In previous articles (including here, here and here), we discussed the different types of filters available, including examples. If you followed along, you should have an understanding of frequency components (i.e., attenuation of high frequencies when using a low-pass filter and the pass frequencies for a high-pass filter). You should also be aware of the four basic hardware filters (LPF, HPF, BPF, and BRF) and their uses, and be able to differentiate between theoretical and practical implementations.
In this article, we will demonstrate real-life applications of filters. The basic principles of hardware filter design are relatively simple, but additional parameters come into play when moving from theory to application. For example, consider the design of a low-pass filter. It seems simple with just two passive components, a resistor and a capacitor. But the cutoff frequency transition period will be extremely slow because this is a first order filter.
The filter transition period is defined as the period between the passband and the endband. The amplitude versus frequency curve goes from high to low, as shown in the figure below. A low-pass filter with a cutoff frequency of 1KHz will pass the frequency components to 10KHz, but with a lower amplitude. A higher order filter can be used to reduce this transition period.
The transition period of a low-pass filter.
That's enough theory. Now, let's implement a low-pass filter followed by a band-reject filter.
The average frequency of a typical male voice is 120 Hz and that of a typical female voice is 200 Hz. For this project, we will use the audio of a human voice, later adding a noise frequency of 10KHz. A low-pass filter is used to transmit low-frequency human voice and stop 10KHz high-frequency noise.
The low pass filter is designed as an RC filter and the cutoff frequency of this low pass filter is 1Khz.
Electronic components:
1. TDA2030 Audio Amplifier
2. 5V source (power bank)
3. Boost Converter for TDA2030
4. Speaker
The configuration…
The low-pass filter setting.
Below you will find a video of the completed project. Speech audio is part of our data, as is 10KHz sine wave noise. The difference between the audio signal and the noisy signal (audio + 10KHz sine wave) can be observed without the low-pass filter.
After applying the low-pass filter, the noise in the audio signal is removed or attenuated.
Notice how clearer the audio is after using the low-pass filter. This means that there were more high frequency noise signals in the audio (speech) signal than in the 10KHz sine wave.
The low-pass filter removes high-frequency components from the audio signal.
The video…
Another experience
A different application is used in the next implementation of the low-pass filter. In electronics several types of converters, including:
- The step-down converter, which converts high voltage to low voltage
- The step-up, which converts low voltage into high voltage.
One of the most famous step-down converters is the LM2596.
Explanation
In this experiment, we will “step down” 22 to 5 volts using the LM2596 module. A 2.5 Ohm resistive load is used to draw current from this module. When drawing a high current (such as two or three amps) from the LM2596, ripples may occur in the output.
Using the LC low pass filter, it is possible to remove these ripples from the LM2596 module.
This image demonstrates 20V input voltage and 5V output of the LM2596.
A 5V DC voltage output from the LM2596.
Now, let's connect a 2.5 Ohm load to the output of the LM2596 module, while drawing about two amps of current from the LM2596 module. Current can be calculated using Ohm's Law.
V = IxR
I = 5/2.5 Amps.
After connecting the load, it consumes a calculated current and a ripple is displayed on the oscilloscope at the 5V output.
Ripples at the output, after connecting a load of 2.5 and 4.10 ohms in parallel.
A close-up view of the ripples on the oscilloscope output screen.
These ripples are 20 kHz as displayed on the oscilloscope. So how do we remove these ripples? Let's try a low pass filter.
Depending on the application, we will need pure DC at the output, and DC has zero frequency components. But a low pass filter should solve the problem. A 1, 2 or 3 Khz filter can be used because the ripple frequency is high. We chose an LC low-pass filter due to the high current requirements of the output.
An LC filter is designed according to the figure below.
A 2.7 KHz low-pass filter
An LC filter on the PCB
Let's connect this designed low pass filter to the output of the LM2596 module and the load.
The complete configuration, including the low-pass filters with the LM2596 module and the load to remove ripples.
After connecting the low-pass filter to the output of the LM2596, the ripples disappear. The project was a success! The low-pass filter removed high-frequency components from the DC voltage. This means the filter is designed correctly and is working well.
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