#### 020170

##### Full Member level 4

**question for FFT**

I've tested simple RC circuit through FFT

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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

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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?