LNAs with integrated notch filters for interstage-filter-less FDD front-ends


Current mobile wireless devices have to incorporate multiple communication bands and standards, respectively. In particular, this problem emerges for smartphones which are meant to be used worldwide. Enabling the mobile terminals to cover all desired applications does not only result in an increased chip area due to more RF frontends, but also in more PCB area for external filters. In the case of frequency division duplex (FDD) systems, common handhelds use duplex filters to separate the receive signal (RX) from the transmit signal (TX).

These filters are not tuneable, thus require one duplexer per frequency constellation. The selectivity provided by the duplexer must reach a certain level such that third order mixing products are suppressed which are caused by the TX signal leaking into the receiver and an external interferer in a certain frequency distance, thus degrading sensitivity of the whole receiver.
Moreover, noise generated on the TX side leaks into the receiver. This is most critical for small duplex distances. The common way to cope with that is to place an external low noise amplifier (LNA) followed by an additional surface acoustic wave (SAW) filter. Finally, this means there are 2 area consuming filters per band on the PCB for RX and one additional for TX.

LNAs with integrated notch filter seem the most promising way to provide enough selectivity even for LTE at the moment. Basically a tuneable integrated notch filter is added to the LNA of the receiver frontend to attenuate the strong TX interferer early in the receive chain such that the following stages do not have to cope with it.

Dirk Bormann < >