## Ping Back Ping Backs

One of the nice things about writing a blog is that when people follow a link to your site you get a link to where they came from, which often leads to interesting places you would not have otherwise found.

I started the RAD Excel blog at the end of 2011. My intention is to post once or twice per month, primarily focusing on Excel and VBA. Through my employment I’m now learning other technologies such as SQL Server and .Net, so you may see posts on those – particularly if they’re linked back to Excel in some way. I hope you enjoy reading the blog as much as I do writing it!

The second is an article from EE times: Analyze FIR filters using high-school algebra, where the author used my polynomial solver UDF to find the real and complex roots of a 14th degree polynomial equation, apparently successfully.

Let’s take our example filter, find the roots and therefore the individual factors. I used an Excel root finderthat seemed to work well.

From EE Times ; http://www.eetimes.com/

Table 1: All the roots of the polynomial in equation [2].

There are fourteen roots – which is good, because it was a fourteenth order polynomial (the fifteenth tap is the constant term, i.e. it multiplies the zeroth power of z) – of which four are purely real and the rest come in complex conjugate pairs. Remember that classic formula for the solutions of a quadratic equation? When the expression inside the square root is negative, that’s where the imaginary part of the root comes from, and the plus-or-minus sign tells you there are two of them, with opposite signs of the imaginary part.