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This free RF Filter program specializes in the design of LC band pass filters, but it also synthesizes low pass, high pass, and notch filters.

Designing a band pass LC filter is almost a lost art today as most of these filters are built with ceramic resonators, and for the obvious reason, size. There are however, many relatively low volume applications not serviced by the ceramics houses that need these filters.

RF filter design requires that you be very good with a simulator, network analyzer, and layout tools. This software will help do the easy part, which is, coming up with a good starting point for your simulator and optimizer.

There are 4 basic ways to build an LC band pass filter. The first is the simplest and taught in most textbooks, which we call the canonical form.

The second method uses the techniques described in Elliptic Approximation and Elliptic Filter Design on Small Computers, Pierre Amstutz, IEEE Transactions on Circuits and Systems, Vol. CAS-25 No12, December 1978.

The third method uses Norton Transforms. Most EE students learn about Norton Transforms in college, but not as they apply to band pass filter design.

The forth, and most popular design method, is the Direct Coupled filter. This method was outlined in Direct Coupled Resonator Filters, Seymour Cohn, Proceedings of the IRE, Feb 1957, pp. 187-196.

Here are examples of these band pass filter topologies. 

3 Band Pass Filter Forms.png

See this page for 70 More 3 Pole Band Pass Topologies

The following is a short discussion on the pros and cons of these four types of band pass filters.

Canonical Filters
The canonical form, or simplest form, is described in most textbooks, but is almost useless in practice. These filters cannot be built unless the bandwidth is greater than about 50 percent. Even then, the component values required to build the filter become impractical. The series inductors are prohibitively large in the sense that their self resonant frequency becomes a major problem. It is also impractical, and usually impossible, to obtain the nominal values needed, not to mention the problem with tolerances.

In general, the only place for a canonical band pass filter is in applications requiring very wide bandwidth. If you are new to RF filter design, take some free advice and don't waste any time with these, except for the time required to understand their limitations. Engineers understood these limitations many years ago, and devised methods that add parts to the design which in turn gives you some latitude in the selection of component values.

Elliptic Filters
If you implement an elliptic band pass RF filter, the IEEE will almost certainly give you a lifetime achievement award, because that's how long it will take to make it work. The problem is component tolerances, there can't be any.

Sarcasm aside, elliptic RF filters are little more than a textbook curiosity. If you are really desperate, and only need one or two of them, they are OK for lab work, but I seriously doubt anybody has ever put an elliptic RF band pass into production. They have very little practical value.

Norton Transformed Filters
As we all learned in EE 101, an ideal transformer with a K turns ratio may be inserted into a circuit at any location. The effect of the transformer is to change the impedance of the circuit by K squared. This program inserts two ideal transformers into the circuit, one on each end, and uses Norton Transformations to take the ideal transformers out. An example is shown here. The interested reader should consult this text for more details: The Analysis, Design, and Synthesis of Electrical Filters, Humpherys, 1970 Prentice Hall.

Norton Transform.png

When the ideal transformer is inserted, the series cap is first replaced by two caps both equal to 2*C2 so that the transformation can be done on both ends. In doing the Norton Transform, the C1*K*K/(1-K) term will generate a negative value for K>1. This negative value is added to the 2*C2*K*K value for a positive result. Either the Tee circuit or the Pi circuit can be used in the final circuit. 

For a given circuit, there is usually a number of different ways to implement Norton Transforms. For example, this program has 4 transforms for 3 pole band pass filters and each transform has an infinite number of potential results depending on K. Which transform works best will depend on the frequency, impedance, and bandwidth of the original circuit.

Norton Transformed filters are not as widely used as Direct Coupled filters. They are probably most used in medium bandwidth applications. They work just as well as a Direct Coupled filter in narrow band applications, but the design method is a bit more complicated so don't get used as often. The impedance inverters used in Direct Coupled filters are inherently narrow band, so "direct coupled" implies "narrow bandwidth".

Direct Coupled Filters
This is the most popular form of LC band pass filter. These filters can be designed in so many ways, it becomes a chore to describe them all. Numerous articles have been written on this design method, so there is no need to labor over the details here. The Iowa Hills RF Filter program has 10 types of tank circuits, and 3 types of inverter circuits, plus it will mix inverter types with matching types. The number of possible filter topologies is huge. Each topology has a slightly different response and its own set of advantages.

Here is a paper we have started that describes how to design Direct Coupled Filters in some detail. It is a work in progress.

In the screen shot below, we show a very popular, and very useful design that uses shunt LC tanks, capacitive inverters, and inductive matches. If you are new to Direct Coupled filters, you should notice that the 3 tank inductors are same "nice value", 22 nH. The beauty of this design method lies in the fact that these tank inductor values are chosen, not predetermined. This is incredibly important because it allows you to minimize the filter's insertion loss and use part values that inductor vendors have available.


RF Filter Screen Shot
RF Filter Program Screen Shot

Available on the  Download Page.

RF = ReFined

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