How to simulate FFT in HSPICE?

[wylee]

hspice fft

I designed a few low pass filters (different order) and I have input a 1Gbps square wave into it

I want to run .FFT from 100us to 200us to probe v(out) and check what are the frequency compenent that passed through the filter

can anyone help me on that?

 

[yorande]

fft in hspice

what's your problem then??just run it.
you've got the statement for it
chosse your window and set the time.

 

[wylee]

fft hspice

I need some example on how to use it coz I have trouble of plotting the power vs freq graph from 1 to 10GHz.

If you know how, can you post a simple example?

 

[leo_o2]

.fft hspice

A example

.fft v(1,2) np = 1024 start = 0.3m stop = 0.5m freq = 5.0k
+ window = kaiser alfa = 2.5

 

[wylee]

hspice .fft

Ok I tried, and I am not getting what I espected

Here is my netlist, I designed a 20kHz 1st order LPF and I inject a 2GHz square wave into it. If I do FFT at the input waveform I should espect to see a fundalmental component at 2GHz and 1st odd harmonic at 6GHz.

But instead I got this (please refer to the attached GIF)


Here is my netlist

* LPF FFT Testing ********************************
.option post

rlpf in out 20k
clpf out gnd 397.89pF

vin in gnd ac=1 dc=1.2 pulse(0.9 1.5 0 100p 100p .9n 2n)

.fft v(out,gnd) np=1024 start = 70u stop = 80u freq = 2G window = kaiser alfa = 2.5
.fft v(in,gnd) np=1024 start = 70u stop = 80u freq = 2G window = kaiser alfa = 2.5

.ic out 1.199
.tran 50p 100u

.ac dec 100 1 10G
.graph par('vdb(out)-vdb(in)')

.end
***********************************************[/img]

 

[derekqiao]

hspice fft

You can try to input an ideal sin wave, if the FFT is right, you can get the result in *.ft0.

 

[xuel]

fft+lpf

Ok I tried, and I am not getting what I espected

Here is my netlist, I designed a 20kHz 1st order LPF and I inject a 2GHz square wave into it. If I do FFT at the input waveform I should espect to see a fundalmental component at 2GHz and 1st odd harmonic at 6GHz.

But instead I got this (please refer to the attached GIF)


Here is my netlist

* LPF FFT Testing ********************************
.option post

rlpf in out 20k
clpf out gnd 397.89pF

vin in gnd ac=1 dc=1.2 pulse(0.9 1.5 0 100p 100p .9n 2n)
~~~~the frequency is 500Mhz not 2G, right?

.fft v(out,gnd) np=1024 start = 70u stop = 80u freq = 2G window = kaiser alfa = 2.5
.fft v(in,gnd) np=1024 start = 70u stop = 80u freq = 2G window = kaiser alfa = 2.5
~~~~~~~~~~~~~~
sim time for fft is 80-70=10u, and your freq bin is 1/10u=100kHz, so your freq range is 100khz*1024/2=50mhz, so you can only see frequency components within 50MHz.
try this:
vin in gnd ac=1 dc=1.2 pulse(0.9 1.5 0 100p 100p .24n 0.5n)
.fft v(in,gnd) np=8192 start = 70u stop = 71u freq = 2G window = kaiser alfa = 2.5

.ic out 1.199
.tran 50p 100u

.ac dec 100 1 10G
.graph par('vdb(out)-vdb(in)')

.end
***********************************************[/img]