3.2 System Level Considerations 59Solution: With nonlinearity, third-order intermodulation between the pair of blockers will cause interference directly on top of the signal. The level of this disturbance must be low enough so that the signal can still be detected. The other potential problem is that the large blocker at –23 dBm can cause the amplifier to saturate, rendering the amplifier helpless to respond to the desired signal, which is much smaller. In other words, the receiver has been blocked.The blocker inputs at – 43 dBm will result in third-order intermodulation compo-nents, which must be less than –113 dBm (referred to the input) so there is still 11 dB of SNR at the input. Thus, the third-order components (at –113 dBm) are 70 dB below the fundamental components (at – 43 dBm). Using (2.59) with Pi at – 43 dBm and [P1 – P3] = 70 dB results in IIP3 of about -8 dBm, and a 1-dB compression point of about –18 dBm at the input. Thus, the single input blocker at –23 dBm, is still 5 dB away from the 1-dB compression point. This sounds safe—however, there will now be gain through the LNA and the mixer. The blocker will not be filtered until after the mixer, so one must be careful not to saturate any of the components along this path.Now after the signal passes through the front end and is downconverted to the IF and passed through the IF filter the spectrum will be as shown in Figure 3.14. In this case, the signal experiences a 20 dB gain, while the two closest blockers experi-ence a net gain of -10 dB and the third blocker experiences a net gain of -30 dB. If no filtering were applied to the system then the IIP3 of the first IF block would need to be roughly -8 dBm + 20 dB = 12 dBm. With filtering, the IM3 products from the two closest blockers must be lower than -93 dBm. Using (2.59), with Pi at –53 dBm and [P1 – P3] = 40 dB, results in IIP3 of about -33 dBm and a 1-dB compression point of about – 43 dBm for the IF block. Thus it is easy to see the dramatic reduc-tion in required linearity with the use of filters. Note that the 1-dB compression point is still 10 dB above the level of any of the blocking tones. 3.2.6 The Effect of Phase Noise on SNR in a ReceiverThe blocking signals can cause problems in a receiver through another mechanism known as reciprocal mixing. For a blocker at an offset of Df from the desired signal, Figure 3.14 Minimum detectable signal and blocker levels after the IF filter.