# Some Subtle Effects in the Measured Frequency Response of Passive Electronic Filters

## Major

Physics

## Submission Type

Poster

## Area of Study or Work

Physics

## Expected Graduation Date

2022

## Location

CNS Atrium, Easel 38

## Start Date

4-9-2022 11:15 AM

## End Date

4-9-2022 12:30 PM

## Abstract

In electronics, filters are used to block certain frequencies while allowing other frequencies to pass. Using capacitors, resistors, and inductors, engineers typically design low pass filters which let low frequencies pass, high pass filters which allow high frequencies to pass, and band pass filters which have a set range of frequencies that they allow through. Such filters are typically labeled by their characteristic frequency, f0. Textbooks claim that these characteristic frequencies f0 are fully determined by the value the resistance R and the capacitance C, as captured by the expression

f0 = 1/2πRC

Such an analysis implies that all that matters in filter design is the product RC, not the individual values of R and C. It also implies that the sequential order of the high pass and low pass circuits ( in the direction of signal flow) is irrelevant. By swapping the placement of the resistor and capacitor, any high pass filter can be converted into a low pass filter, or vice versa.

In our experiments, we have found that, for most cases, these predictions did match our experimental results. But for certain values, the switching of components produced results that disagree with theoretical predictions. In this abstract, we show just a paired example of two filters, both made by using a 2.2 KΩ resistor and a 2.8 nF capacitor. The data for the low pass filter (graph on the left) fully agrees with theory but when the components were switched (graph on the right), it responds in a way not shown in any textbook!

During this project, we are exploring a range of filter circuits, and we hope to explain the discrepancies between data and what is currently shown in most electronics textbooks. This might have significant implications for the pedagogy of filter design.

Some Subtle Effects in the Measured Frequency Response of Passive Electronic Filters

CNS Atrium, Easel 38

In electronics, filters are used to block certain frequencies while allowing other frequencies to pass. Using capacitors, resistors, and inductors, engineers typically design low pass filters which let low frequencies pass, high pass filters which allow high frequencies to pass, and band pass filters which have a set range of frequencies that they allow through. Such filters are typically labeled by their characteristic frequency, f0. Textbooks claim that these characteristic frequencies f0 are fully determined by the value the resistance R and the capacitance C, as captured by the expression

f0 = 1/2πRC

Such an analysis implies that all that matters in filter design is the product RC, not the individual values of R and C. It also implies that the sequential order of the high pass and low pass circuits ( in the direction of signal flow) is irrelevant. By swapping the placement of the resistor and capacitor, any high pass filter can be converted into a low pass filter, or vice versa.

In our experiments, we have found that, for most cases, these predictions did match our experimental results. But for certain values, the switching of components produced results that disagree with theoretical predictions. In this abstract, we show just a paired example of two filters, both made by using a 2.2 KΩ resistor and a 2.8 nF capacitor. The data for the low pass filter (graph on the left) fully agrees with theory but when the components were switched (graph on the right), it responds in a way not shown in any textbook!

During this project, we are exploring a range of filter circuits, and we hope to explain the discrepancies between data and what is currently shown in most electronics textbooks. This might have significant implications for the pedagogy of filter design.