Sampling frequency of digital low pass filter

[sabu31]

Hi all,

I want to filter out dc component of a signal. I am using a low pass filter with cut-off frequency of around 10Hz. I would like to know what should the nominal sampling frequency of this filter.

Thanking you.

 

[andre_luis]

Question too vague...
You didn't mention for what application, with what constraints, etc...

 

[sabu31]

Question too vague...
You didn't mention for what application, with what constraints, etc...

Presently I am simulation stage in matlab. It will be used for active power filters application. I would like to know some references regarding effect of sampling frequency.
Say what would be the effect if sampling frequency is kept at 1kHz as compared to 10kHz. The signal contains order order harmonics (50Hz fundamental along with dc components).

 

[oscarcot]

Hi all,

I want to filter out dc component of a signal. I am using a low pass filter with cut-off frequency of around 10Hz. I would like to know what should the nominal sampling frequency of this filter.

Thanking you.

Hello Sabu,

It's true, you didn't describe your problem at all. By "filter out", I understand you want to remove DC. If that is the case, you should use a high-pass filter and not a low-pass. In that case, the sampling frequency should be at least as high as two times the signal bandwidth fs= 2*BW.

If what you want is actually the DC, then use the low-pass. By default, I would advice you to use fs= 10*fc. If the high frequency noise is too high, the alias might produce an ofset on your DC signal.

Good luck,
Oscar

 

[sabu31]

Hello Sabu,

It's true, you didn't describe your problem at all. By "filter out", I understand you want to remove DC. If that is the case, you should use a high-pass filter and not a low-pass. In that case, the sampling frequency should be at least as high as two times the signal bandwidth fs= 2*BW.

If what you want is actually the DC, then use the low-pass. By default, I would advice you to use fs= 10*fc. If the high frequency noise is too high, the alias might produce an ofset on your DC signal.

Good luck,
Oscar

Thanks Oscar,

I would like to retain DC component only . I want to know about aliasing effect. As you mentioned about fs=10*fc. If I keep cut off frequency at 10Hz, and sampling at 100Hz. What would signal components such as 150Hz, 250Hz etc contribute to filtered dc signal.

 

[oscarcot]

Thanks Oscar,

I would like to retain DC component only . I want to know about aliasing effect. As you mentioned about fs=10*fc. If I keep cut off frequency at 10Hz, and sampling at 100Hz. What would signal components such as 150Hz, 250Hz etc contribute to filtered dc signal.

Hi Sabu,

much clearer now, you want to keep the DC component of a signal with 50 Hz and harmonics. I guess you're actually trying to simulate a rectified power supply, like a half-way rectifier. Then take ten times the 50 Hz frequency of the power supply. Most harmonics will be dead by 500 Hz. fs= 10*(f_AC). If you try lower cut-off frequencies you'll get an offset. There will be a frequency (approaching 500 Hz I guess), where there is no longer influence from the sampling rate fs.

Give it a shot. Good luck,
Oscar

 

[KlausST]

Hi,

simple answer:
(Nyquist Shannon)

To avoide alias frequencies: your sampling frequency needs to be more than twice of the max. frequency that is included in your analog signal.

* So either you use a high sampling frequency,
* or you use an appropriate LPF in your analog signal.

Klaus

- - - Updated - - -

Added:

Most harmonics will be dead by 500 Hz. fs= 10*(f_AC)

The european regulations for mains frequency voltage signal quality go up the 30-40th harmonics.
This is far above 500Hz.

And this is only for voltage.
Usually the harmonics in the current are worse.

So it depends on your application /specification.

Klaus