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# Problems with testing RC circuit through FFT

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#### 020170

##### Full Member level 4
question for FFT

I've tested simple RC circuit through FFT

##########################################

Vdd vdd gnd 2.5

.option nomod fftout

.param fi = 1.0meg fs = 10meg fo = ' fs * (23/256)'

* Fin / Fs = Prime number / number of point

* input signal frequency = 1Meg, Sampling frequency = 10Meg
* output signal Frequency = Fs * ( 23 / 256 )

*va 1 gnd sin (0 1 fo)

va 1 gnd sin (0 1 fo)
* offset/amplitude/Frequency

R1 1 2 1K
C1 2 gnd 1p

* 100n * 256 = 25.6u

* 25.61u - 10n = 25.6u

* 10n?

.fft v(2) start=10n stop=25.61u np=256 format=norm dc=off

.op
.tran 10p 27u
.end

########################################

I didn't write some freq or window in FFT commmand.

is it right?

in my thought, simple RC circuit noise floor is determine like this

noise = kT/C

okay, noise is determined by C not R

in theory, when I increased the value of C, noise floor is decreased in FFT. right?

but I have the result that opposite of my thought

what is my problem?

Re: question for FFT

020170 said:
I've tested simple RC circuit through FFT

##########################################

Vdd vdd gnd 2.5

.option nomod fftout

.param fi = 1.0meg fs = 10meg fo = ' fs * (23/256)'

* Fin / Fs = Prime number / number of point

* input signal frequency = 1Meg, Sampling frequency = 10Meg
* output signal Frequency = Fs * ( 23 / 256 )

*va 1 gnd sin (0 1 fo)

va 1 gnd sin (0 1 fo)
* offset/amplitude/Frequency

R1 1 2 1K
C1 2 gnd 1p

* 100n * 256 = 25.6u

* 25.61u - 10n = 25.6u

* 10n?

.fft v(2) start=10n stop=25.61u np=256 format=norm dc=off

.op
.tran 10p 27u
.end

########################################

I didn't write some freq or window in FFT commmand.

is it right?

in my thought, simple RC circuit noise floor is determine like this

noise = kT/C

okay, noise is determined by C not R

in theory, when I increased the value of C, noise floor is decreased in FFT. right?

but I have the result that opposite of my thought

what is my problem?

No noise parameter will be involved in tran simulation. So, you get a wrong results.

sixth

Re: question for FFT

I hope that someone fix my problem before tell me I was wrong

okay. noise parameter is not included.

what do I have to do in order to have correct value?

thanks

Re: question for FFT

020170 said:
I hope that someone fix my problem before tell me I was wrong

okay. noise parameter is not included.

what do I have to do in order to have correct value?

thanks

Using tran analysis to evaluate the noise effect is not a correct method. Of course, you can decribe the noise source of your circuits in time domain with behavioral language and then do tran analysis. But it seems be a very hard work. The correct method is to do small signal analysis using .noise analysis.
sixth

Re: question for FFT

you mean I have to use AC analysis?

but typically, someone use tran analysis in FFT analysis to measure distortion.

I want to know how to measure distortion in FFT.

and I hope that someone verify my method.

in my thought, most bottom line means noise floor in FFT graph.

therfore when I increased the value of C in simple RC circuit,

noise floor is decreased and SNR will be great value.

but the result is not. why?

thanks

question for FFT

FFT can analysis the nonlinear distortion but not the thermal noise.
If you want to view the distortion, you should use the nonlinear circuit such as MOSFET. Added a large signal to the input of common open loop MOS amplifier and then you can find the distortion.

Re: question for FFT

okay I see

but is it right FFT test in my circuit?

okay this is CS-amp FFT netlist

################################################

Vdd vdd gnd 3.3

.param fi = 1.0meg fs = 10meg fo = ' fs * (23/256)'

* input signal frequency = 1Meg, Sampling frequency = 10Meg
* output signal Frequency = Fs * ( 23 / 256 )

*va 1 gnd sin (0 1 fo)

vin vin gnd sin (1.4 1 fo)

* offset/amplitude/Frequency

R1 Vdd Vout 10K
M1 Vout Vin gnd gnd nch W=1u L=0.35u

C1 out gnd 1p

* 100n * 256 = 25.6u

* 25.61u - 10n = 25.6u

* 10n?

.fft v(Vout) start=10n stop=25.61u np=256 format=norm dc=off

.op
.tran 10p 27u
.end

#############################################

in this time, I ask you that my FFT method is right or not

thanks

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