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28th February 2007, 13:01 #1
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spectre thd
Hi all,
I want to simulate the THD using spectre, I've done the Transient analysis
and want to use Calculator > Special functions > thd; there're four parameters must fill in, (the first two must start and stop frequency I think, Number of Samples and Fundamental)
What's the relation between them and THD? And how do I find out the THD in Caculator?
Thanks!

28th February 2007, 13:01

28th February 2007, 14:48 #2
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thd in spectre
If you have in your transient responce current i.e I2, then you first press wave,it and then you press the waveform in transient responce so the expression is in the buffer.After that select from special functions thd.At this point you have to set as you said 4 parameters. At boxes From & To you have to write something in time, for example 3u to 5u(meaning usec).These values depends on the frequency of your output waveform.So it is good to select two periods of your signal. At fundamental choose 0 .Now for number of samples you can write 64,512,1024 but it depends on the step you have in transient analysis.Check at cadence help for calculator.I think you will find it at calc.pdf . There you will find the expression in order to select the right No of samples. If you have problem send a photo with your signal or just send the frequency of your signal.
If your waveform is voltage, press wave,vt and then the waveform
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1st March 2007, 02:15 #3
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cadance calc.pdf
Thanks laoud!
Would you pls send me the calc.pdf? There's no cadence manul in my computor.

1st March 2007, 02:35 #4
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thd analysis spectre
when u plot a waveform, you can find the calculator in your waveform window. it is in the tools buttom. You can find the Thd function in the calculator

1st March 2007, 03:25 #5
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cadence spectre calc.pdf
And the this is the step by step method
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19th February 2010, 09:36 #6
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How to calculate THD in spectre
the result got from thd simulation is the real value, or with unit%?
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22nd February 2010, 15:04 #7
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Re: How to calculate THD in spectre
Originally Posted by chichi

22nd February 2010, 18:55 #8
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Re: How to calculate THD in spectre
thanks..i just find it so strange that not only THD but also HD3, they does not behave the normal way, i mean normally, the third harmonics increase faster than signal amplitude when increasing the input signal, so HD3 should decrease till 0 with increasing output amplitude(so called IIP3 point),but in my case the curve is like a wave, very bad.lz se the atattched

23rd February 2010, 09:32 #9
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Re: How to calculate THD in spectre
Originally Posted by chichi
Originally Posted by chichi
Originally Posted by chichi
But may be this is my misinterpretation?

23rd February 2010, 09:40 #10
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Re: How to calculate THD in spectre
Hi, right the x axis is the HD1, while the y axis is HD1HD3, the distance between thrid order harmonics and fundamental signal.

23rd February 2010, 09:40

23rd February 2010, 10:54 #11
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Re: How to calculate THD in spectre
Ok, then it seems you are still far, far away from the IIP3 point, which is HD1HD3=0dB (see the wiki). So you should simulate the input range 20dB ≤ HD1 ≤ +10dB !
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19th May 2010, 15:23 #12MohEllayaliGuest
Re: How to calculate THD in spectre
hello
the result got from thd simulation is the real value, or with unit%?

31st May 2010, 16:47 #13
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Re: How to calculate THD in spectre
yes it is, as can be seen in the wavescan user guide (calculator functions) for THD
just multiply it with 100 and you can use it to calculate ENOB or SN(D)R

31st May 2010, 22:52 #14
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How to calculate THD in spectre
from the calculator use the Fourier transform then sum a few terms HD2, HD3 is usually sufficient
good luck

15th September 2010, 06:32 #15
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Re: How to calculate THD in spectre
hello, is the thd in cadence calculator expressed in percentage?

15th September 2010, 12:08 #16
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Re: How to calculate THD in spectre
See the replies above yours!

16th September 2010, 08:53 #17
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Re: How to calculate THD in spectre
but why my THD always seem to be about 8%, my specs requirement is 0.92%, what do u think is the reason?

16th September 2010, 08:53

16th September 2010, 10:53 #18

30th September 2010, 05:58 #19
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Re: How to calculate THD in spectre
Hello everyone
I am using the calculator in cadence to find the settling time for the output voltage waveform.
There is this calculator function(Settlingtime) in cadence which I believe would be useful to me.
However, there are 4 blanks to be filled up:
Initial value type:
Initial value:
Final value type:
Final value:
Kindly advise. I tried doing it but the answer does not seem sensible.
Advance thanks

20th December 2011, 08:42 #20
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