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17MHz 的 tone 會不會是系統OSC or XTAL 的 16.667MHz clock feedthrough noise?" \' i/ ~- n& [9 M
" N# G) c' l/ H" a. V- V因為即使是under sampling, alias image in first order term: (FS=54MHz, Ft=44MHz, Nyquist band=27MHz)/ Y6 G+ U% w, S3 @5 N# ~: M
FS-Ft=54-44=10MHz% d" Z; x' o7 l' c8 K
FS+Ft=54+44=98MHz (folding 回 Nyquist band=10MHz)
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2nd order term:% D& l/ _/ t" @5 ~1 p
2FS-Ft=108-44=64MHz (folding 回 Nyquist band=10MHz)3 d4 Y x2 b: t' C# Z/ b- ?
2FS+Ft=108+44=152MHz (folding 回 Nyquist band=10MHz)
# O. m4 p, a6 X* Y, s, D t$ F|FS-2Ft|=54-88=34MHz (folding 回 Nyquist band=20MHz)
; j* D& u! s+ t, C5 ^FS+2Ft=54+88=142MHz (folding 回 Nyquist band=20MHz)
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$ `* I# D. g$ J6 d! v3 ~( f, ]2nd~7th Harmonic:6 i, S" q; S# L+ e0 v4 r- i
2nd = 20MHz
7 N4 t5 ~5 w' M5 ]3rd = 30MHz (folding 回 Nyquist band=24MHz)
7 Z/ H$ B. x+ E8 W4 K+ _# R4th = 40MHz (folding 回 Nyquist band=14MHz)6 p) K& l: F9 r, r/ C% @$ U E2 A) f
5th = 50MHz (folding 回 Nyquist band=4MHz): u( T8 g4 P4 z$ ]0 o m
6th = 60MHz (folding 回 Nyquist band=6MHz)2 D' Z/ v1 L. h3 z1 O
7th = 70MHz (folding 回 Nyquist band=16MHz) z5 ?1 s0 ?# m+ t2 M9 i9 Y
你附的Frequency domain plot 在6th之後已在noise floor之下了, 故之後的harmonic可略去不計,
L) n; K, d m7 C8 I. ^9 R# O但是就是沒有17MHz的theoretical noise source.' | M+ t8 U/ p. I( S
來個Maxim AN928 anti-aliasing filter的文件:2 k" u3 p9 x% G0 M8 h# U
. u6 m. c6 W! R$ I. s: [, K[ 本帖最後由 DennyT 於 2007-11-10 12:00 AM 編輯 ] |
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