3.2 System Level Considerations 57signal will be mixed down to IF1 at 70 MHz. Thus, fLO is adjustable between 972 MHz and 998 MHz to allow signals between 902 MHz and 928 MHz to be received. The image occurs 70 MHz above fLO. The worst case will be when the image frequency is closest to the filter frequency. This occurs when the input is at 902 MHz, the LO is at 972 MHz, and the image is 1,042 MHz. The required filter order n can be calculated by solving (3.28) using fBW = 26 MHz and Df = 70 + 44 + 13 = 127 MHz as follows:´==´DdBBW25.0520 log(2 /)Anf fSince the order must be an even number, a sixth order filter is used and total attenuation is calculated to be 59.4 dB.Now with the original image rejection specification, the -40-dBm image signal will be attenuated by 50 dB so after the filter, this signal will be -90 dBm. This will be mixed on top of the desired signal, which has a minimum power level of -75 dBm, giving a signal to distortion ratio of -75 dBm – (-90 dBm) = 15 dB. This is higher than the required SNR of 9.5 dB so this should be enough image rejection. 3.2.5 Blockers and Blocker FilteringThe receiver must be able to maintain operation and detect the desired signal in the presence of other signals, often referred to as blockers. These other signals could be of large amplitude and could be close by in frequency; for example, they could be signals in other channels of the same radio band. This is an example of the so-called near-far problem that occurs when the desired signal is far away and one or more interfering signals are close by, which are larger than the wanted signal. A large blocker must not saturate the radio and therefore the 1-dB com-pression point of the radio must be higher than the blocker power level to avoid saturating the radio. The intermodulation products of blockers can also be a very big problem. Con-sider the case where a desired channel is detected at its minimum power level. Two close by channels are also received at their maximum receive power. If these signals are at frequencies such that their IM3 products fall on top of the desired signal, they Figure 3.12 Signal spectrum for filter example.