Stepped impedance lowpass filter - how to compensate lengths?

[nctnico]

If the objective is to actually make a filter of higher than about third order ... the first step is to run some boards and model the parasitics that will always be present. Extracting them from a 7th order filter without a major test equipment budget ... not going to work.

I tend to disagree. Sonnet is pretty good as a simulation tool. For example: in order to verify my Elliptic microstripline software tool I build 2 filters (both 1GHz low pass) on FR4 and it turns out the real filters match the simulation just fine (within manufacturing tolerances). With an RF generator and/or a spectrum analyser (even if they are rather crude) you can get a long way and you don't need to get real fancy since microstrip can be used from around 800MHz. With the RF generator directly coupled to the spectrum analyser you can create a baseline (zero reference). Anything else in between will cause a change compared to the baseline.

[G0HZU]

Yes, Sonnet is very good. I've been using it at work for 12 years or so. I've always used it with Eagleware/Agilent Genesys because you can install Sonnet and use the Sonnet engine within a Genesys project.

http://www.keysight.com/main/editorial.jspx?cc=GB&lc=eng&ckey=1824231&nid=-34275.0.00&id=1824231&cmpid=zzfindeesof-genesys-sonnet

With Genesys you can draw the schematic and design the PCB and then you can export the PCB to Sonnet. The results from Sonnet automatically get exported back to Genesys and you can plot the results for the entire circuit on the PCB layout. It's a very powerful combination.

I have a lot of faith in its accuracy and I usually design filters on Rogers 4003 or 4350 in various laminate thicknesses. Sonnet is good at simulating filters or couplers that use multilayers as well. I also use Genesys+Sonnet to simulate the PCB layout of circuits like LNAs or oscillators up to many GHz. It's an old friend now and I trust it more than the Agilent Momentum EM simulation tool we also have at work.