SparkFun Forums

[jremington]

Hi, All:

I know some of you use versions of SPICE for circuit simulation, so perhaps you can shed some insight on my question.
I need a bandpass filter for an ultrasonic receiver (40 kHz) and am experimenting with a design that I found on line (attached). It works in theory but two different simulations with LTSpice give conflicting results.

1. If I simulate the circuit using transient analysis with a 100 mV, 40 kHz AC signal source, the gain is about 5, or 15 dBV. See image bandpass1.png attached. The gain drops off on either side of 40 kHz as expected.

2. If I do a "small signal" AC analysis, the circuit is reported to have a maximum gain of about -5 dB at the peak, 40 kHz.

Why the difference? All the components are linear to my knowledge, so I don't see why linearizing the circuit should have such a large effect. I've attached the LTSpice input file, but the SparkFun forum doesn't allow .asc extensions, so it is named bandpass.txt

Thanks for any insight you can offer!

bandpass.txt

bandpass1.png

bandpass2.png

[skimask]

Isn't it an apple/oranges comparison?
IIRC, transient analysis makes you set the initial conditions, whereas small signal analysis assumes "zero's" for initial conditions. (or I might be backwards there, not at my main machine)
Which one is correct? I dunno...have to play with it...

[UhClem]

This is most likely due to your using an input of 100mV. I switched this to "AC 1" and also 1V for the transient analysis and the gain at 40KHz is about 15dB.

I would poke around in LTspice help to see what it says but that doesn't work too well under Linux.

[jremington]

UhClem, you are right, thanks! LTSpice does give a gain of 15 dB for 1VAC input. Maybe the small signal analysis just assumes 1 VAC input.

The built in help function doesn't seem to work on my Win7 system either. I thought of posting on the LTSpice user forum on Yahoo Groups, but it seems difficult to use. I couldn't even make much sense out of the results of the search function.

[skimask]

Works over here. What version does it report? I got 4.20p

[jremington]

Same version. When I hit "Help Topics" nothing happens. What happens on your system?

[Valen]

...
The built in help function doesn't seem to work on my Win7 system either. I thought of posting on the LTSpice user forum on Yahoo Groups, but it seems difficult to use. I couldn't even make much sense out of the results of the search function.

It does work on mine! 4.20i (EDIT: I guess I'm not up to date if there is 'p') But I can't remember if I did something special with the help file.

I did install the application in a self-created folder on my drive! I purpousefully did not installin the default location in Program Files (with or without x86). This was done because the updater wants to change files in there. And Windows (since Vista) will not allow this inside a Program Files folder without User Account Control consent by an administrator. Installing into a custom folder avoids the failing of updates.

[EDIT2] Just updated with menu:\Tools\Sync Release to 4.20s

[jremington]

I did install the application in a self-created folder on my drive!

That is probably the key. I let the program go into the Program Files folder and update doesn't work either. I'll try reinstalling the package somewhere else. Thanks for the suggestion!

[skimask]

I'm on Win7Home. LTSpice installed to Program Files x86.
When running in my normal account (eg. non-Admin), updates won't work, but the help still works.
Re-logging over to the admin account makes the updates work.