Recipe for creating microstrip filters?

Does it really make any sense to try such transformation ? After all, there are different loss mechanisms in lumped LCR circuits and at least with low cost PCB materials when used for microstrip filters.

Why not start from scratch and design a low pass filter followed by a band stop filter, possibly with an isolation stage (amplifier) between the sections ?

What I want to build is a 1.5GHz low pass filter with a reasonable sharp cut-off.

Nico Coesel Inscribed thus:

Try "Puff" ! I haven't used mine for a few years but I seem to recall that it was ideal for this sort of thing. The manual had an example design for a filter. HTH.

Baron Inscribed thus:

Preface: I haven't designed a microstrip (or whatever) filter yet, myself.

My impression of such filters is like this:

Suppose you want a, say, 6 pole bandpass filter, very narrow. You need 3 L's and 3 C's. The general design of such a filter is a parallel resonator, coupled to another parallel resonator, using a series resonator between (for a Pi design). A very sharp bandpass means the impedance of each resonator must be very different from the transmission line impedance, while the poles are kind of on top of each other (give or take pulling interactions). So the parallel resonators need a very low impedance to successfully shunt the line, while the series resonator needs a very high impedance to keep coupling to a minimum, except in the narrow frequency band where it's desired.

But with microstrip or what have you, it's very difficult to get such a large impedance ratio, so your filter Q (sharpness) is way down and you need more stages instead. This is not done with discrete components, because you can wind an arbitrarily good inductor, and one expensive inductor is better than matching three, smaller, custom inductors.

As I'm sure you're already familiar with, the basic idea of microstrip (or whatever) is to alternate between high and low impedance segments, where the low impedance segments look like low-Z parallel resonators and the high impedance segments look like high-Z series resonators. Or vice versa. Using the impedance of a resonator as the corresponding quantity, it should be very easy to calculate a simple bandpass by trace widths, of course you'd need to model it to verify dimensions are correct and the poles are in the right place.

A lowpass filter doesn't need large impedance ratios (high Q resonators), at least until the higher order poles. Getting a sharp corner could be challenging in that case, but using more stages always works.

You can save on trace width by giving it some height over the ground plane -- you can cut out a hole to give the field some room, but I don't know how to calculate the cutout required. Would also kill EMC.

Tim

I figured out myself! I found another paper which had an example of using wide traces for capacitors and thin traces for inductors:

I decided to follow the same path as the authors and see where that would get me.

The next big problem was finding a piece of software which could calculate the inductance and capacitance of a copper strip to the same results as in the paper. That turned out to be harder than one would expect. Even the tool you can download from Rogers doesn't work properly! The problem is that there are two formulas and which one is right depends on the ratio between the width of the track and the height of the substrate. Most tools only work when the trace width is less than the height of the substrate.

I was just about to give up when I found this web page. The microstrip calculator gives the same results as in the paper:

Now I could start to translate the lumped element diagram I got from a program called SVCfilter (

It looks kinda cool though:

On a sunny day (Mon, 24 Dec 2012 19:18:01 GMT) it happened snipped-for-privacy@puntnl.niks (Nico Coesel) wrote in :

Nice posting., nice board too.

There is actually already a program named 'wcalc' on Linux, so beware of overwriting: wcalc - a natural-expression command-line calculator

I tried that other thing in wine, but it wont run.

Thanks :-)

overwriting:

I gave up on running Windows software in Wine. Too much doesn't work and the Wine crowd seems to focus on games and office. Instead I installed virtualbox and installed Windows in a virtual machine. There is a Windows license sticker on the Linux box so why not :-)

It may be worthwhile to get a copy of the ARRL UHF Experimenter's manual.

The microstrip filters that I recall seeing in the amateur radio literature, and being billed as easy to make and reproduce (to the extent that they're called "no tune") use resonators made up of U-shaped traces

A quick search on "no-tune microstrip" got nothing, so I think you need to go old-school and buy a book, or Google around to see if you can figure out the right search terms.

Google for KK7B

One of his design is at

I recall doing some simulations on those hairpin filters as an experiment a few years ago. It turned out that they won't work at all at 300MHz. Since then I acquired some equipment to be able to measure in the GHz region.

How didn't they work? Low Q? Your simulation software blew up? You couldn't get the resonators coupled tightly enough? What were you simulating them on? What gives you confidence that your simulation results reflects real life?

I take it that you're working in the 300MHz range? That should be a low enough frequency that you could use lumped-constant components for most things. Have you tried making your filters out of all 50-ohm line, and just adjusting the length for the correct equivalent capacitance and inductance?

At least in that paper. You could probably get a higher Q with less coupling between the resonators (by moving them apart).

I am a bit confused, since I have always assumed that loaded Q, unloaded Q and insertion loss are all related.

Googling for various kinds of resonators, there appears to be a huge number of papers how to make microwave resonators with Q _less_ than 5 (apparently for some wide band services). Traditionally, the unloaded Q has been quite low due to the PCB material losses.

Right -- there are [at least] two "Q"s one could define in a filter circuit. The one that causes power loss* is the Q of the components alone -- simple resistive (or equivalent) losses. The other is the impedance of any given component in relation to the circuit impedance at that point -- typically, the line impedance. An inductor with a Q of 100 and a reactance of 50 ohms at some frequency, connected to a transmission line of 50 ohms, has an overall Q of about 0.99 (i.e., about 1/100th less than 1.0).

Tim

It was a long time ago but I recall the resonators started resonating at a multitude of 300MHz. Something like 1.2GHz (which makes sense). I used Sonnet for simulation. If the simulation results wouldn't be close to real life they probably would be out of business real quick.

I'm not specifically working in a range :-) At that time I choose