Low pass filter (LC) design

[vikash23]

Hi I am using a PI filter 22nF-22uH-22nF for cutoff frequency of 400KHz. I calculated this from online tutorial. I used a function generator to see how it works actually. I used 1MHz frequency from function generator and feed through one end of PI FIlter (22nF-22uH). I used Oscilloscope to measure the output cutofff frequency on the other end of PI Filter (2uH-22nF) and to see the function genetaor output (1MHz as well).

I am not able to see the output cutoff frequency 400KHz. Where am I going wrong. Why I was not able to see the cutoff frequency ?

Help me out.

 

[BradtheRad]

According to my simulation, your component values are in the approximate range to do what you wish.

Screenshot:



Some amount of load is required, when using a passive filter. The tutorial may not have made this plain.

I used 50 ohms because that is a typical value.

The image was snapped while the scope trace was at 401 Hz. It shows the output amplitude is down around 50 percent attenuation. Different loads will produce different curves.

 

[FvM]

The filter specification is incomplete without a source and load impedance specification. For 50 ohm source and load impedance, the said filter corresponds to Chebyshev I with about 350 kHz cut-off and 1.5 dB passband ripple. If you have different source and/or load impedances, the filter has to redesigned.

 

[vikash23]

Hi,

Thank you for your reply. Could you please tell me more about impedance. I am using a card slot as a load in my design and so to avoid any noise I am using a PI filter. How do I calculate the impedance ? If my filter is not impedance matched (50 ohm) how will that affect my systems. ? if possible could you please tell me more about how to reduce switching noise of a voltage regulator. ? My voltage regulator switching frequency is 1MHz and using the above filter I was able to reduce the output voltage ripple and not the switching noise. Any suggestions ?

Thanks in advance.

 

[FvM]

I was able to reduce the output voltage ripple and not the switching noise.

Means exactly what? Switch frequent ripple would be considered as "switching noise" too. Can you clarify with a sketched waveform?

If you mean short high frequent ringing, it should be filtered by a well designed low-pass. If you still see it after the filter, it most likely bypasses the filter. Or there are too much parasitics in the filter elements.

 

[vikash23]

Green is after filtering (22nF 22uH 22nF) and red is before filtering. I used DSP as load. 3V3 input and 1V4 output. I can see some spikes on the pink signal. Is that the switching noise ?



blue is after filtering (22nF 100uH 22nF) and green is before filtering. 24V input and 3V3 output.



blue is after filtering (22nF 22uH 22nF) and green is before filtering. 24V input and 3V3 output.



All the above power supply are not related to each other.

 

[sibushri]

hey
im doing a project in dsp

i need to design a circuit and implement it in agilent ads

pls sombody help me