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17MHz 的 tone 會不會是系統OSC or XTAL 的 16.667MHz clock feedthrough noise?- I! D/ ]7 B& O9 U
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因為即使是under sampling, alias image in first order term: (FS=54MHz, Ft=44MHz, Nyquist band=27MHz)/ A' q. |/ Q2 [8 \' n6 j! Z
FS-Ft=54-44=10MHz
& u: t4 ~9 j- _+ p' M, s. ~FS+Ft=54+44=98MHz (folding 回 Nyquist band=10MHz)1 R, \1 s, s, ], m$ s* Z
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2nd order term:4 A( l( x6 f" ^
2FS-Ft=108-44=64MHz (folding 回 Nyquist band=10MHz)
7 w, `; N& w+ N: V2FS+Ft=108+44=152MHz (folding 回 Nyquist band=10MHz)- ?4 |$ @% [) {% _
|FS-2Ft|=54-88=34MHz (folding 回 Nyquist band=20MHz)8 g: G& P; _2 z1 o) \7 a: U
FS+2Ft=54+88=142MHz (folding 回 Nyquist band=20MHz)
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2nd~7th Harmonic:8 w+ z# b0 w; E7 e4 J
2nd = 20MHz. J" x* A( K C) w. L+ @
3rd = 30MHz (folding 回 Nyquist band=24MHz)4 C8 h4 i% S* l- O6 U5 X9 y
4th = 40MHz (folding 回 Nyquist band=14MHz)
, b# o+ J' `7 _! C$ T5th = 50MHz (folding 回 Nyquist band=4MHz)+ s1 ]7 E: y3 B
6th = 60MHz (folding 回 Nyquist band=6MHz): x, {, s! M' U4 S9 b$ o6 o1 I
7th = 70MHz (folding 回 Nyquist band=16MHz)
: k# l6 T/ g7 E0 `5 U' S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,
* `+ \' n* D0 R w4 y& {: J/ I+ f但是就是沒有17MHz的theoretical noise source.5 ^* |* j% g- J4 j
來個Maxim AN928 anti-aliasing filter的文件:
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[ 本帖最後由 DennyT 於 2007-11-10 12:00 AM 編輯 ] |
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