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Tunable notch Filters for audio frequency

Dear Team,
I have a requirement of tunable notch filter for which I tried two IC's LTC1060 & LTC1067. Below is my requirement description
Requirement description: Tunable notch filter from 1KHz to 5KHz.

Problem/Issue: Whistling noise in the audio frequency band 1KHz to 5KHz
Solution required: To identify and eliminate the noise in the frequency band of 1KHz to 5KHz.
To have a practical solution to identify the noise frequency, we planned to have LTC1060 or LTC1067 in which the notch frequency can be tuned using which we can identify the whistling noise frequency.
So we have taken the circuits as mentioned in the datasheets and tried to simulate the same in LTSpice. The response is not as what is specified. 
Please support to get the required tuning of notch frequency.
Problem faced during simulation:
  1. LTC1060: The simulation did not show any notch in the graph.
  2. LTC1067: I could get a notch but it is not at the frequency as specified in the datasheet which is 1.02KHz, but as per simulation I got it at 4.04KHz. Also as I change the clock frequency, there was no change in the notch frequency. But as per datasheet it is a clock tunable notch filter. Kindly explain
Please let me know where I have gone wrong in the circuit or simulation. Also if I need to modify the circuit to my requirements what are the changes which needs to be taken care off.
The circuits which I have simulated are in the respective datasheets LTC1060 Pg-18 & LTC1067 Pg-20
Awaiting your earnest response.
Kindly do the needful at the earliest.
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  • Hi Philip, thanks again! This really helped.

    BTW: I see you are creating the notch subtracting a BPF from the original signal and you are adjusting the gain of the original signal in the rest.  Isn't this possible to adjust the gain of the BPF instead? I'm asking this because with the actual design, I'm changing the gain outside the notch zone.

    Thanks again, Julian

  • Yes, adjusting  the BPF gain will adjust the notch attenuation at fnotch

    Note: The notch summing resistors must be equal if the BPF gain is adjusted. 

  • Hi Philip, at the end I have assembled the board with the filter but something isn't working.

    The Oscillator is working and is being regulated in the required frequency range. The output is a square of 0-5V amplitude and the V+ of the LTC1067 is +9V with respect to V-.

    Unfortunately, the notch isn't working and I found the output of the bandpass filter is null. 

    The simulation has shown that INV_A should be DC AGND and this is fine but BPA has DC at 300mV below AGND instead of signal centered in AGND. This is for any oscillator/signal frequency, for example, oscillator at 51kHz.

    Do you have any suggestions?

    Thanks in advance, Julian

  • Hi Julian.

    If the LTC1067 is operating with a single 9V supply then AC couple the 0-5V clock signal.

    The clock input levels for 9V supply are Hi >=7V and Lo<=2.5 biased at V+/2.

  • Good point! I´m sorry for my dumb question. Now it is well working with everything at 5V across a regulator.

    I will try lather to amplify the oscillator with a transistor to make this work at 9V

    Thanks