Problem of Image Figure 4.9.Problem of image in heterodyne downconversion. [View full size image] Desired Image Signal LPF (Dim 01F 0 ⊙F cosOLot 0L0 ① ωjim=①in+2ωIF ))))
Problem of Image
Down-converted Spectrum Figure 4.10.Downconversion of two channels for (a)@Lo<@1,(b)@Lo slightly above @1,(c)@Lo midway between @1 and w2,and (d)@Lo>02. [View full size image] -02 +01 +0 +02 -0L0 =0o 9 -204 1+ (a) +0 +LO -0 +01-L0 0 0 -01+0儿0
Down-converted Spectrum
High-side and Low-side Injection High-side injection @Lo win, Super heterodyne Low-side injectionωLo<ωim, Infra heterodyne
High-side and Low-side Injection High-side injection �!" > �#$, ◦ Super heterodyne Low-side injection �!" < �#$, ◦ Infra heterodyne
Example 4.5. The designer of an IEEE802.11g receiver attempts to place the image frequency in the GPS band,which contains only low-level satellite transmissions and hence no strong interferers.Is this possible? Solution The two bands are shown in Fig.4.11.The LO frequency must cover a range of 80 MHz but,unfortunately,the GPS band spans only 20 MHz.For example,if the lowest LO frequency is chosen so as to make 1.565 GHz the image of 2.4 GHz,then 802.11g channels above 2.42 GHz have images beyond the GPS band. Figure 4.11.Attempt to make the GPS band the image of an 11g receiver. GPS Band 11g Band 1.565 1.585 2.400 2.480 f (GHz) ))
Example 4.6. A dual-mode receiver is designed for both 802.11g and 802.11a.Can this receiver operate with a single LO? Solution Figure 4.12(a)depicts the two bands.We choose the LO frequency halfway between the two so that a single LO covers the 11g band by high-side injection and the 11a band by low-side injection [Fig.4.12(b)].This greatly simplifies the design of the system but makes each band the image of the other.For example,if the receiver is in the 11a mode while an 11g transmitter operates in close proximity,the reception may be heavily corrupted.Note that also the IF in this case is quite high,an issue revisited later. Figure 4.12.(a)11g and 11a bands,(b)choice of fLo. 11g Band 11a Band 和市”中中、 2.40 2.48 5.15 5.355.7255.825 (GHz) (a) fLo f (b)