SAWless零中频接收机之考虑与剖析

Why do we need external filter

In most wireless radios a weak desired signal can be accompanied by interferers

that can be signi?cantly 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.

Blockers can drastically degrade the receiver performance through compressing its gain and hence increasing its noise ?gure, as shown below[3] :

As shown below[8], stronger the blocker, more the gain reduction.

According to cascade noise figure formula[4,6] :

In general, less the LNA gain, higher the noise figure and worse the sensitivity. Take SKY74092-11 of SKYWORKS for example, its P1dB is -5 dBm, and may be saturated by the 0 dBm blocker as mentioned above[2].

In Zero-IF (ZIF) architecture receiver, RF downconverts to baseband directly. And DC offset is one of nonlinear effects as well[2,3]. Thus, if the strong blocker saturates receiver, the DC offset due to receiver nonlinearity also aggravates sensitivity.

Besides, in ZIF architecture, even though a strong out-of-band blocker, once amplified by the LNA, may find a path to the LO-input port of the mixer, thus once again producing self-mixing, which is DC component at the mixer output and aggravates sensitivity as well[5,8]. As shown below :