# Circuit analysis in LtSpice

Discussion in 'Homework Help' started by Michael777, Aug 26, 2014.

1. ### Michael777 Thread Starter New Member

Aug 25, 2014
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I am new to LtSpice and trying simulate the following circuit.As per my calculations,the rise time tau(t=R*C) is about 0.12ps. I am not sure whether I need to run a transient analysis (.tran 0.12p). What spice directive must I use to find the voltage across R2? Any suggestions and corrections will be helpful.

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2. ### ericgibbs AAC Fanatic!

Jan 29, 2010
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hi,
I would say you are pushing to the limits of LTS.

This is the best I could get.
E

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3. ### Michael777 Thread Starter New Member

Aug 25, 2014
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I am new to the subject and I'm trying to demonstrate capacitive coupling in a circuit, where the voltage across R2 as a function of the voltage drop across R1. R2 being the load in the circuit2, R1 and voltage source belonging to circuit1, and the C, the capacitance between the two circuit wires terminating on the same ground plane. Thanks for your reply

4. ### MikeML AAC Fanatic!

Oct 2, 2009
5,450
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At these rise/fall times, I would be very surprised that the equivalent circuit of your stray coupling would be so simplistic. At these speeds, everything is inductive, even the ground plane....

I see no evidence that you are anywhere close to LTSpice's numerical resolution. I was able to model your voltage source with rise times down to 1e-5f = 0.00001fs = 1e-20s, and I don't see any limit in sight...

Also, there is something wrong with your circuit. Notice that the current that flows through R2 is totally independent of if R1 is there or not! You have an ideal voltage source with zero series resistance, which never happens in reality... Either model the voltage source as having source resistance, or replace it with a current source, as in the attached simulation. Notice that this changes your imagined time constant...

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Last edited: Aug 26, 2014
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5. ### Michael777 Thread Starter New Member

Aug 25, 2014
8
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Yes you are right, the series resistance must be taken into account in reality.I see where I went wrong, I assumed that the voltage drop across R2 depended on R1 but I realize that it depends on the capacitance. Thank you for the pointer.