Academic Master


Generating Digital Signals with the Addition of Noise, and Building a Digital Threshold Detector


The primary objectives of this lab are:

  • To generate digital signals with the addition of noise
  • To build a zero-crossing threshold detector


This experiment is about generating digital signals with noise content added to them and making a digital threshold detector. Thus, this lab will help us understand the concept of noisy signals and assist in comprehending the underlying design of digital threshold detectors. This report, however, caters to the part of the experiment till the generation of a noisy digital signal. The simple square wave is used to simulate a digital signal and noise generator, and the adder is used for the addition of noise in the digital waveform.


Noise is the unwanted content that gets added to a signal during storage, transmission, or processing. In real scenarios, no signals that are transmitted are free of noise. Noise causes the signals to distort, and in case of a significant noise content, a signal might lose its meaning, and hence, errors are caused. There can be numerous causes of noise; for example, noise can be caused by heating electronic equipment or electrostatic discharges. The first part of this lab will help generate digital signals with added noise using National Instruments LabVIEW and NI ELVIS II+ Emona Telecom Trainer.

The second part of this lab deals with building the zero-crossing threshold detector, which is used to detect the transition of the signal from the positive voltage level to the negative voltage level and vice versa (, 2018). A comparator embedded in the lab workstation (NI ELVIS II+ Emona Telecom Trainer) will be used to build a digital threshold detector, by providing the noisy signal at one of its inputs and connecting the other input to the ground.

The following discussion will only be related to the first part of the experiment, which is generating a digital signal with the addition of noise.


A square wave will be generated with a frequency of 1 kHz and amplitude +/- 2.5 Volts to simulate a digital signal. This wave then needs to be added with noise. The final signal to be produced is a square wave with an amplitude of +/- 2.5 Volts and a signal-to-noise ratio 5.


The first step is to generate a square wave using a function generator. The amplitude of the function generator is set to 2.5 volts and the frequency knob is set at 1 kHz. In waveform settings, the square wave is selected, and sweep settings include a start frequency of 100 Hz, stop frequency of 1 kHz, step to be 100 Hz, and step interval to be 1000 milliseconds as shown in figure (1) below.

Figure 1: Setting the function generator to produce a 1 kHz square wave

The next step is to add noise to the output of the function generator (FG). The square wave generated is then fed into the adder using a red male-to-male connector. On the second input of the adder, the -20 dB output of the noise generator (NG) is connected using a white male-to-male connector as shown in Figure (2) below.

Figure 2: Connecting FG output to Input A of Adder (Red Link) and connecting -20dB output of NG to Input B of Adder (White Link).

To display the generated square wave with added noise on the oscilloscope, the output of the adder is connected to channel 0 of the oscilloscope using the red probe as shown in Figure (3).

Figure 3: Connecting Channel 0 to the output of Adder.

Results and Discussion

The output of this whole experiment is the 1 kHz square wave with an approximate signal-to-noise ratio of 5. The resultant waveform is displayed on the oscilloscope as shown in the figure below.

Figure 4: Output signal with the SNR approximately equal to 5.

The oscilloscope shows the distorted square wave due to the added noise using a noise generator. This result indicates that the noise content causes the signal to distort and the increased level of distortion is the source of errors in signal detection.


We can conclude this discussion by the statement that the noise that gets added to the signal during its storage, transmission, or processing causes the signals to distort as seen in the experiment results.

Reference List

  • (2018). Zero Crossing Detectors. [online] Available at: [Accessed 11 Feb. 2018].



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