Function "lowpass()" Behavior Dependent on Sampling Rate?

Hi folks,

I'm trying to determing the bandwidth of amplifier I'll need, as part of control system.

So I wrote up a simulator script and got this waveform:

Fig.1: Waveform from simulator output

Then I tried to apply a lowpass filter to the waveform, and see how the signal will be warped by the limited bandwidth.

I tried to use the "lowpass()" function provided in the signal processing toolbox:

Fig.2: Call of the lowpass function

Here comes the wierd part, the output do not change when I changed the variable "bnd_f", which should be the cut-off frequency. But when I changed the simulator time resolution (i.e. 1/sampling rate), the output from "lowpass()" changed.

Fig.3: The changed value res_t is highlighted

Fig.4: Resolution = 1ns

Fig.5: Resolution = 5ns

Fig.6 Resolution = 10ns

I'm so confused by what's going on. Any suggestion is highly appreciated!

Many Thanks,

Anyu

[Anyu]

Seems like I posted the wrong picture for Resolution = 5ns, here's the correct one:

Replace Fig.5: Resolution = 5ns

Answer 1

[Rishav]

Hi Anyu,

Yes, the 'lowpass' function in MATLAB is dependent on the sampling rate of the input signal.

Please refer to the following documentation to know more about the 'lowpass' function:

https://www.mathworks.com/help/releases/R2023a/signal/ref/lowpass.html

[Anyu]

Thank you for your reply!

In that case, can you please suggest some way I can test the impact of bandwidth on the signal? The target signal is output from simulator, and contained in a 1x20001 vector in workspace.

Thank you very much!