CIC output spectrum ....help please

[gggould]

Hi all,

The following is CIC output spectrum (Ycic) obtained from passing a 16MHz SDM signal (Ysdm) through CIC decimation filter (Hcic).

1. I would expect the Ycic=Hcic*Ysdm, but where does those humps around 1.8MHz come from?
My guess is alias, but I don't know if it is from aliased_Ysdm * Hcic , or Ysdm * aliased_Hcic, or aliased_Ysdm * aliased_Hcic ...

2. Why doesn't the 250KHz signal tone get aliased to 1750KHz??

Thanks

 

[zorx]

1. Can you post separate figures and corresponding parameters of input signal, CIC filter and output signal?
When you say 16MHz input signal, do you mean that it occupies frequency band of 16MHz, or the sampling frequency is 16MHz ?
When the sample rate conversion is performed, the signals bandwidth is unchanged, only the sampling frequency is changed, so plotting those signals on the same plot (on the same frequency axis) can be confusing.

 

[gggould]

Sorry about the confusion. The input to SDM is 250KHz and oversampled by 16MHz clk before going into the CIC. Then CIC decimate to 2MHz.

 

[mbello240]

hello,
pls i'm stuck trying to generate the SDM signal as input to the CIC filter,any help?is there a direct way of generating the signal in matlab