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I want to make the time-step and stopping-time in a trasient simulation scale automatically by using the parameter values (.PARAM lines) in the netlist. Here is a minimal ngspice etlist to demonstrate my use case.

How to access parameter values in transient simulation and interactive interpreter

.PARAM R = 1kOhm
.PARAM C = 1nF
.PARAM pi = 3.14159265359
.PARAM fb = {1/(2*pi*R*C)}  ; Q1. Why does pi need to be defined above?
.PARAM amplitude = 5V
.PARAM pointsPerCyle = 20
.PARAM numCycles = 1.5

VDD 1 0     SIN(0V {amplitude} {fb})
R1  1 2     {R}
C1  2 0     {C}

.TRAN {1/159kHz/20}         {1.5/159kHz}
* .TRAN {1/{fb}/{pointsPerCyle}}        {numCycles/fb}  ; Q2. How to access the parameter values defined above in tran sim?
.PLOT TRAN V(1) V(2)

.CONTROL
    run
    plot V(1) V(2)
    * plot {V(1)/{amplitude}} {V(2)/{amplitude}}    Q3. How to access the parameter values defined above in interactive interpreter and/or ngspice scripts?
    * print $fb
    * echo $R

.ENDC

.end

The netlist will run as is, producing a transient simulation plot. The commented lines are where the errors occur. So I have 3 questions here.

  1. Why do I need to define pi in order to calculate some parameters? If it is already defined, I cannot access it at this point in the netlist (.PARAM lines)? Why?
  2. How do I access the .PARAM values to define the time-step and stop-tim in a trasient simulation?
  3. How do I access the .PARAM values in interactive interpreter and/or ngspice scripts?

Thank you!

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  • \$\begingroup\$ It might be useful to show an example of where you can't access pi, it may be how your referencing it. \$\endgroup\$
    – Voltage Spike
    Aug 31, 2016 at 20:14
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    \$\begingroup\$ I often specify .param variable names in the .tran expressions, using the {} braces you already are using. I do it all the time, so I have no idea what you are asking. It just works. I will often use .param to set f=159k, for example. Then apply in in the formula as just f. Just works fine. \$\endgroup\$
    – jonk
    Aug 31, 2016 at 20:19
  • \$\begingroup\$ @laptop2d, The included netlist shows an instance where pi cannot be used in a calculation unless explicitly defined by the user as another parameter (line 5 and 6). \$\endgroup\$
    – Aamir
    Aug 31, 2016 at 20:31
  • \$\begingroup\$ @jonk I also expect it to just work. But it doesn't. Could you please execute the included netlist after commenting the line .TRAN {1/159kHz/20} {1.5/159kHz} and uncommenting the line * .TRAN {1/{fb}/{pointsPerCyle}} {numCycles/fb} ; Q2. How to access the parameter values defined above in tran sim? \$\endgroup\$
    – Aamir
    Aug 31, 2016 at 20:34
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    \$\begingroup\$ NGSpice seems to be fussy about syntax for expressions involving parameters. You might find `{numCycles/fb}` works where {numCycles/fb} doesn't (etc) \$\endgroup\$
    – user16324
    Aug 31, 2016 at 20:50

1 Answer 1

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I also had the problem for some time, the answer is to use CSPARAM.

Description from NGspice manual:

description from NGspice manual

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