Side Project Scientist

A website about my projects.

Coffee Can Radar Low Pass Filter

Sat 09/05/20 07:26:18 pm
Categories: Electronics

The next part of the Coffee Can Radar is a Fourth Order Active Low Pass Filter.

UPDATE 9/12/2020 In terms of peak-to-peak voltage the circuit is performing according to the design specifications.

The original design specified a MAX414CPD+ quad op-amp. It is no longer manufactured so the course author suggested using a TI LM324 instead. The filter circuit is designed to pass frequencies up to 15 kHz. The amplitude of signals at 15 kHz should reduce by 3 dB and amplitude should further decrease as signal frequency increases.

Here is the low pass filter circuit (on the right) next to the previously assembled modulator circuit (on the left):

I used on of my signal generators to test it out:

In the following pictures the yellow curve is test signal sent into the filter and the purple curve is the filter output.

As expected, for a low frequency of about 30 Hz there is no attenuation:

Also as expected, for a frequency close to but below the threshold, in this case about 9 kHz, there is no attenuation:

Above the threshold, at about 20 kHz, there is a 2 dB reduction in the signal:

UPDATE 9/12/2020 In terms of peak-to-peak voltage the reduction is -6.57 dB.

An increase to about 27 kHz reduces the signal by 3 dB:

UPDATE 9/12/2020 In terms of peak-to-peak voltage the reduction is -13.65 dB.

UPDATE 9/12/2020 I checked the attenuation at about 15 kHz. In terms of peak-to-peak voltage the reduction is -3.86 dB:

I don’t know why I am getting the 3 dB reduction at 27 kHz instead of 15 kHz. I may have an error in my circuit or the substitute op-amp may have different performance than the original op-amp the author used. I don’t know whether the attenuation level at 15 kHz is critical or whether the levels I have are good enough. I plan to rewire the circuits to clean up the breadboard anyway so I’ll check everything as I go through that task. For now I think the low pass filter is a success and I’m glad I finally had a practical use for the signal generator.

UPDATE 9/12/2020 Based on the results of calculating attenuation in terms of peak-to-peak voltage and the additional test at 15 kHz, the low pass filter is working correctly.