Philippa Young

Designing an Analogue Low Pass Filter System for a Quantum Ion Trap.

Simulating Filters

By far the easiest way to design a filter is to use a filter design program. The two I used were ‘Filter Wiz Pro V4’ for active filters and then ‘AADE Filter Design V4.4’ for the passive filters.

Active Filters.

Filter Wiz Pro V4 is a very useful tool in designing active filters, it guides you through the process step by step and provides you with a response curve, a parts list and a circuit diagram.

Details of how to use this program can be found here.

By simulating a range of different response types and order numbers I found that the best option was the Chebyshev Type I 8th order. The following graph shows the response of the filter and the circuit diagram.

As can be seen in this circuit diagram each filter needs a very large number of components, so the PCB may be very large.

Passive filters.

AADE Filter Design V4.4 is a much more basic package and you input your attenuation and cut off frequencies and it will create your filter ‘ladder’ and provide a list of optimum values of the components. From this you can plot a curve of the response.

After looking at this program it became clear that a passive filter would be just as efficient as an active one. Although there are more orders the orders themselves are made up of far less components and so overall would be easier to design and build.

The following shows a typical filter which could be output by the package.

The top image shows the circuit diagram of a 10th order Chebyshev low pass filter and parts list which is provided by the filter design package.

The graph below shows the filter response which cuts off to -80dB by 2MHz.

Details of how to use this program are here.

<< Back to overview

Next: Designing the PCB sourcing parts >>