Input Impedance plot using Spice

Jun 04, 2006 3 Replies
R

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



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R

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
    formatting link
    *
********************************************************* *============== 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:

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K

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 snipped-for-privacy@anasoft.co.uk

formatting link
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"

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R

Kevin,

Thanks a lot. WIll try that.

Ramakrishnan

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