| This technology was developed in response to
the need for very low loss RF filters at 900 MHz and above,
for mobile phone applications. In contrast to other devices,
the IEF uses elements that are connected electrically. |
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The device circuit is a sequence of resonators connected alternately
in series and in parallel, as indicated for a simple case in Fig.1.
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The device is designed such that,
in the passband, the series resonators have low impedance
and the parallel resonators have high impedance, thus giving
low loss. Outside the passband the resonators behave like
capacitors whose values determine the rejection. The resonators
are usually long transducers with strong internal reflections,
plus a grating at each end, so that each resonator is basically
a one-port resonator as in Fig.2. |
Using a strong-coupling substrate, such as 42° lithium
tanatalate, this filter can give very low loss, e.g. 1 dB
at 1 GHz, with up to 5% bandwidth. However, the stopband
rejection is not generally as good as other filter types.
A typical Ladder frequency response is shown in Fig.3.
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