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Design variables in FFT and THD measures

baenischfau

Hello,

I'm currently designing a Class-D output stage and I have come across some

minor problems with the configuration with two of my measurement, namely FFT and THD.

As I always know the frequency(s) of my input signal I wanted to configure the FFT and the

THD measurement to always fit to the period of the output signal. So I defined two variables,

one for the start of the interval and one for the stop, both are integer multiples of the period.

I included those design variables into my measurements as discribed in a different post in this

forum using the desVar() function. For THD this yields in

 

thd(v("/outp" ?result "tran") desVar("startint") desVar("stopint")  16384 desVar("sigfreq")

 

for FFT I used the same syntax.

However both measurements do not work. I checked a lot of other measurements with the

desVar() method, they all work fine.

 

Anyone out there with some pointers ?

 

Thank you 

 

Best Regards

 

Andi

 

Parents
  • baenischfau

     Hi Andrew,

     

    Thanks for the hint, however it's still not working.

    What I have tried:

    I use the same set of three variables

    sigfreq = 256k

    startint = 5u

    stopint = startint+5*1/sigfreq 

    Staring from 

    thd(v(.....) 5u 24.531u 36000 256000)

    I replaced the frequeny setting 256k for the THD measurement with VAR("sigfreq") -> Measurement evaluates fine

    If I replace the starttime with VAR("startint") the measurement already fails,

    drForwardFFT: argument #2 should be a number

     

    What can I do now ?

     

    Best Regards

     

    Andi

     

Reply
  • baenischfau

     Hi Andrew,

     

    Thanks for the hint, however it's still not working.

    What I have tried:

    I use the same set of three variables

    sigfreq = 256k

    startint = 5u

    stopint = startint+5*1/sigfreq 

    Staring from 

    thd(v(.....) 5u 24.531u 36000 256000)

    I replaced the frequeny setting 256k for the THD measurement with VAR("sigfreq") -> Measurement evaluates fine

    If I replace the starttime with VAR("startint") the measurement already fails,

    drForwardFFT: argument #2 should be a number

     

    What can I do now ?

     

    Best Regards

     

    Andi

     

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