AS3460
ANC Tuning Guide
Demo Kit Manual
• PUBLIC
UG000480
• v3-00 • 2021-Nov-08
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│ 57
This leads (without derivation) to the ideal filter transfer function:
Equation 2:
H
F
(s) = − 𝐻
𝑀
(𝑠)
−1
𝐻
𝐴
(𝑠)
−1
𝐻
𝐿
(𝑠)
−1
The three transfer functions on the right hand sid
e are often grouped together as the “Driver to
Feedback Microphone” transfer function:
Equation 3:
𝐷𝐹𝐵𝑀 = 𝐻
𝑀
(𝑠)𝐻
𝐴
(𝑠)𝐻
𝐿
(𝑠)
These can be obtained by a single measurement, described in the App Note on “Characterization”.
The ideal feedback filter would be the perfect inverse of the DFBM transfer function, but it is not
possible to design such a filter. Only for a limited bandwidth, typically located between 20 Hz and
1 kHz it is possible to match the phase of the ideal filter close enough to achieve cancellation.
This leads to the goals when designing a feedback ANC-filter:
●
Match the phase of the ideal filter as close as possible for as much bandwidth as possible
●
Where the phase matches close enough, try to get as much filter gain as possible
●
Where the phase does not match, try to reduce the gain as much as possible to ensure stability
Since the phase and gain of a filter can generally not be controlled separately, it is not a trivial task to
find the optimal filter.
Additionally, the DFBM transfer function is subject to changes depending on the fit and user handling
of the earphones and it should be guaranteed that the control loop remains stable in all use cases.
6.1.2
Tuning Guide
The tuning of the feedback path typically is an iterative process. One initial filter design will be
developed and then tested for performance and stability. Since there is no simulation for all misuse
cases, each filter needs to be hand tested for stability and if necessary adjusted until both ANC-
performance and stability are satisfactory.
A typical design strategy would be:
●
Bring down the filter phase close to the ideal phase at a frequency as low as possible, for
example by boosting low frequencies using a shelf or a wide peak
●
At the same time do not produce too much low frequency gain to ensure stability and little
overshoot
●
Overshoots at high frequencies can by suppressed using a low pass and notch filters at critical
frequencies
●
Performance bandwidth and strength can be extended by using peak filters
