NO 1/f noise on my PSD

[swkimaz]

Guys, please help me clear this issue.
I have a bit stream from sigma delta and input is DC.
I am using straight FFT and periodogram(or walsh) to plot PSD.
But flicker noise does not appear on my PSD.(It should be in)
I think there is a way to plot 1/f noise for DC input.
Please let me know.

 

[wenadinho]

If u are doing some fft on transient signal analysis waveform, flicker noise won't appear. Try using pnoise feature from spectreRF

 

[swkimaz]

Thanks wenadinho but my bit stream is from chip in Lab. And I am using Matlab to make plot PSD with the data. It is not done on spice-like simulator.
Any idea?:idea:

 

[steer]

First, make sure your spectrum covers low enough frequencies. If it does not go low enough, then 1/f noise may be masked by quanization noise.
Second, make sure your sigma-delta loop does not lose noise shaping abilities down to low frequencies. Badly designed sigma-delta might have so high low-frequency noise that you are not able to see any other noises.
Third, make sure the noise coming from digital portion is not dominating. If digital interference is too high, you can not see anything else.
Fourth, make sure your reference voltage is clean enough. Noisy reference ruins sigma delta low-frequency noise, so you can not see other noises.
If all the above is not a problem, then read further.
Does your sigma-delta ADC use auto-zeroing integrator at the input? If so, 1/f noise is partially eliminated, while other part is spread into white noise.
Try to increase your DC input (or AC input in case of band-pass sigma-delta), then you might see 1/f noise coming from your reference.