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# low pass filter design- how to start the calculation?

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#### popoinzaghi

##### Newbie level 3
low pass filter design

Hi, im still new in rf design for filter.Actually i've got some coursework for low pass filter design.Im dont know how to start the calculation as the question provide:
i) dielectric constant of 2.2
ii) thickness of 0.787mm
iii) metal thickness is 8µm
iv) physical size should not exceed 7×7 cm² (including feeding lines)

The final filter design must comply the specification illustrated by the figure in my attachment.I dont really understand the figure.Maybe anyone can give some idea & explanation.

Re: low pass filter design

What it means is that if you graph the transmission, the line should be within the lighter area on the figure/graph scale. The key points are the insertion loss at the lower frequency up to the "corner" then the attenuation should drop rapidly in the vertical light area in the center of the graph and then the attenuation should be greater than the light area on the right side of the figure.

This method of specification is commonly called a "mask."

Re: low pass filter design

ok..but i still dont understand how to calculate the order of the filter,n since the cutoff frequency is not given.

Re: low pass filter design

popoinzaghi said:
ok..but i still dont understand how to calculate the order of the filter,n since the cutoff frequency is not given.

Hi popoinzaghi,

There are a lot of filter design programs (partly with demo versions for free) which you can use to determine the order required.
All you have to do is to insert the information from the attenuation graph (mask) into the program. Then you have the choice between several approximations which fulfill the mask.

Internet links: filter-solutions.com, microchip.com, schematica.com, linear.com.

low pass filter design

Although you didn't tell, the problem formulation seems to suggest a microstrip or similar distributed filter.

The cut-off frequency is actually given by the filter mask that you posted.

Generally, the problem wouldn't make sense, if you haven't been provided in the course with respective methods to solve it. You should know about these prerequisites, but unfortunately we can't.

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