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I've been trying to run some relatively simple simulations but am running into strange behavior, which I've narrowed down to using the non-ideal capacitor parameters in LTSpice (in both an old and an up to date version). This is what I'm running:

The problematic simulation

And this is the result I'm getting:

Simulation Results

I don't understand what's happening at the 500MHz mark, why is the current suddenly increasing again?

Here's the contents of the LTSpice ASC file:

Version 4 SHEET 1 2036 680 WIRE 1232 -320 1152 -320 WIRE 1568 -320 1488 -320 WIRE 1568 -304 1568 -320 WIRE 1152 -208 1152 -320 WIRE 1232 -208 1232 -320 WIRE 1488 -208 1488 -320 WIRE 1568 -208 1568 -224 WIRE 1568 -112 1568 -144 WIRE 1152 -16 1152 -128 WIRE 1232 -16 1232 -144 WIRE 1488 -16 1488 -128 WIRE 1568 -16 1568 -32 FLAG 1152 -16 0 FLAG 1488 -16 0 FLAG 1568 -16 0 FLAG 1232 -16 0 SYMBOL Misc\\signal 1152 -224 M0 WINDOW 123 24 132 Left 2 WINDOW 39 0 0 Left 0 SYMATTR Value2 AC 1 SYMATTR InstName V1 SYMBOL cap 1216 -208 R0 SYMATTR InstName C1 SYMATTR Value 10µ SYMATTR SpiceLine Rser=10m Lser=10n SYMBOL Misc\\signal 1488 -224 M0 WINDOW 123 24 132 Left 2 WINDOW 39 0 0 Left 0 SYMATTR Value2 AC 1 SYMATTR InstName V2 SYMBOL cap 1552 -208 R0 SYMATTR InstName C2 SYMATTR Value 10µ SYMBOL res 1552 -320 R0 SYMATTR InstName R1 SYMATTR Value 10m SYMBOL ind 1552 -128 R0 SYMATTR InstName L1 SYMATTR Value 10n SYMATTR SpiceLine Rser=0 TEXT 1224 56 Left 2 !.ac dec 1000 1 10G 
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1 Answer 1

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I think LTspice adds a nonzero parallel capacitance to capacitors when you set the series inductance to a nonzero value. If you set C1's parallel capacitance to 0, the second resonance should disappear.

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    \$\begingroup\$ Yes. This seems to be an LTspice thing- most other flavo[u]rs of SPICE just don't have the series inductance option (hence no need for a fix). Here, Mike Engelhardt explains why he didn't implement series inductance in QSPICE even though he did in LTspice. \$\endgroup\$ Commented Nov 5, 2024 at 15:12
  • \$\begingroup\$ Sneaky basterds! That does indeed fix it, and the commend from @Spehro'speff'Pefhany explains why. Very helpful, thank you to both of you. :) \$\endgroup\$ Commented Nov 6, 2024 at 10:38
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    \$\begingroup\$ @DavidKoster If you get a chance, then please accept the answer as shown here: meta.stackexchange.com/a/5235 \$\endgroup\$ Commented Nov 11, 2024 at 20:01

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