Hi,

I’m trying to design a 50 Hz notch filter using the Fliege topology. In my testing, each individual stage achieves about 20 dB attenuation. Based on this, I expected around 60 dB when cascading three stages. However, after combining them, I’m only seeing about 23 dB.

Between the stages, I’ve added voltage follower circuits. Each stage has a Q factor of 10. The design works well in simulation, but in practice, I’m not getting the expected results.

The circuit diagram of each Fliege filter stage is attached. The component values are:

• R1 = R2 = 300 kΩ
• C1 = C2 = 220 nF
• R3 = R4 = 14.5 kΩ
• R5 = R6 = 1 kΩ

I’d really appreciate any guidance or suggestions on what might be causing this issue.

 

1. Link to a description of the topology.

2. Complete schematic.

ak

 

What tolerance parts did you use?
The tolerance could cause the notch frequency of the three filters to be sufficiently different to give the results you see.

Did you measure the rejection of each filter stage individually at the same frequency?

 

EDIT; re-read the original post and noticed that the issue is that his expectations for a higher notch depth are not being met.