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Analog EDA

Simulate circuits in a spreadsheet with some 'ladderal thinking'

April 23, 2010 | Kendall Castor-Perry | 222900659
Kendall Castor-Perry explains how to use a ladder analysis technique to simulate circuits in a spreadsheet
Page 1 of 5

Click here to see all articles by Analog Guru Kendall Castor-Perry

So, filter fans, I hope you tried out my recent spreadsheet-modified simulation schematic method (Excel tunes up your schematic files). Its a nice way of creating small circuit chunks with calculated component values, especially when the sums cant be carried out using the math capabilities of your simulators preprocessor.

But say you now need to adjust component values to achieve some system goal, such as frequency response or predefined time behaviour, and theres no closed method for working out those values. You have to spend quite a bit of money to get proper circuit optimization features in a SPICE-compatible simulator; none of the free or affordable packages support it (QUCS looks intriguing, but its SPICE compatibility is very poor). And in todays busy world, its hard to find the time to write bespoke analysis and optimization programs. I count myself lucky that I had to do lots of that in the past because its such a good way of learning about practical circuit calculations.

In An Excelent Fit, Sir! we saw that the Solver tool in a spreadsheet can mold the
coefficients of a factorized transfer function to fit a frequency response specification. Thats useful if you can define the transfer function first, and then implement it with a suitable topology. Now, if youre comfortable turning transfer functions into component values and interconnections, then congratulations, youre a filter designer!

Lets assume, though, that this time youve got to optimize the response of an existing circuit without altering the topology, and that it isnt a simple cascade of second-order biquad blocks. Maybe its like Figure 1 the bandpass filter from Fainting in Coils. You can download it at http://www.docstoc.com/docs/35527292/FW9-bandpass-filter-_LTSpice-asc-format_ . It should arrive as a .txt file, and you'll need to change the extension to .asc before running LTSpice on it.

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