# Narrowband filter on 50Hz?

1. ## Narrowband filter on 50Hz?

On line, we have 230V 50Hz and sometimes control frequency 180Hz, the controll signal is 100 weaker.
I need to evaluate the presence of the control frequency. I use MCU and Goertzel algorithm, its work, but sometimes there is a problem and it would be convenient to suppress 50Hz with an analog filter .

I designed a filter that suppresses 50Hz against 180Hz by -60dB.
That's the theory. SMD ceramic capacitor have manufacturing tolerance and temperature and time changes.
I don't need -60db just me i -30dB.
The question is whether such a thing will work in reality and will be usable in the long term?

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2. ## Re: Narrowband filter on 50Hz?

I have never had problems with notch filters like that. There may be minor degradation of performance over many years but it should still block to below your 30dB threshold. Assuming this is to remove AC power line signal, don't forget that there may be slight variation on frequency anyway so you don't want the notch to be too narrow or it will fail to work.

Brian.

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3. ## Re: Narrowband filter on 50Hz?

Hi,

The filter setting is accurate and stable.
No need for trimming
Simple hardware.

Klaus

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4. ## Re: Narrowband filter on 50Hz?

A simple HPF might work better.

The values in the twin-tee ( post #1 ) do not look right though. The single capacitor should be about twice the value of the series ones and the series resistors should be twice the value of the single one.

Using 51K + 51K + 100nF and 47nF + 47nF + 25.3K may work better. 25.3K = 22K and 3.3K in series.

Placing the network in a feedback loop will sharpen the response.

Brian

5. ## Re: Narrowband filter on 50Hz?

The powerline is not always a perfect sinewave. Frequently its peaks are squashed causing harmonics like 100Hz and 150Hz on a 50Hz powerline.

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6. ## Re: Narrowband filter on 50Hz?

What Audioguru said.
Powerlines have significant harmonics, so perhaps it is best that you use a bandpass filter to recover your frequency of interest.

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