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179

answers:

1

I wish to plot the EMI noise generated by high frequency switching circuits in simulink. The spectrum scope shows a plot in dB vs frequency. I want to plot this in log frequency scale.

+5  A: 

If I understand correctly, you have data containing Frequency -> dB and you want to plot it using the log of the frequency?

For example if you have the following:

Freq   dB
  63   70
 125   75
 250   71
 500   75
1000   65
2000   77
4000   79
8000   74

You want the log of the freq:

Freq   dB     Log(Freq)
  63   70     1.8
 125   75     2.1
 250   71     2.4
 500   75     2.7
1000   65     3.0
2000   77     3.3
4000   79     3.6
8000   74     3.9

And plot it in a graph:

80 |
 - |                           x
76 |                        x
 - |            x     x           x  
72 |
 - |         x     x
68 |
 - |                     x
64 |

     |----|----|----|----|----|----|
    1.0  1.5  2.0  2.5  3.0  3.5  4.0

If so, you can just take the data and convert each frequency to the log of the frequency.

Gamecat