Matlab: plotting one period of a discrete signal

[Steve Collins]

I have a 3 rad/s sine wave that I am sampling every 2 seconds.

The slow sampling has created an alias of -0.14 rad/s.

I have plotted this over the top of the original continuous time signal, please see attachment.

My problem is that I only want to plot the first period of the discrete signal, i.e. end at 44.88 seconds, but I want to plot the original CT signal for the full 60 seconds.

I'm going around in circles and getting some strange results. Hopefully somebody out there can shed some light.

Cheers

Steve

 

[berkeman]

I have a 3 rad/s sine wave that I am sampling every 2 seconds.

The slow sampling has created an alias of -0.14 rad/s.

I have plotted this over the top of the original continuous time signal, please see attachment.

My problem is that I only want to plot the first period of the discrete signal, i.e. end at 44.88 seconds, but I want to plot the original CT signal for the full 60 seconds.

I'm going around in circles and getting some strange results. Hopefully somebody out there can shed some light.

Cheers

Steve

Are you doing this to illustrate aliasing on purpose? What is the context of this work? It seems like you can plot whatever you want -- what is the issue? Sorry if I'm missing the obvious...

 

[Steve Collins]

Are you doing this to illustrate aliasing on purpose? What is the context of this work? It seems like you can plot whatever you want -- what is the issue? Sorry if I'm missing the obvious...

I'm in the process of making a GUI that prompts a user for signal frequency, sample time and length.

If the user enters a sampling rate that is insufficient I want to lay the first cycle of the aliased signal over the original CTsignal to, as you say, illustrate aliasing.

As you can see from my attachment I have successfully overlaid the aliased signal, but I would like to only display the primary period.

 

[Steve Collins]

I managed to sort my problem out…. sort of.

I found the time taken to make one revolution

Circle time= (1/frequency) x 2 x π

And then made a vector from zero to

circle time x (1/sampling time)

 

[rezeew]

Thank you for sharing your question and concerns. It seems like you are dealing with some issues related to sampling and aliasing in your signal processing. To address your problem, I would suggest taking a step back and reviewing the fundamentals of sampling and aliasing in discrete signals.

First, it is important to understand that sampling is the process of converting a continuous signal into a discrete signal by taking measurements at specific time intervals. This is usually done to make the signal easier to process and analyze. However, sampling can also introduce errors and distortions in the signal, such as the aliasing you have observed.

Aliasing occurs when the sampling frequency is not high enough to accurately capture the original continuous signal, resulting in a distorted representation of the signal. In your case, the sampling frequency of 2 seconds may not be high enough to capture the 3 rad/s sine wave accurately, causing the alias of -0.14 rad/s.

To address your issue of plotting only the first period of the discrete signal, you can use the "xlim" function in Matlab to specify the range of the x-axis for your plot. This will allow you to plot only the desired portion of the signal. However, keep in mind that this will not change the underlying issue of aliasing in your signal.

To prevent aliasing, you can increase the sampling frequency or use a low-pass filter before sampling to remove high-frequency components of the signal. This will ensure that the sampled signal accurately represents the original continuous signal.

I hope this helps to shed some light on your situation. It is always important to have a good understanding of the fundamentals when dealing with signal processing to avoid unexpected results. Good luck with your research!