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."
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
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
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.
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.
Posted January 13, 2014