Input Impedance plot using Spice

[rkrishnan]

Hi,

I am starting to learn Spice, specifically for simulating some amateur
radio circuits. I am using ngspice, a version of the original Berkeley
Spice. I have a very silly doubt, sorry for my ignorance.

I have simulated a lowpass LC filter. The netlist is given below:

*============== Begin SPICE netlist of main design ============
R101 0 n3 50
C102 0 n3 330pF
C101 0 n2 820pF
L102 n2 n3 1.3uH
L101 1 n2 1.3uH
R102 2 1 50
V101 2 0 AC
C103 0 1 330pF
..END

I would like to plot the input impedance Vs frequency of this filter
using spice. How do I do that?

Thanks
Regards
Ramakrishnan

 

[rkrishnan]

I solved it by a crude method : by adding a 1 ohm resistor in the
input and plotting the soltage across it (which will also be the
current, as R = 1 ohm) and using nutmeg commands real and imag. The new
netlist is:

* gnetlist -g spice-sdb -o norcal40A-harmonic-filter.net
norcal40A-harmonic-filter.sch
*********************************************************
* Spice file generated by gnetlist *
* spice-sdb version 12.27.2005 by SDB -- *
* provides advanced spice netlisting capability. *
* Documentation at http://www.brorson.com/gEDA/SPICE/ *
*********************************************************
*============== Begin SPICE netlist of main design ============
R101 0 n3 50
C102 0 n3 330pF
C101 0 n2 820pF
L102 n2 n3 1.3uH
L101 v2 n2 1.3uH
R103 1 v1 49
V101 1 0 AC
R102 v1 v2 1
C103 0 v2 330pF
..END

In the ngspice prompt:

let Rin = real((v1)/(v1-v2))
let Iin = imag((v1)/(v1-v2))
plot Rin
plot Iin

This plots the real and imag part of input impedance.

Is there a better, direct method to find the input impedance?

73
Ramakrishnan, VU3RDD

rkrishnan wrote:

Hi,

I am starting to learn Spice, specifically for simulating some amateur
radio circuits. I am using ngspice, a version of the original Berkeley
Spice. I have a very silly doubt, sorry for my ignorance.

I have simulated a lowpass LC filter. The netlist is given below:

*============== Begin SPICE netlist of main design ============
R101 0 n3 50
C102 0 n3 330pF
C101 0 n2 820pF
L102 n2 n3 1.3uH
L101 1 n2 1.3uH
R102 2 1 50
V101 2 0 AC
C103 0 1 330pF
.END

I would like to plot the input impedance Vs frequency of this filter
using spice. How do I do that?

Thanks
Regards
Ramakrishnan

 

[Kevin Aylward]

rkrishnan wrote:

Hi,

I am starting to learn Spice, specifically for simulating some amateur
radio circuits. I am using ngspice, a version of the original Berkeley
Spice. I have a very silly doubt, sorry for my ignorance.

I have simulated a lowpass LC filter. The netlist is given below:

*============== Begin SPICE netlist of main design ============
R101 0 n3 50
C102 0 n3 330pF
C101 0 n2 820pF
L102 n2 n3 1.3uH
L101 1 n2 1.3uH
R102 2 1 50
V101 2 0 AC
C103 0 1 330pF
.END

I would like to plot the input impedance Vs frequency of this filter
using spice. How do I do that?

Use an ideal 1A current source to ground as the drive signal. The
voltage at the non ground lead is then equal to the input impedance.

Kevin Aylward
431infoEXTRACT@anasoft.co.uk
http://www.anasoft.co.uk
SuperSpice, a very affordable Mixed-Mode
Windows Simulator with Schematic Capture,
Waveform Display, FFT's and Filter Design.

"There are none more ignorant and useless,than they that seek answers
on their knees, with their eyes closed"

 

[rkrishnan]

Kevin Aylward wrote:

Use an ideal 1A current source to ground as the drive signal. The
voltage at the non ground lead is then equal to the input impedance.

Kevin,

Thanks a lot. WIll try that.

Ramakrishnan