Author : Criterion Why do we need external filter In most wireless radios a weak desired signal can be accompanied by interferers that can be significantly stronger[1]. For example, as liiustrated below, if phone1 transmits maximum GSM power, i.e. 33 dBm. Let’s assume the antenna efficiency of phone1 and phone2 are both 50%(i.e. 3dB loss). Consequently, the phone2 receiver circuit will suffer from 0 dBm interferer. We usually call the strong interferer blocker or jammer. 1Author : Criterion Blockers can drastically degrade the receiver performan ce through compressin g its gain an d hen ce in creasin g its n oise figure, as shown below[3] : As shown below[8], stron ger the blocker, more the gain reduction . 2Author : Criterion Accordin g to cascade n oise figure formula[4,6] : In gen eral, less the LNA gain , higher the n oise figure an d worse the sen sitivity. Take SKY74092-11 of SKYWORKS for example, its P1dB is -5 dBm, an d may be saturated by the 0 dBm blocker as men tion ed above[2]. 3Author : Criterion In Zero-IF (ZIF) architecture receiver, RF down con verts to baseban d directly. An d DC offset is on e of n on lin ear effects as well[2,3]. Thus, if the stron g blocker saturates receiver, the DC offset due to receiver n on lin earity also aggravates sen sitivity. Besides, in ZIF architecture, even though a stron g out-of-ban d blocker, on ce amplified by the LNA, may fin d a path to the LO-in put port of the mixer, thus on ce again producin g self-mixin g, which is DC compon en t at the mixer output an d aggravates sen sitivity as well[5,8]. As shown below : 4Author : Criterion In addition , if the blocker has high phase n oise, which also con tributes t...
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