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- 16th February 2017, 10:46 #1

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## Difference between PSD ( power density spectrum ) and Spectrum measurement in Cadence

Hello there,

I am doing a transient analysis on a 3rd order SD modulator ( MASH topology ) and i want to plot the power spectrum of the quantization noise.

Thing is, that i am confused on how to do that.

My first try was sending the output sequence to Measurement tab---> Spectrum and ploting the curve.

My second try was sending the output sequence to the calculator and using the psd() function , afterwards use db10() function and finally plotting.

The 2 above mentioned curves have the same slope but a constant difference between them of about 40 dB ( e.g the first curve has peak at -20 dB and the second at -60 dB).

Could you enlighten me as to which is the correct way?

Thank you

- 16th February 2017, 12:12 #2

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## Re: Difference between PSD ( power density spectrum ) and Spectrum measurement in Cad

Use correct terminology.

There is no tool Cadence.

You use db20( dfft() ) not db10( psd() ) there.

However they have to be**almost**same if you use Cadence OCEAN functions.

The second plot is dB10(psd()) and is generated if I send the output data to the calculator and use the psd() function.

The 2 plots are far from same though, they have around 40 dB constant difference.

- 16th February 2017, 12:12

- 16th February 2017, 12:23 #3

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## Re: Difference between PSD ( power density spectrum ) and Spectrum measurement in Cad

**Use correct terminology.**

There is no tool Cadence.

**Surely answer my questions in your previous thread.**

MASH ( SD modulator) power spectrum dc term

You use db20( dfft() ) not db10( psd() ) there.

However they have to be**almost**same if you use Cadence OCEAN functions, if you set parameters correctly.

http://www.designers-guide.org/Forum...1242749411/3#3

**Surely answer my questions in your previous thread.**

MASH ( SD modulator) power spectrum dc term

Show us all results and their settings correctly with including the following.

SciLab power spectral density of quantization noiseLast edited by pancho_hideboo; 16th February 2017 at 12:28.

- 16th February 2017, 14:13 #4

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## Re: Difference between PSD ( power density spectrum ) and Spectrum measurement in Cad

I post again in case my previous posts weren't submitted correctly.

For all the following plots , the number of accumulator bits are N=14 and the input fractional word is 2037.

1) Scilab , using fft.

2)Scilab, using pspect() function ( equivalent to periodogram function in Matlab )

3) Cadence Virtuoso, using fft() ( from Measurement tab---> Spectrum )

4) Cadence Virtuoso, using psd() function ( from calculator )

5) Ideal curve, plotted in Scilab

- 16th February 2017, 14:13

- 16th February 2017, 15:43 #5

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## Re: Difference between PSD ( power density spectrum ) and Spectrum measurement in Cad

Show us arguments of all functions.

- 16th February 2017, 16:16 #6

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## Re: Difference between PSD ( power density spectrum ) and Spectrum measurement in Cad

Ok, for all cases N=14 and input 2037 as before. Also sampling frequency is 1Ghz (10^9), let's call it Fs.

"Cout" is the output carry sequence with range [-3,4] from the 3rd order SD modulator and length 2*2^(N+1)=65536 samples ( 2 full sequence cycles as with an odd input , max sequence 2^(N+1) is guaranteed).

__For SciLab:__

Frequency construction : f=Fs*(1:length/2)/length -- ( frequency vector from 0 to Fs/2)

**1) SciLab plot using fft()**

spectrum=abs(fft(Cout)/(length/2))

spectrum=spectrum(1:length/2)^2

plot2d("ln",f(1:length-1),10*log10(spectrum)) -- (argument "ln" means that the x axis is logarithmic and the y axis is normal )

**2) SciLab plot using pspect() function**

spectrum=pspect(length/2,length,'hn',Cout)

plot2d("ln",f,10*log10(Cout(1:length)))

The arguments in pspect() function are : offset of each data window,number of points in window, window type (here Hanning), sampled vector)

**3) Cadence Virtuoso using fft()**

Start time : x ns

Stop time : (x + 2*length) ns

sample count/Freq= 2*length -- ( same result if i chose "length" sample count , but the maximum frequency would be Fs/2 )

Start/end Freq= 15.26k to 1G

Window type : Rectangular -- ( little to no difference with Hanning)

**4) Cadence Virtuoso using psd()**

Start/Stop : Same as above

Number of samples : 2*length

Window : Hanning

Smoothing factor : 1

Windows size : 2*length

Detrending Mode : Mean

Coherent gain : default

Coherent gain factor : 1

**5) Ideal curve**

spectrum= 1/(12*2^(N+1))* (2*sin(pi*f/Fs))^6 -- ( 3rd order SD modulator )

plot2d("ln",f,10*log10(spectrum))

- 16th February 2017, 16:16

- 16th February 2017, 16:43 #7

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## Re: Difference between PSD ( power density spectrum ) and Spectrum measurement in Cad

See MASH ( SD modulator) power spectrum dc term

Sorry, I don't have Scilab, so I don't know definition of psd functions of Scilab correctly.

- 16th February 2017, 16:50 #8

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## Re: Difference between PSD ( power density spectrum ) and Spectrum measurement in Cad

http://https://searchcode.com/codesearch/view/25398262/

This is the source code of pspect() function in SciLab ( that as far as i understand is the equivalent to psd())

- 16th February 2017, 17:06 #9

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## Re: Difference between PSD ( power density spectrum ) and Spectrum measurement in Cad

Currently psd() is not recommended to use in MATLAB.

https://jp.mathworks.com/matlabcentr...psd-and-pwelch

http://www.designers-guide.org/Forum...1405235104/2#2

pwelch() give dBV/Hz, on the other hand psd does not.Last edited by pancho_hideboo; 16th February 2017 at 17:12.

- 17th February 2017, 13:54 #10

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## Re: Difference between PSD ( power density spectrum ) and Spectrum measurement in Cad

Last edited by pancho_hideboo; 17th February 2017 at 14:02.

- 17th February 2017, 13:54

- 17th February 2017, 14:03 #11

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- 17th February 2017, 19:08 #12

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## Re: Difference between PSD ( power density spectrum ) and Spectrum measurement in Cad

- 20th February 2017, 09:07 #13

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## Re: Difference between PSD ( power density spectrum ) and Spectrum measurement in Cad

As you can see on the screenshot that i uploaded on the other thread, i have the correct values on the required fields.

( Start time : 2.125 ns

Stop time : (2.215 + 2*2^15) ns )

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