
If the antennas are installed very close (<10 metres), vertical antenna separation should be considered to increase the path loss (and to reduce the need for filtering).
The filters can be configured as single channel or RX or TX combiners. Usually a TX combiner is fitted with a dual isolator on each channel. The combiner has at least one cavity filter per channel. In addition, extra cavity filters might be needed on some channels depending on the frequency and RX-TX antenna separation.
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