# Filter design for frequencies in range 0-60Mhz

First of all excuse me for asking a seemingly vague question. But i am going nowhere in the problem i am trying to solve.

I am working on a powerline communication system where i need to upconvert in 28-56Mhz range and later downconvert it to 0-28Mhz so that it works in the coupler range.So i need to design a band pass filter in 28-56Mhz and a low pass filter in the 0-28Mhz range. I need to design an analog filter for the same. I have no experience of filter design. I have been looking online for resources but wherever i go there is an explanation on chebyshev, butterworth and elliptic filters.Then you have 1st order,2nd order and higher order filters equations. My problem is from all this i am unable to decide which type of filter to use and what order to use. So i need to understand practical design constraints and examples of how to design given certain constraints and then boiling down to a solution. So could anybody help me in how to approach the filter design problem and suggest either examples or resources to go forward.

As you stated there are some parameters associated with filters such as type of filter (butterworth, elliptic, chebyshev I,chebyshev II, bessel,...), order of filter, etc. (Note that there are lots of considerations in filter design bases on the application)

Let's first talk about the order of filters: Filters with higher orders let you have a filter with better quality. For example in lowpass filter if you want to have a sharper transition band you need to design a filter with high order filers. Order of a passive analog filter is determined by the number of elements you use in filter (resistor, inductor, capacitors). This was all I knew about the order of the filter.

Now let's talk about the most applicable kinds of filter types. The most important difference between filter types is the band they have ripples. Note that the priority of filters is due to the application and when we say one filter is better than the other we must state that in what aspects (order, ripples, poles, zeros, stability, group delay, phase response, minimum phase, ...) this is better than that one. Picture below compares 4 important types of filters designed with the same order. As you can see, Elliptic filter has ripples both in pass-band and stop-band while it has the sharpest transition, and butterwoth filter has no ripples but has least sharpness among filters of the same order.

• I understand the characteristics of different types of filters. But how do i decide how much ripple my application can tolerate.Similarly how much phase response and group delay can i work with is also a question. Any example designs and implementations will also be helpful. Maybe a book that explains analog filter design from basics could be helpful maybe. – Karan Talasila Aug 22 '13 at 1:16
• Unfortunately, in the context you said I don't have any information so I refer you to Design of Analog Filters 2nd Edition. I hope it helps you – Mahdi Khosravi Aug 22 '13 at 4:13

First, you are trying to design an RF filter. So when you Google for filters, try searching on terms like "Direct Coupled Filters" or "RF Filters" or "Norton Transform Filters" or "Seymor Cohn". Cohn authored the most important RF fiter design paper ever published (around 1950).

Second, don't worry about the prototype. With these sorts of filters, you essentially have one choice, the Butterworth. The reason for this is that the filter's response will be governed more by the components you use than the prototype. It is virtually impossible to obtain a true Chebyshev response from a filter constructed with inductors that typically have a Q of less than 30. You can't achieve a true Butterworth response either, but it is the best place to start.

Third, if you are going to do this, you need an RF circuit simulator. You absolutely must simulate these circuits using accurate component models. Once you do, you will quickly understand what I said about the prototype. You will also find that, depending on the bandwidth of your bandpass filters, you can't implement more than about 3 poles. The component losses will destroy the response.

Forth, if you are designing an up converter and down converter, you need to make some careful decisions regarding the frequencies you use. i.e. Whether to use low side or high side injection. It can have a significant affect on the filter requirements. So to get started, you should first understand your filtering limitations, and then decide on a mixing strategy.

I hope this helps, but the task you describe is a great deal of work, even for experienced professionals.

You may want to consider using either a crystal, SAW, or ceramic filter, rather than trying to design your own.

• Thanks for the reply. The problem with butterworth was the transition band was just too huge for me to go forward with it. the other major problem was bandwidth . I had to design a 28Mhz bandpass filter in the frequency range of 28-56Mhz which according to what i read was almost impossible. So i gave up and am not using the filter. I have seperated the frequencies by a large value and hoping that the signal from one band will not interfere with the other. – Karan Talasila Oct 18 '13 at 16:00