LTspice AC analysis question

[John Coloccia]

I don't have any trouble getting the AC analysis to work, but I've always wondered about something.  If I have more than one AC source, how does it know which source I want to sweep?  I mean, in a sense I guess the question doesn't make sense from the standpoint of having separate AC inputs sweeping at the same time, but what about the case where I'm doing something such as simulating a rectified power supply (AC source number 1) powering a circuit that has an AC input I want to sweep?  I guess even if I got it to only sweep one source, the rectified power rail never actually achieves steady state...there's always some ripple, so what happens?

I never do anything like this.  I usually just simulate small sections of the circuit, mainly just for fine tuning various filters, but it just got me to thinking that I really don't fundamentally understand the general behavior of the AC analysis beyond the simple analysis I normally do.

Maybe someone who's a lot more skilled than me could explain exactly what it's doing.  Does it just always sweep all the AC sources, and that's that?  It seems like that's the most straight forward implementation.  Anyhow, I just really want to understand exactly what the behavior is.

Thanks!

[c4757p]

AC analysis only works a single frequency, so all AC sources must be swept simultaneously. AC analysis involves converting reactances like capacitors and inductors into the equivalent reactive impedance and then solving the circuit as if they were complex "resistors", and this requires the single frequency to be known.

Typically, in your example, you'd analyze the two separately: analyze PSRR with the power source being swept and the input replaced with 0V, and analyze transfer function with the power source being set to 0VAC (plus whatever DC you require).

[Jay_Diddy_B]

John Coloccia,

Can you post your LTspice model and indicate what you would like to know?

If you are concerned about secrecy, post something similar.

You will have to zip the .asc file, because the forum does not allow .asc as an attachment.

Regards,

Jay_DiddyB

[John Coloccia]

AC analysis only works a single frequency, so all AC sources must be swept simultaneously. AC analysis involves converting reactances like capacitors and inductors into the equivalent reactive impedance and then solving the circuit as if they were complex "resistors", and this requires the single frequency to be known.

Typically, in your example, you'd analyze the two separately: analyze PSRR with the power source being swept and the input replaced with 0V, and analyze transfer function with the power source being set to 0VAC (plus whatever DC you require).

This makes perfect sense. I think I sort of answered my own question as I was typing it.  Thanks for explaining it so simply!

[John Coloccia]

Maybe you can answer another question while we're at it.  How the heck do I pan around in the schematic capture view.  This drives me nuts.  I'm sure there's some simple way to do it, but I can't seem to figure it out.

[mikerj]

There should be scroll bars if the schematic is larger than the window.  I usually just hit the "Zoom full extents" button, and then zoom back into the part I'm interested in.

[c4757p]

Maybe you can answer another question while we're at it.  How the heck do I pan around in the schematic capture view.  This drives me nuts.  I'm sure there's some simple way to do it, but I can't seem to figure it out.

Nope. Not really. LTspice doesn't even support middle-button pan. I tend to zoom out and back in.

[John Coloccia]

There should be scroll bars if the schematic is larger than the window.  I usually just hit the "Zoom full extents" button, and then zoom back into the part I'm interested in.

That's how I've always done it.  I figured that there must be some "pan" tool somewhere like every other schematic capture.  Every now and then go looking for one and never find it.  C'est la vie.  Thanks!

[uChip]

There is a pan tool.  I'm not saying it's great, but if you click on the magnifying glass (the one with no +, -, or x) and then click in your schematic, it will redraw the image such that the click point is centered.

I think it's still easier to use the scroll wheel to zoom out and then zoom in again.

  - Chip

[Ian.M]

If you are zoomed in, with a plain cross cursor, you can simply drag the background of the schematic around to pan it. If you have any of the tool cursors active, simply right click the background to return to the cross cursor.