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Calculation of THD in cadence using DFT

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ICdesignerbeginner

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Hi

I am simulating a low frequency differential OTA circuit. And I want to calculate THD. I am using calculator in cadence. First i take the transient output of THD then take thd . E.g. dB20(thd (VT1-VT2) ) . I am using rectangular window its showing -40dB

When i am using DFT of the transient output its showing 57 dB which one is correct. My fundamental frequency is 100Hz.

The inputs applied at the OTA circuit is vsin fr9m analig. But i have read in different forums about placing port from analog lib at input. Which input should I use.
 

Why you use transient simulation than DFT ?? Is PSS+THD simulation combination not suitable ??
 

BigBoss said:
Why you use transient simulation than DFT ?? Is PSS+THD simulation combination not suitable ??
Click to expand...

Thank you for your reply. I new this way of finding THD so I started this way. But I could not figure out which method is right using THD directly from calculator or by using transient+DFT.

If I use PSS then I have to use the port source from analog lib but for fully differential structure. Do I have to place to two port : one at input negative input terminal and other at positive input terminal and two port at output. Can you help me in making the setup for fully differential structure because on different links I am just able to find single ended structure.

Thankyou
 

ICdesignerbeginner said:
Thank you for your reply. I new this way of finding THD so I started this way. But I could not figure out which method is right using THD directly from calculator or by using transient+DFT.

If I use PSS then I have to use the port source from analog lib but for fully differential structure. Do I have to place to two port : one at input negative input terminal and other at positive input terminal and two port at output. Can you help me in making the setup for fully differential structure because on different links I am just able to find single ended structure.

Thankyou
Click to expand...
Differential circuit is not an issue.You can use ideal balun,ideal transformer etc. to manipulate the simulation environment.You don't have to use PORT element, you can also use such ideal VSIN, ISIN components.
 

Balun I thought is used for Rf circuits because its available in RF analog library. I am working for low frequency will it be ok using balun in my case?
 

hello

Is there anyone who can help me in finding THD. I dont know what setup do I have to do for THD calculation?

When I am using calculator for thd its showing other results. When I am taking transient of output and then doing dft its giving some other result when I am using pss its giving some different values can some one help?
 

Thanks Frankrose. I am using PSS but when I am sweeping the amplitude (-1 to 1V)its giving different THD. When on a fix amplitude lets say 100mV its giving different thd. The harmonics should be same whether I am sweeping or placing it fixed at one amplitude. While comparing it with thd function in calculator is giving totally different thd.
 

I don't know anything about your project, your testbench, but try to set a simple one transistor testbench and repeat your calculation methods on that. If you still don't find a solution come back here with your results.
 

ICdesignerbeginner said:
Thanks Frankrose. I am using PSS but when I am sweeping the amplitude (-1 to 1V)its giving different THD. When on a fix amplitude lets say 100mV its giving different thd. The harmonics should be same whether I am sweeping or placing it fixed at one amplitude. While comparing it with thd function in calculator is giving totally different thd.
Click to expand...
What is that ?? Joke ??
THD will absolutely be different while you're increasing the Input Amplitude or moving the Operating Point of the device.Have you never heard "Non-linearity" ??THD is a function of the harmonic levels depending on Input Amplitude and Non-linearity.
Even you change the OP,or Input Amplitude, or Load and Source Impedance,or Frequency in any case, in anyway, THD or Harmonic Levels due to Non-linearity will always be different.
 

I havent calculated THD so I am confused whats happening. I have read about two tone and single tone while calculating thd. Which one is better. Do we have to apply two tone or single tone for thd calculation. The circuit is ota for low frequency application.
 

ICdesignerbeginner said:
Balun I thought is used for Rf circuits because its available in RF analog library.
Click to expand...
Wrong.

Ideal Balun is no more than combination of VCVS(Voltage Control Voltage Source) and CCCS(Current Control Current Source).

ICdesignerbeginner said:
I am working for low frequency will it be ok using balun in my case?
Click to expand...
Of course.

https://www.designers-guide.org/Analysis/diff.pdf

Code: 
`include "constants.vams"
`include "disciplines.vams"

`ifdef CDS_MMSIM6_0_KERNEL_OR_LATER
   //(* instrument_module *)
   //(* ignore_hidden_state *)
`endif

module ideal_balun(c, d, p, n);
inout c, d, p, n;
electrical c, d, p, n;

parameter integer balun_en = 1; //0:Disable, Others:Enable

real vd1;

analog begin
   if(balun_en != 0) begin
/*
      V(p, c) <+ 0.5 * V(d);
      V(c, n) <+ 0.5 * V(d);

      I(d) <+ -0.5 * I(p, c);
      I(d) <+ -0.5 * I(c, n);
*/
      vd1 = 0.5 * V(d);
      V(p) <+ V(c) + vd1;
      V(n) <+ V(c) - vd1;

      I(d) <+ -0.5 * (I(p) - I(n));
      I(c) <+ -(I(p) + I(n));
   end
   else begin
      V(d, p) <+ 0.0;
      V(n) <+ 0.0;
   end //if
end //analog

endmodule

ICdesignerbeginner said:
I have read about two tone and single tone while calculating thd.
Click to expand...
Surely read.
You can not define THD by two tones drive.
 
Last edited:

I have designed an OTA with THD = 56dB when I am using it in 3rd order OTA its THD is degraded to 35 db. It means while implementing filter the THD will always deegrade. We have to design a very good THD OTA near around 80 dB so that in designing filter its THD if degraded reaches somewhere around 58 dB. Is this the case or I have something done wrong? I thought the THD will remain same equal to a single OTA even after designing filter?
 


I have used 6 OTAs for designing 3rd order LPF OTA-C filter. THD of the filter is degraded by cascading OTAs. For a single oTA THD is 58dB
 

Obtaining linearity in OTA is quite difficult. And too good THD for a single OTA is quite difficult.
 

Yes there are many things which I dont know and I am in learning phase. I am trying to learn.

In many research articles high gain OTA is designed to be used in low frequency OTA-C filter. Is there any use of having high gain while designing LPF filter.What I know high gain is not required cutoff frequency is a concern. Is high gain of use?
 

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