Filter design

[hapetter]

I want to clean up a 0 - 230VAC, 0-120Hz signal. How can I design a filter for this?
Im new into filter so I need to understand some basics.

1. I do understand that a low pass filter cuts off high frequency disturbanses. At what level of frequency should the cut off be?

2. 1 or 2 pole filter? What is the difference?

3. What kind of filter would fit this application

Thanks.

Best Regards
Hans Pettersson

 

[audioguru]

A one pole filter has a gradual attenuation of high frequencies.
A two pole filter is twice as good.
A four pole or eight pole filter is much better at removing high frequencies.

If the frequency is stable (North American or European mains) maybe you want a high-Q tuned circuit instead of a lowpass filter. It is made with an inductor and a capacitor.

 

[stevez]

It's possible that an isolation transformer might be useful here. Most are designed for 50/60 Hz operation - not sure how they might behave at 120 Hz.

 

[audioguru]

It's possible that an isolation transformer might be useful here. Most are designed for 50/60 Hz operation - not sure how they might behave at 120 Hz.

I just noticed that the signal is from 0Hz to 120Hz.
A transformer will saturate at low frequencies and will not work at 0Hz (DC).

 

[stevez]

Yes, good point on the near DC performance of a transformer. The point I wanted to make is that a transfomer can act as a filter because it's response can often be in a relatively narrow range.

 

[Nigel Goodwin]

He's never given the slightest clue as to what he wants - I suspect it's probably nothing like everyone is assuming?.

 

[fsmyth]

Check here for some examples:
Line Level Filters

 

[hapetter]

Thanks. My aplication is to measure the frequency from a Hydro generator PT signal (Potential transformer). I dont have to measure 0Hz, but as low as possible.

The voltage will be from 0 at startup and 220VAC at nominal speed. The frequency will be 50/60Hz at nominal (depending on the grid). 0 at startup and 120 at 100% overspeed. I need to measure the whole range. 0.1-220VAC and 5-120Hz.

Im using a microcontroller to calculate the frequency. I have control over that part, but I need help to design the voltage limiter and filter, as well as the signal isolator.

1. I need to isolate the PT signal with an isolation transformer. I like to integrate it into a PCB if possible?

2. The isolated signal must be cleaned for high frequencys. A 2 pole low pass filter is suggested. Any suggestion on how to design?

3. The voltage must be clamped so it not damage the microcontroller. The current must be low. I think less then 1mA. Clamp diodes are suggested. How is this done?

Thanks.

 

[MrAl]

Hi,


Just wondering, do you want to use op amps or keep it totally
passive?
Op amps give sharper responses, but then passive only keeps the
parts count low and no power supply needed.

 

[hapetter]

I think of passive filters for now.