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FilterDE: Free Software for Distributed Element
Lowpass and Bandpass Filter Design

Justin Crooks sent me an e-mail alerting me to a free* distributed element filter program he just released called "FilterDE." It was designed to assist in the development of distributed element RF microwave filters, consisting of series and open shunt transmission line elements for low pass filters, or parallel coupled lines for band pass filters. For low pass filters, FilterDE uses an iterative optimizer, enabling the user to design a filter by specifying the desired performance of the filter and the number of poles. For band pass filters, it uses edge coupled filters based on Chebyshev Polynomials. FilterDE uses ideal, loss-less models for distributed elements. This means that there will be higher insertion loss and more rounded transition regions."

FilterDE: Free Software for Distributed Element Lowpass and Bandpass Filter Design - RF Cafe

I downloaded and ran FilterDE and entered parameters for the lowpass filter to match the user's manual. This is v0.2 of the program and there are minor differences between the manual and the program, like for instance v0.2 allows the user to select a specific order for the filter after specifying the frequency parameters. The output screen is formatted in four quadrants as shown above (5-pole lowpass). The filter physical layout, a table of element impedances, and S21 and S11 plots are provided. It is up to the user to calculate trace widths and lengths for each filter element depending on the substrate parameters. The right column of the table references a Note 1, which although not specifically designated, appears to be below the table (it is an online calculator for determining trace width).

The lowpass filter is calculated based on an optimizing algorithm rather than a standard pass band transfer equation for Butterworth, Chebyshev, etc. The bandpass filter designer, on the other hand, generates element impedances for the Chebyshev equation. Only odd order BPF designs are available because as you probably know, an even order Chebyshev filter with equal input and output impedances cannot be generated without compensating element values for the necessary impedance transformation. Maybe the next version will accommodate even order designs (not a big deal if it doesn't, though). The cursor in the plots can be moved with your left and right arrow keys.

* Well, it's not totally free. The price you pay is a popup advertisement at the end of the design process for Signal Hound spectrum analyzers. From their website, "Streaming a 20 MHz digitized RF bandwidth to your PC at 140 MB/sec is finally here at an affordable price! It’s done over a USB 3.0 SuperSpeed link to your PC or laptop. The BB60A sweeps up to 24 GHz/second and is optimized for broadband, time-stamped, and real-time applications requiring RF spectrum analysis and/or RF recording." Prices start at just south of $1k. Tracking generators are also available for sweeping amplifiers, filters, etc.

Signal Hound spectrum analyzers - RF Cafe   Signal Hound BB60A output screen - RF Cafe

Posted  January 13, 2014
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