On Tuesday 12 July 2005 11:05 am, Harold D. Skank wrote: > I have created a state variable BP filter. I can run the ac > analysis using ngspice, and that seems to work very well. It > has allowed me to specifiy the circuit values to get the band > shape that I need. Now though, I would like to see the > circuit response to a 5-cycle truncated sine wave input, > since this is the "real" signal that the filter will see in > operation. How do I specify this as a voltage input and how > do I actually run the simulation?
Why don't you try gnucap? Gnucap has a "generator" that lets you set up things like this. .generator freq=1k delay=.001 width=.005 vin (1 0) generator On Tuesday 12 July 2005 11:18 am, Stuart Brorson wrote: > 1. Model the truncated sine wave as a piecewise linear > signal, use a PWL voltage source as the excitation, and then > use a .tran analysis to examine the result. This will give > you a bunch of spurious harmonics whose magnitude and > frequency will depend upon how finely you discretize the > signal, but perhaps you don't need to worry about them in > your analysis. That's really rough unless you use lots of points. Gnucap adds a "FIT" that is like "PWL" except that you can specify the order of interpolation. Specifying an order of 3 makes it fit with cubic splines. You can also specify the boundary conditions. You can specify a knot by giving the same point twice. Still, I don't recommend this for what you are doing. On Tuesday 12 July 2005 11:18 am, Stuart Brorson wrote: > 2. Use a voltage controlled switch to connect an AC voltage > source to your filter for the desired time, and then > disconnect it afterwards....... >....... I believe that ngspice will support > voltage controlled switches too. However, I don't have > first-hand experience with this. Of course it does. It starts with "S". Spice 3 does, so as far as I know all Spice derivatives except some really old ones do. Another possibility is to use a poly or a "B" device as a multiplier. On Tuesday 12 July 2005 11:29 am, Gene Heskett wrote: > Just off thet op of my head, Harold, a state variable BP > filter is usually going to have too high a Q factor to > adequately respond to a 5 cycle burst. ............ Tone burst tests are often used to evaluate the performance of filters. A state variable filter can have any Q you want, even very low. I might use a cheaper design for low Q, but nothing beats the 3-op-amp design for ease of tuning.
