Hi,

I'm currently working on the design of an active power filter and i've to use a notch filter to eliminate 50Hz component. I've managed to get a twin T notch circuit which in simulation is clearly eliminating the 50 Hz component. Unfortunately, there is an attenuation b/w the input and output voltage.

I'ld like to know if it is the typical behaviour of a notch filter.

I've attached a schematic as well as the graph of the input and output voltage.

Thank you n Best wishes

 

Here is a link to an article on the subject - http://www.radio-electronics.com/info/circuits/opamp_notch_filter_2/opamp_twin_t_notch_filter.php.

You will notice using a trim pot instead of a fixed voltage divider to vary the filter Q.

You might want to resolve the resonance formula using lower value resistors to see if the signal attenuation problem is related to that.

 

I noticed your notch was designed to attenuate appoximately 50 Hz [ 49.798 Hz ] but your signal generator is set to 150 Hz ... or am I reading your schematc diagram incorrecly?

The scope output should replicate the input, only much lower amplitude, at the frequency you are rejecting.

 

Yes. You can see the phase-shift caused because the input signal is 150Hz and the notch is tuned to 50Hz.

 

Thanks been there; indeed the pot has resolved my problem.

About the 150Hz signal generator

Sorry, i am new to forum and i can see that i'vent explain myself clear.

Well in fact, i want to eliminate the 50Hz fundamental component and pass all the harmonics. This is why i set the generator to 150Hz to see if i could recover the harmonic signal without damping.

Thanks a lot.

 

Marie,

Not a problem.

 

You need to increase the Q of the filter so it rejects 50Hz but passes 100Hz and 150Hz without loss. The output voltage divider adjusts the Q.