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THD of low pass filter for ultra low frequency

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[moved] dft and thd of a low pass filter in Cadence

I have designed a low pass biquad for cutoff 14Hz.
I have done its dft and thd.
I got thd 125.814. What does this value mean. In the help guide of cadence calculator, it say the formula returns voltage percentage. Please help me to understand this.
With dft I got the follpwing graph
dft.jpg
When I use the log scale
I get this graph
dft log .jpg
Please help me to understand this graph.
These spikes at 111Hz, 130 Hz, 230Hz and 250Hz.
The input signal frequency is 10Hz.
Thanks in advance
 

thd of low pass filter for ultra low frequency

Hello
I have designed a low pass filter with -3db cutoff at 14Hz.
I have done its thd, using calculator function in cadence 6.1
I have taken transient output wave applied the formula using calculator, kept fundamental frequency same as the input signal frequency, 8192 samples, took period such that I had at least two wave periods.
The formula returns me a value - 125.8 thd for 100uVpp.
The value remains unchanged even if I change the input ac amplitude.
I was expecting the thd will be a percentage, and it should vary with ac amplitude.
Please help with how to do thd in cadence 6.1
I think I am doing something wrong.
Thanks in advance
 

Re: thd of low pass filter for ultra low frequency

What noise figure are you expecting and thus what SNR are you expecting?

What gain?

I hope your input bias is in the acceptable range and output is in the linear range.
 

Re: thd of low pass filter for ultra low frequency

Hi,

What input frequency and what sampling frequency?
What order is the filter?

took period such that I had at least two wave periods.
Click to expand...
Can you get an FFT plot of your measurement? Do you see the bin for your input frequency, and the bins for the overtones?
What amplitudes do you see?

Klaus
 
Re: thd of low pass filter for ultra low frequency

I changed the period such that now it includes two cycle.
Also, I changed the transient analysis to moderate; with that, I get THD =1.055 (I guess it is a percentage) which means -39.6dB THD (-40dB approx).
1) However, If I change the transient analysis to conservative, the THD becomes 372.151m!!!
2) And keeping the transient analysis moderate, if I take the period output transient signal as 0.04s to 0.24s THD becomes 1.26.
Does this means that signal has more noise for initial few mili sec and then noise subsides??
Please help me understand both the phenomenons
Thanks
 

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