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aaron_do

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Simple RC filter time domain
Feb 26^th, 2007, 7:54pm

Hi all,

if i have a simple filter, e.g.

Vin ---vvvvv----------Vout
R |
|
C ==
|
|
GND

and Vin is a ramp with an AC signal on top. Does the filter have the correct characteristics before it reaches the DC solution? i.e. the ramp is still charging up the output node to the DC value, so is the AC response of the filter still 1/(1+RsC). I'm not sure if the filter example I chose is right (maybe too simple). I'm just asking for a general answer.

thanks in advance,
Aaron

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aaron_do

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Re: Simple RC filter time domain
Reply #1 - Feb 26^th, 2007, 8:20pm

Let me just add a different circuit too cos i think the other circuit may be too simple.

Iin
GND----(->) ---------------------------Vout
| |
C1 == >
| > R2
GND >
| node Vx
C2 ==
|
GND

The circuit is such that node Vx takes much longer to charge up than node Vout. So does node Vout retain is steady state filtering characteristics when Vx is charging up?

Oh yeah one more thing...what simulation would i run in spectre to test this? I need the AC response at different time steps.

thanks,
Aaron

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Andrew Beckett

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Re: Simple RC filter time domain
Reply #2 - Feb 27^th, 2007, 3:40pm

Not sure I quite get what you're asking here (perhaps because I couldn't quite make out the ASCII art circuit diagrams - you can upload png files, BTW). However, you can run a transient which runs an ac analysis at specific times. Look at the acnames and actimes parameters for transient. An example would be:

Code:

tran tran stop=100u acnames=[myac] actimes=[10u 20u 30u 40u]
myac ac start=1 stop=1G dec=20

Regards,

Andrew.

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aaron_do Senior Fellow Offline Posts: 1398	Re: Simple RC filter time domain Reply #3 - Feb 27^th, 2007, 8:48pm Hi Andrew, thanks for the reply. I've never really tried chaning the netlist for the simulation. I usually just set it up directly using the analog environment interface. Anyway I tried to change the netlist but I couldn't find any results. Could you elaborate on how I should go about this? thanks, Aaron
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mg777 Senior Member Offline Posts: 131	Re: Simple RC filter time domain Reply #4 - Feb 27^th, 2007, 9:07pm I get the sense that you are intuitively reinventing QPSS. I'm pretty sure there will be no simulation performance gain for linear systems - proving this may require the equivalence between an FFT and inverting the Vandermonde matrix of natural response. M.G.Rajan www.eecalc.com
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Ken Kundert Global Moderator Offline Posts: 2386 Silicon Valley	Re: Simple RC filter time domain Reply #5 - Feb 27^th, 2007, 10:23pm None of this is necessary. Your circuit is linear time invariant. The AC analysis will produce the same result regardless of the time at which it is run. -Ken
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aaron_do Senior Fellow Offline Posts: 1398	Re: Simple RC filter time domain Reply #6 - Feb 28^th, 2007, 2:11am Hi all, thanks for the replies. I was asking because my filter was not acting like a filter and I noticed that one of the nodes had not reached the DC value yet. Just for future reference, how would I actually test the effect I was talking about anyway? thanks, Aaron
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Andrew Beckett

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Re: Simple RC filter time domain
Reply #7 - Feb 28^th, 2007, 4:29am

I must admit I thought what Ken did - I couldn't see why you would do this - the response will be independent of the operating point, since it is linear time invariant.

Anyway, there's no need to do the actimes in the netlist. It can be done from the GUI.

If you have a recent enough IC version, setup the ac analysis as usual. Then on the transient options form, go down to the field actimes - and enter space-separated times. The acnames field will open up, and you would specify the names of the ac analyses you want to run at these times. So enter "ac" in that field (the ADE interface will call the instance name of the analysis the same as the name of the analysis itself - so enter ac, xf, noise etc).

If you're using an older version where acnames/actimes doesn't appear on the transient options, scroll down to the bottom of the transient options form, and in the additional options field, type:

Code:

acnames=[ac] actimes=[10u 20u 30u]

If you haven't got the additional parameters field (which appeared in IC5141 USR2), you'd have to go to a netlist-based approach (perhaps with an include file for the analyses). You'll need an MMSIM version of spectre to handle this.

Of course, with your circuit, you'll just see the same frequency response multiple times...

Regards,

Andrew.

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aaron_do Senior Fellow Offline Posts: 1398	Re: Simple RC filter time domain Reply #8 - Feb 28^th, 2007, 5:26am thanks Andrew I know it must have taken a while to get that answer down. I get the point about the circuit being LTI (thanks Ken)... too many hours in front of the computer made my thoughts get a little too wild. thanks all, Aaron
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mg777 Senior Member Offline Posts: 131	Re: Simple RC filter time domain Reply #9 - Feb 28^th, 2007, 6:42am The reasoning in your original question is quite meaningful for a non-linear circuit. For example, add a diode in series with the source. M.G.Rajan www.eecalc.com
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