Interpolation is used every day in engineering, linear is just one method. This is the linear interpolation function: We seek to find the Y value for an arbitrary X value between these two points, such that it is on the straight line that is drawn between them. X1 and Y1 are "real" data points, so are X2 and Y2. Especially after they look it up on Wikipedia! Below is a close-up of an interpolated data point. Try to see what extrapolating the response of a bandpass filter does outside the passband sometime if you need proof! What is interpolation?Īny high school graduate should be able to tell you the formula for linear interpolation. Extrapolation is wishful thinking, and can get you in trouble in microwave engineering. Extrapolation is faking data points outside the dataset. Interpolation is "faking" data points that are contained inside a wide dataset. Note 2: when interpolating S-parameter data, is recommended that you interpolate magnitude and angle (you could call this "radial" interpolation) rather than real and imaginary data. We don't discuss that here, but you can get spline data out of Excel. Note 1: fitting a "spline" to data is often much more accurate than linear interpolation. Simple in concept, a royal pain to do in a spreadsheet. Interpolation means fitting Y-value data to to an X-value that is somewhere between two data points, using a straight line. BTW, what do you call the function when your kid wants help with their math homework? That's MathDAD! MathCAD has it (and tons more cool functions!), MathCAD calls it the LINTERP function. Go to our download area and get the Linterp_101 spreadsheet that we used to create this page.īefore we even get into the obtuse EXCEL calculation, let's point out that there is no excuse for Microsoft Excel not to have this function built in. Also when you have manufacturer's S-parameters, they are not always at the exact frequency you want. In microwave engineering, this happens in power bench measurements all the time (you only get a limited set of calibration data with a power head). There are many times in engineering where you find yourself interpolating between data points. I couldn't find this anywhere else and it was fantastic help!" "I just wanted to drop a quick note that your advice about using Index and Match Functions just helped me finish a project. Here's a quote from Josh who sent us an email in Febrary 2015: This page has one of our highest page views rates, because it is useful well beyond the microwave engineering community. Click here to go to our page on using linear interpolation to solve "one-way" equations
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