11/14/2021 3F_13_Multiple IF stages.docx 1/8 Jim Stiles The Univ. of Kansas Dept. of EECS Multiple IF Filters Say you are tasked with designing an FM radio receiver, and the specification calls for a very high level of image rejection (e.g., 60 dB). You are pondering two design options. Option 1: High-side tuning, with an IF center frequency 0IFfof 27 MHz. 88 108 142 72 82 FM band Image band for fIF=27 ( )psfT 3rd-order band for fIF=27 162 RFfMHz11/14/2021 3F_13_Multiple IF stages.docx 2/8 Jim Stiles The Univ. of Kansas Dept. of EECS The 3rd-order band extends into the FM band! Option 2: Low-side tuning, with an IF center frequency 0IFfof 23 MHz. Each option provides sufficient image rejection, but the 3rd-order rejection is poor/awful. RFfMHz147 42 88 108 107 62 FM band Image band for fIF=23 ( )psfT 3rd-order band for fIF=2311/14/2021 3F_13_Multiple IF stages.docx 3/8 Jim Stiles The Univ. of Kansas Dept. of EECS Divide FM into lower and upper sub-bands For both options, you could decrease the IF center frequency to improve 3rd-order signal rejection, but this will cause the image rejection to be unacceptably reduced. You need a solution—and you need it fast! A possible solution reveals itself—if we again consider FM sub-bands. Consider design option 1: RFfMHz88 108 142 72 82 FM bands Image bands for fIF=27 ( )psfT 3rd-order bands for fIF=27 162 9811/14/2021 3F_13_Multiple IF stages.docx 4/8 Jim Stiles The Univ. of Kansas Dept. of EECS Option 1 works pretty well for the lower sub-band Looking closely, we find that this solution works well for the lower FM sub-band. Note for this option, suppressing 3rd-order products is easier for the lower FM band. RFfMHz88 108 142 72 77 Low FM band Low Image band for fIF=27 ( )psfT Low 3rd-order band for fIF=27 162 9811/14/2021 3F_13_Multiple IF stages.docx 5/8 Jim Stiles The Univ. of Kansas Dept. of EECS Now for design option 2 And for design option 2: Looking closely, we find that this solution works well for the upper FM sub-band. RFfMHz127 52 147 42 88 108 107 62 FM bands Image bands for fIF=23 ( )psfT 3rd-order bands for fIF=2311/14/2021 3F_13_Multiple IF stages.docx 6/8 Jim Stiles The Univ. of Kansas Dept. of EECS Design option 2 works well for the upper sub-band Note for this option, the suppressing 3rd-order products is much easier for the upper FM band. RFfMHz127 52 147 42 88 108 62 High FM band High Image band for fIF=23 ( )psfT High 3rd-order band for fIF=2311/14/2021 3F_13_Multiple IF stages.docx 7/8 Jim Stiles The Univ. of Kansas Dept. of EECS We tune to only one station at a time Thus, a solution is to use two Intermediate Frequency filters—one when tuning to a station in the lower FM sub-band, and the other when tuning to a station in the higher FM sub-band. RFfMHz127 52 147 42 88 108 62 High FM band High Image band for fIF=23 ( )psfT High 3rd-order band for fIF=23 Low 3rd-order band for fIF=27 Low FM band Low Image band for fIF=2711/14/2021 3F_13_Multiple IF stages.docx 8/8 Jim Stiles The Univ. of Kansas Dept. of EECS 27 MHz IF for RF below 98 MHz; 23 MHz IF for RF above 98 MHz For this example, your final design might be: IF filters Preselector filter S P D T S P D T Tuning and Control 023IFMHzf = 88 108−027IFMHzf
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