A frequency agile LNA for cognitive radio applications in the UHF band

A frequency selective LNA for the UHF band is presented that uses an N-Path filtering concept to realize frequency agile filtering between 600 MHz and 800 MHz. A frequency generator like an on-chip PLL can be used to choose the filter band without the need for an additional post-production calibration step. The LNA concept uses the advantages of the source degenerated as well as the shunt feedback LNA. The characteristic of the LNA and the filter function is described in detail. Measurement results from a prototype chip in 90 nm CMOS prove the presented theory. At 700 MHz the LNA offers a gain of 18 dB without filtering and 17 dB with filtering applied. The noise figure is as low as 3.1 dB and increases to 3.9 dB with filtering. The linearity in terms of the intermodulation point IP3 is as high as 0 dBm. A single filter gives 9.5 dB selectivity at 40 MHz frequency offset. Using only a source inductor, the LNA area occupation is lower than 0.2 mm².