The Noise Source Block acts as a source of thermal noise. In this example, we have a user defined bit sequence connected to an NRZ pulse generator which will represent the data signal. We will be combining a noise source with our data signal to show how noise affects it. We will then observe the effects of changing the noise properties.
About The Component
The noise source block acts as a source of thermal noise. In this example we have a user defined bit sequence connected to an NRZ pulse generator which will represent the data signal. we will be combining a noise source with our data signal to how noise will affect it. We will then modulate the data signal with a with a carrier signal, use a photo diode to receive the optical signal and then send it through a low pass filter to extract the data signal. We will then check the eye diagram to see how the added noise will affect the eye diagram.
Calculating the project with initial values we can see that the noise doesn’t show on our encoded signal and the eye diagram looks perfect as well. This is because the power of our noise source is relatively low compared to the signal strength. Double clicking the noise source brings up the noise source properties. In this window we can modify the noise power to make the value larger.
 Calculating the project and clicking the visualizer we can see all of the signals. Selecting signal + noise will show us the signal added to the noise. This time we notice the noise that is added to the NRZ pulse. Clicking the BER analyzer, we can view the eye diagram and this time the eye diagram is slightly noisier and now has minimal signal distortion.
Double clicking on the noise block and Going into the noise window we can select add noise to signal. Running it and going into the visualizer we can see that this simply adds the noise and the signal together and this time they display as 1 signal instead of being displayed as separate signals. In the properties of the noise source we can select PSD which allows us to chose the power spectral density instead of average power. If we were to leave the rest of the settings the same and calculate the project, we will see that the power of the noise is so high that it drowns out the rest of the signal. this is because we went from having and average power of 10 dbm to having and spectral power density to 10 dbm. Typically, in psd mode, you would expect to have lower values of noise power. Changing the noise power to -91 dbm we can see that we are starting to see the shape of our NRZ pulse and the noise is less overwhelming. Changing the noise power to -101 dbm we can see that the signal resembles the nrz pulse more clearly and it is easier to distinguish a 1 from a 0. Looking at the eye diagram we can see that it is less noise and has less distortion.
In the noise source properties, we can also change the units of power. Changing the units to watts will show us the equivalent power of noise in watts. In the simulation window we can change sample rate by clicking beside the box. Pressing evaluate will show what sample rate is currently defined as. Sample rate is currently defined by the global parameter sample rate. It can also be changed to be dependent on other layout parameters or a function. Double clicking anywhere on the work space we can see and change any of the global parameters including the sample rate.
Going back into the noise source and pressing evaluate script we can now see the new value of sample rate. In the random numbers tab, deselecting generate random seed will make the generator not use a randomly generated seed to create the random noise and will instead use the defined random seed index to generate the noise. This means if we run the random noise generator multiple times, the noise generated will be exactly the same because the random seed stays the same every time it is run.