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Simulators >> Circuit Simulators >> SHA evaluation
https://designers-guide.org/forum/YaBB.pl?num=1232444102

Message started by seahs on Jan 20^th, 2009, 1:35am

Title: SHA evaluation
Post by seahs on Jan 20^th, 2009, 1:35am

The output waveform is as follows. It looks quite good. But the measured THD is quite bad. I added the component 'fourier' in analogLib to measure THD. The result is as follows:

Harm: Absolute Absolute Relative Relative
Magnitude Phase Magnitude Phase
1: 276.589 mV 50.4506 Deg 0.000 dB 0 Deg <- normalizer
2: 43.0674 uV 16.6644 Deg -76.154 dB -33.7862 Deg
3: 161.77 uV 125.855 Deg -64.659 dB 75.4046 Deg
4: 5.56332 mV 92.6739 Deg -33.930 dB 42.2232 Deg
5: 82.87 uV 160.102 Deg -70.469 dB 109.652 Deg
6: 4.03262 mV -10.6745 Deg -36.725 dB -61.1252 Deg
7: 167.665 uV -40.5823 Deg -64.348 dB -91.033 Deg
8: 37.2955 uV -177.267 Deg -77.404 dB -227.718 Deg
9: 27.7985 mV 21.0579 Deg -19.956 dB -29.3927 Deg
10: 333.301 uV 87.8913 Deg -58.380 dB 37.4407 Deg
Total harmonic distortion = 10.354 % (-19.6978 dB).

Is there something wrong with the measurement, it should not be so high. somebody told me that I do not need to care about the even harmonics because " If you do fourier transform of a signal ranging from say, -V to +V ( i.e., differential), you will find that the output spectrum will be devoid of any even harmonics." . Is that true?
thanks for any advice.

Title: Re: SHA evaluation
Post by Berti on Jan 20^th, 2009, 3:29am

Hi Seahs,

What means "measured"? Do you mean simulated or really measurements of a physical chip? It's not so clear to me.

Cheers

Title: Re: SHA evaluation
Post by Ken Kundert on Jan 20^th, 2009, 8:44am

The Fourier analysis is reporting the distortion in the entire waveform, but presumably the circuit that follows the sample & hold is only sensitive to the settled values of the signal. To model this, you should follow your circuit with an ideal sample & hold and apply the Fourier analysis to the output of the ideal sample & hold. You can find ideal sample & holds by going to the Verilog-AMS section of this website.

-Ken

Title: Re: SHA evaluation
Post by Ken Kundert on Jan 20^th, 2009, 6:00pm

What is the nonperiodicity reported by Spectre?

-Ken

Title: Re: SHA evaluation
Post by seahs on Jan 27^th, 2009, 12:42am

Berti wrote on Jan 20^th, 2009, 3:29am:

Hi Seahs,

What means "measured"? Do you mean simulated or really measurements of a physical chip? It's not so clear to me.

Cheers

Hi, Berti
I mean the simulated result using Cadence Spectre.

Title: Re: SHA evaluation
Post by seahs on Jan 29^th, 2009, 12:08am

Ken Kundert wrote on Jan 20^th, 2009, 6:00pm:

What is the nonperiodicity reported by Spectre?

-Ken

Sorry for the late reply.

Total harmonic distortion = 10.7841 % (-19.3443 dB).
RMS value of computed spectrum (excluding DC) = 277.742 mV.
RMS value of computed spectrum (including DC) = 277.742 mV.
Nonperiodicity (first/last point mismatch) = 94.3452 uV (34.1656 m%).

Title: Re: SHA evaluation
Post by seahs on Jan 29^th, 2009, 12:23am

Ken Kundert wrote on Jan 20^th, 2009, 8:44am:

Btw, How to use the Verilog-AMS in spectre?

Title: Re: SHA evaluation
Post by Ken Kundert on Jan 29^th, 2009, 9:07am

I did not mean to suggest you should use Verilog-AMS. Rather that you visit the Verilog-AMS page on this website (http://www.designers-guide.org/VerilogAMS/). There you will find a Verilog-A model of a sample and hold.

-Ken

Title: Re: SHA evaluation
Post by ywguo on Feb 9^th, 2009, 8:03pm

Hi,

I'd like to calculate the FFT of the SHA output, one point for each cycle after the output settling down. The DFT function in Calculator is easy to use.

Yawei