Creating a "virtual short" in simulations

From typing "spectre -h options" at the command line:

102 lshort=0 H Fixed value inductors with L<=lshort are shorted.
lshort can additionally be applied to variable value
inductors using the option 'short_cut_var_elem=yes'.

Thank you - this seems like the right path.

However, I try to specify lshort=10n, under additional arguments in the spectre simulator options, and then run a transient simulation. However, it doens't seem to short the inductor. Am I using the lshort correctly, or did I misinterpret?

Dear jehh,

I was going to suggest a slightly different approach. Have you considered including an ideal resistor in parallel with each inductor with a value set by a variable, for example, "rshort"? In this fashion, when you want to include the inductors in your simulation, you set "rshort" to very large value. For  simulations where you do not wish to include the inductors, you set "rshort" to a very small value (1 milliohm, as an example).

Shawn

In earlier versions of Spectre, the option was named lcut instead of lshort. You can check with "spectre -h options" if this is the case.

In earlier versions of Spectre, the option was named lcut instead of lshort. You can check with "spectre -h options" if this is the case.

Note that in some pretty old versions (I tried even MMSIM15.1) lshort wasn't advertised but did work (it was an alias for lcut, I think). In SPECTRE 17.1 we removed lcut and made lshort the documented option. I tried it with old versions, and it was fine.

I would expect if lshort was not working in the version you were trying that it would tell you in the log file. No mention was made of the simulation version being used, so it's hard to guess what the issue might really be.

Andrew

Hi Andrew,

I am using spectre_17.1-ISR14_64 (also, lshort does show up, when I look type "spectre -h"). The only mention of lshort in the log file, is under global options sections, see setion below >

===================

Global user options:

psfversion = 1.4.0
vabstol = 1e-06
iabstol = 1e-12
temp = 27
gmin = 1e-12
rforce = 1
maxnotes = 5
maxwarns = 5
digits = 5
cols = 80
pivrel = 0.001
sensfile = ../psf/sens.output
checklimitdest = psf
lshort = 2e-05
save = allpub
tmioutput = /scratch/jehh/simulation/test_libraries/ind_short_test/maestro/results/maestro/ExplorerRun.0/1/test_libraries:ind_short_test:1/netlist/input
degfile = /scratch/jehh/simulation/test_libraries/ind_short_test/maestro/results/maestro/ExplorerRun.0/1/test_libraries:ind_short_test:1/netlist/input
scalefactor = 0.855
reltol = 0.001
tnom = 27
scalem = 1
scale = 1

========================================================

And the input.scs netlist is very simple, but still contain the inductor >

========================================================

// Point Netlist Generated on: Aug  5 12:45:33 2020
// Generated for: spectre
// Design Netlist Generated on: Aug  4 15:20:31 2020
// Design library name: test_libraries
// Design cell name: ind_short_test
// Design view name: schematic
simulator lang=spectre
global 0
parameters fs=10000000
*** INCLUDE OPTIONS REMOVED FOR LEGAL REASONS ***

// Library name: test_libraries
// Cell name: ind_short_test
// View name: schematic
L0 (vout vin) inductor l=1u
C0 (vout 0) capacitor c=1u
R0 (vout 0) resistor r=1K
I1 (vin 0) isource type=pulse val0=0 val1=1 period=1/fs delay=10n \
        rise=200p fall=200p width=0.5/fs
simulatorOptions options psfversion="1.4.0" reltol=1e-3 vabstol=1e-6 \
    iabstol=1e-12 temp=27 tnom=27 scalem=1.0 scale=1.0 gmin=1e-12 rforce=1 \
    maxnotes=5 maxwarns=5 digits=5 cols=80 pivrel=1e-3 \
    sensfile="../psf/sens.output" checklimitdest=psf lshort=20u
tran tran stop=10u errpreset=conservative write="spectre.ic" \
    writefinal="spectre.fc" annotate=status maxiters=5
finalTimeOP info what=oppoint where=rawfile
modelParameter info what=models where=rawfile
element info what=inst where=rawfile
outputParameter info what=output where=rawfile
designParamVals info what=parameters where=rawfile
primitives info what=primitives where=rawfile
subckts info what=subckts where=rawfile
save I1:sink L0:1 C0:1 R0:1
saveOptions options save=allpub

Is there any obvious thing I am doing wrong?

BR,

JEHH

Of course the inductor still appears in the netlist, this option does not change the behavior of the netlister, after all. However, the inductor should behave like a short in the simulation (which you would see in the simulation results). If the option is listed in the log file (as in your case), it is supposed to work.

Hi Frank,

Sorry if I seem like a young buck. ;) Below I tried to insert the simulation result. I am pulsing a current through an inductor, at node /vin - this node is seen to spike at several kV, unexpected with the inductor shorted.

If you have any suggestion on how to investigate further, they are very welcome.

BR,

JEHH

Please contact customer support. I did some experiments, and in SPECTRE16.1 versions the lshort appears to work with this example, but in later versions it doesn't. The inductor is still listed in the circuit inventory (which may be OK), but I still see the behaviour of the inductor present in the circuit.

Andrew

Thank you, Andrew,

I will do that.

BR,

JEHH