Hi, Sheldon,
Thanks for your reply.
1) I didn't use fourier command, and don't know hwo, instead, I use fourier analysis in GUI, as shown in the following. The plus terminal should be connected output, not ground.
2) the period is automaticlly set by the fourier analysis. At the end of the analysis, it showed 307.434 m% mismatch between period begins and ends. ls this causing spectrum leakage?
Thanks.
========================
Fourier Analysis `FOUR0'
========================
Fundamental frequency = 968.75 kHz.
Fundamental period = 1.03226 us.
Fourier components of V(net016) for the interval from 8.96774 us to 10 us:
DC = 412.951 mV.
Harm: Absolute Absolute Relative Relative
Magnitude Phase Magnitude Phase
1: 412.944 mV -95.4565 Deg 0.000 dB 0 Deg <- normalizer
2: 245.753 uV -155.438 Deg -64.508 dB -59.9814 Deg
3: 154.551 uV -127.679 Deg -68.536 dB -32.2226 Deg
4: 136.333 uV 70.9494 Deg -69.626 dB 166.406 Deg
5: 167.488 uV 127.225 Deg -67.838 dB 222.682 Deg
6: 70.1281 uV 50.7072 Deg -75.400 dB 146.164 Deg
7: 143.164 uV -4.80259 Deg -69.201 dB 90.654 Deg
8: 85.7771 uV -167.751 Deg -73.650 dB -72.2944 Deg
9: 57.4369 uV 85.7145 Deg -77.134 dB 181.171 Deg
10: 58.2287 uV 58.7229 Deg -77.015 dB 154.179 Deg
11: 96.982 uV -140.591 Deg -72.584 dB -45.1347 Deg
12: 112.673 uV -151.425 Deg -71.281 dB -55.9682 Deg
13: 48.6384 uV -81.8236 Deg -78.578 dB 13.633 Deg
14: 73.3771 uV 139.605 Deg -75.007 dB 235.062 Deg
15: 95.3559 uV -131.078 Deg -72.731 dB -35.6217 Deg
16: 75.5083 uV -27.9219 Deg -74.758 dB 67.5347 Deg
17: 27.7394 uV 40.3701 Deg -83.456 dB 135.827 Deg
18: 10.836 uV 114.367 Deg -91.620 dB 209.823 Deg
19: 29.4197 uV 24.4814 Deg -82.945 dB 119.938 Deg
20: 75.3786 uV 78.4791 Deg -74.773 dB 173.936 Deg
21: 40.8751 uV 159.28 Deg -80.089 dB 254.737 Deg
22: 88.73 uV 12.2185 Deg -73.356 dB 107.675 Deg
23: 58.7199 uV 111.711 Deg -76.942 dB 207.168 Deg
24: 96.3994 uV -151.858 Deg -72.636 dB -56.4017 Deg
25: 65.5622 uV -14.3222 Deg -75.985 dB 81.1343 Deg
26: 56.5478 uV 63.265 Deg -77.270 dB 158.721 Deg
27: 110.388 uV -157.099 Deg -71.459 dB -61.6427 Deg
28: 87.8713 uV -56.0377 Deg -73.441 dB 39.4189 Deg
29: 127.44 uV 70.3307 Deg -70.212 dB 165.787 Deg
30: 193.419 uV 173.36 Deg -66.588 dB 268.817 Deg
Total harmonic distortion = 138.224 m% (-57.1883 dB).
RMS value of computed spectrum (excluding DC) = 412.945 mV.
RMS value of computed spectrum (including DC) = 583.996 mV.
Nonperiodicity (first/last point mismatch) = -1.26953 mV (-307.434 m%)
sheldon wrote on May 23rd, 2007, 4:47am:Driveforce,
Some comments:
1) You using a fourier command like
FOUR0 ( out 0) fourier fund=968710.6461 harms=??
As mentioned in one of the previous appends, it would be better
to add the normharm property to the symbol
FOUR0 ( out 0) fourier fund=31250 harms=1024 normharm=31
This will allow a direct comparison of the DFT and Fourier analysis
results. Currently the analysis use different fundemental frequencies.
2) The period appears to be a little off, (31/1024)*32MHz = 968750Hz,
1/(40us-39.9677us)=968710.6461 Hz and this could result in spectral
leakage that requires windowing. The Fourier component does not
include a windowing function so you need to be careful about frequency
selection.
3) An ideal DAC has quantitazation noise not distortion harmonics so you
need to compare snr ( or sinad) in order to verify the results.
4) For non-ideal DACs, the results would not be expected to match since
the DFT samples the waveform, i.e., that is it is difficult to capture the
the effect of glitch impulse on the response. The Fourier component
integrates over the waveform and is more suited for this measurement.
Best Regards,
Sheldon