Problem with Spice - it won't allow me to change bounds of graph

You are posting this question to the wrong newsgroup.

Try the cad newsgroup. However, once you've done the sweep, if you put the cursor over circuit nets, it'll turn into a little probe. If you then right click, it shows the voltage on that node. If you hover over a component, another cursor shows up, and you can find the current. Holding down the alt key while clicking on a component gives you the power dissipated in that component.

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   Robert Monsen
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Robert Monsen
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Perhaps I wasn't paying attention.

If you select an area, it will only show that area. If you then right click on the waveform display window, and select "Zoom to Fit" from the popup, it'll show the entire waveform.

If you right click on the waveform window, and select 'Autorange' it may be better. Perhaps you got into manual limit mode somehow.

There is a manual that talks about this stuff. Use the help menu. There is also a yahoo group devoted to LTSpice, which may have more answers for you.

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   Robert Monsen
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Robert Monsen

Hi - I'm trying to simulate a circuit with spice (which I'm very new to). I first set a DC sweep on the input (V3):

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2005_time_11_38_36.jpg

Then I hit the run button and choose node 6 (the output) when it prompts me, giving me this graph:

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2005_time_11_39_02.jpg

Then I try to change the horizontal axis bounds:

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2005_time_11_39_15.jpg

Which for some odd reason does not work:

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2005_time_11_39_19.jpg

Any idea as to what I am doing wrong? Thanks for your help!

-Michael J. Noone

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Michael Noone

Robert Monsen wrote in news: snipped-for-privacy@comcast.com:

I don't think you understand - the problem is that it will only display on the x axis v3=9.9-10, even though it's set to range from 0-10.

-M. Noone

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Michael Noone

Using that source, I cannot replicate your problem. (I did use my finger on the mouse rather than my tongue!) Zoom- to-fit is active. Zooming works as Mr. Monsen describes.

Maybe it would help to describe what you are actually expecting to see. The trouble may reside there.

Looking at your circuit, I find myself wondering if you have dropped your 1 mS response time requirement. What that unreal when you mentioned it awhile back?

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--Larry Brasfield
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Larry Brasfield

This still seems to be the case, but maybe it will clear up once you get a time-varying source per below.

Right click on V3, hit the "Advanced" button, select the PWL radio button (in "Functions" pane), and fill in the piece-wise-linear specification of an excitation function for which you would like to see the transient response.

As for suggestions about how to deal with the response you get, I've stated my piece on that in earlier posts.

Likewise.

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--Larry Brasfield
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Larry Brasfield

The time at which your PWL input does anything (100 Seconds) is far beyond the duration of you simulation (100 mSeconds).

Often, a good sanity check is to use the plotting facility to look at your inputs and verify they are doing what you expected.

You're welcome.

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--Larry Brasfield
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Larry Brasfield

You PWL spec presents a ramp as the setpoint for the amplifier to follow. This is not what people usually use to explore response time. (It can be, for some systems, but just not often compared to step response.)

I observed similar behavior. The circuit appears to have DC convergence problems in the transient simulation and AC simulation, so I've attributed that strange starting behavior to that issue without studying it closely. I trust the simulator's result once the transient simulation settles at a believable operating point. I take that as occuring after the strange, unrequested initial flailing ceases.

I would suggest an input like this: PWL(0, 0.1, 40m, 0.1, 40.05m, 10, 200m, 10) to see your step response past the time taken for the true initial conditions to be achieved.

You're welcome.

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--Larry Brasfield
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Larry Brasfield

Use a behavioral current source ('bi' in the LTSpice top-level component listing) with current controlled by the voltage seen at the current terminals and another voltage of your choosing, perhaps a PWL source describing your R(t) function.

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--Larry Brasfield
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Larry Brasfield

[step vs. ramp cut]

Here are the changes I would make:

  1. Eliminate or greatly reduce R2.
  2. Degenerate the M1,R1 gain stage to get a more stable and predictable gain versus operating point. A source resistor would be a good addition for this.
  3. Set the zero established by C1,R5 to be near the lowest frequency pole created in the M1,R1,M2 stage.
  4. Adjust the M1,R1 gain for best loop response. (Note this likely interacts with step 3, so it will could be a little tedious.)

Perhaps lost in that maelstrom was a simple circuit I posted with about 8 uS response time. You should hope to get 1 mS, perhaps with revision of your current configuration once the above 4 changes are properly applied.

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--Larry Brasfield
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Larry Brasfield

Robert Monsen wrote in news:p_idnUl snipped-for-privacy@comcast.com:

Hi again mr Monsen - I read through the pertaining sections in the LTSpice manual, but I couldn't find anything about what I was doing.

Zoom to fit is greyed out for some odd reason:

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_2005_time_07_58_48.jpg

There are "autorange Y axis" and "autoranging" menu items when I right lick on the waveform window. The autoranging item seems not to do anything, while the other just auto ranges the y axis.

If you want to try it out for yourself (I've duplicated this same problem on 2 computers now), this is the schematic:

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Thanks again,

-Michael

Reply to
Michael Noone

"Larry Brasfield" wrote in news:0bX2e.28$ snipped-for-privacy@news.uswest.net:

Hmm this is odd indeed. I was expecting for the x axis to have a range of V3=0-10 volts in 01.V increments. I was then hoping to see a nice little linear line that would be something like Y=30X. Is this what you are seeing? Instead - All I see is the x axis having a range of v3 = 9.9-10V, in 0.1V increments.

I am still hoping for 1ms response time though. I was hoping the simulator could tell me if that was a possibility - but I can't seem to get it to show me anything besides V3=9.9-10V!

Any suggestions?

Best regards,

-Michael

Reply to
Michael Noone

"Larry Brasfield" wrote in news:y_X2e.30$ snipped-for-privacy@news.uswest.net:

Hi - I tried that:

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2005_time_13_35_28.jpg

And got this:

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2005_time_13_35_50.jpg

Any idea what I'm doing wrong?

Thanks again,

-Michael

Reply to
Michael Noone

"Larry Brasfield" wrote in news:eBY2e.33$ snipped-for-privacy@news.uswest.net:

Ah - you're quite right! I changed it to 1 second and it seems to work fairly well:

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2005_time_14_40_27.jpg

A couple questions though: First of all - it has a strange spike at the very beginning. Is that just due to the components initializing? Also - it looks to me that it takes about 30 ms to stabilize after the input has changed. Does that seem about right?

Thanks for your continuing help with this project,

-Michael

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Michael Noone

"Larry Brasfield" wrote in news:eBY2e.33$ snipped-for-privacy@news.uswest.net:

Oh - I almost forgot. One last thing: Is there any way to add a time- varying load in Spice? I clicked around for a good while but couldn't figure out how to add anything but a constant valued resistor.

-Michael

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Michael Noone

"Larry Brasfield" wrote in news:VAZ2e.37$ snipped-for-privacy@news.uswest.net:

I understand that - I just wanted to be sure I was getting a linear (though delayed) output.

I already have all the components needed (well, different FETs, but similar enough that I think they will work) sitting around the lab anyways, so I'm going to see how it performs in the real world. If I get similar results, is there any way to modify this circuit so as to improve response time? I looked back at my original post from two weeks ago and couldn't find anything along those lines, though it's possible that I missed something due to the very large size of that topic (over

150 posts! eek)

We think alike - That's almost exactly how I found a response time of

30ms

-Michael

Reply to
Michael Noone

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