Hi Aaron: Thanks for your reminding. I am pretty sure my input is quite low compared to the full scale.
Thanks again. Wan ________________________________ From: Aaron Parsons [mailto:[email protected]] Sent: Wednesday, 13 January 2010 3:19 PM To: Dan Werthimer Cc: [email protected] Subject: Re: [casper] DC part of FFT output Another thing to think about is that for signals that are strong relative to the number of bits you are using to represent them, you can see the effects of the asymmetric distribution of values around zero for two's complement numbers. For example, in the CASPER correlators, where we use four bits to represent values, we could have values ranging from 7 to -8. Since +8 does not exist to offset -8 values that sometimes occur, we see an average DC offset if we do not saturate negative values at -7. Prohibiting the -8 value keeps the distribution of values symmetric and solves our DC bias problem in the correlator. On Jan 12, 2010, at 7:37 PM, Dan Werthimer <[email protected]<mailto:[email protected]>> wrote: hi wan, what are the levels of this output? if they are the least significant bits, then this is likely from round off noise. best wishes, dan On 1/12/2010 6:45 PM, <mailto:[email protected]> [email protected]<mailto:[email protected]> wrote: Hi All: I use a matlab sine wave generator to generate a sine wave as an input to PFB and FFT. But I find I get a little DC output and a few small steps from FFT output. I am pretty sure there is no DC added into the input signal. So where these DC could be from? And why there is some small steps on the noise floor? For the details of spectrum output, please see the attached. Thanks Wan
