A Really Good Notch Filter

When you think notch filter you usually think twin T:

Twin T filter

In this R1 = R2 and C2 = C3, C1 is 2 * C2 and R3 is R1 / 2. The notch is at 1/2piRC.

My favourite though is the Bridged differentiator:

Bridged Differentiator

The reason for this is that it much easier to adjust. With the twin T you have to adjust three components in perfect harmony, a nearly impossible task.
With the Bridged differentiator R1 determines the Q and needs to be exactly 6 times R2 + R3 to get a good notch. To adjust the frequency you vary the ratio between R2 and R3.
The capacitors have to be perfectly matched to get a good notch but I found a set of three on e-bay that my 4-digit capacitance meter couldn’t tell apart.
The frequency of the notch is determined by the formula:

For my mains hum filter the capacitors were 100nF, R1 was a 430k plus a 100k trimmer, R2, R3 were a 1k and a 56k and a 20k trimmer between them with the wiper to ground,
You only need to adjust R1 once. with the right R2, R3 this can go from 30Hz to 150Hz and beyond.
This functions just as well as a twin T and is lot more friendly.

Leave a Reply

Your email address will not be published. Required fields are marked *