關於undersampling ADC的問題

jeffsky

設計的undersampling ADC用54MHz sample rate採樣44MHz signal，出現17MHz的雑訊，請問各位先進可否指點？
或者能否提供相關資料？因爲之前曾經看到過類似的解釋現在急需卻找不到了。：（

jeffsky

I am really thankful to DennyT's reply, it is a reasonable and constructive explanation on this issue. I will try to pull higher supply voltage to see if it can be relieved or removed or not. Thank you very much.

DennyT

有此一說:
當Fin接近於Fs/2時, ADC sample的電壓slew at full scale, 此時奇數點之間的壓差其實很小 (偶數點亦同), 但是相鄰兩點間的壓差卻很大, 測試上又稱為ADC envelope test, 而在此測試中被引進來的 "beat frequency" 會被視為noise, 使SNR下降.

* ?* D% w% Q5 Q也就是說, 若ADC內front end的PGA or buffer Amp slew rate不足, 此缺陷便很容易在此測試中被突顯出來.
# b: E9 Y$ i, b0 j
就系統面而言, 拉高ADC的AVDD看看有沒有救, (ADC PAD_VDD反而要調低, 除了降EMI外也可拉低系統noise floor).

[ 本帖最後由 DennyT 於 2007-11-12 10:29 PM 編輯 ]

jeffsky

Thank you for your kind reply.
* j+ w# }5 Y  c* n  }) P( _I have ever seen a paper or something else to say that when the signal frequency approaches the sample rate, the beat frequency is easy to appear at Fsig-Fs/2, for example, this frequency is 17MHz. It is as if sampling frequency is 27MHz. It is really a puzzle.

DennyT

17MHz 的 tone 會不會是系統OSC or XTAL 的 16.667MHz clock feedthrough noise?

- e3 |# B( L5 T& ]* ~因為即使是under sampling, alias image in first order term: (FS=54MHz, Ft=44MHz, Nyquist band=27MHz)
FS-Ft=54-44=10MHz
FS+Ft=54+44=98MHz (folding 回 Nyquist band=10MHz)
7 j5 b+ g  T# o9 U5 [
2nd order term:
4 ?2 @7 ]/ y9 u2FS-Ft=108-44=64MHz (folding 回 Nyquist band=10MHz)
2FS+Ft=108+44=152MHz (folding 回 Nyquist band=10MHz)
|FS-2Ft|=54-88=34MHz (folding 回 Nyquist band=20MHz)
) v4 }1 e/ V2 b5 ]  G; i6 mFS+2Ft=54+88=142MHz (folding 回 Nyquist band=20MHz)

& s. `6 a7 r5 g+ u$ y2nd~7th Harmonic:
2 M; t/ |- L: I2 @5 ]7 O4 w2nd = 20MHz
; l2 p+ v6 w- k8 N. m* y0 m3rd = 30MHz  (folding 回 Nyquist band=24MHz)
3 ^# z2 L% z+ ]9 i4th = 40MHz  (folding 回 Nyquist band=14MHz)
5th = 50MHz  (folding 回 Nyquist band=4MHz)
# ^7 O9 W, C4 r  q4 y3 b% v  P6th = 60MHz  (folding 回 Nyquist band=6MHz)
  ?$ I9 E9 Q  g" c7th = 70MHz  (folding 回 Nyquist band=16MHz)
( H9 \1 k8 ]: E* L: `# \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,
# w6 H( a! S" b1 Q但是就是沒有17MHz的theoretical noise source.
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 ]

jeffsky

原帖由 jeffsky 於 2007-11-8 02:07 PM 發表
設計的undersampling ADC用54MHz sample rate採樣44MHz signal，出現17MHz的雑訊，請問各位先進可否指點？
或者能否提供相關資料？因爲之前曾經看到過類似的解釋現在急需卻找不到了。：（

0 x9 {& v6 G: j, P0 ^, E# }! n？？？？？？？？？？？？？？？？？？？？？？？？？？？？？？？？？？？？？？？？/