TUNABLE_PHASE_SHIFT_NETWORK

Published:2009/7/17 2:25:00 Author:Jessie | From:SeekIC

Frequently, the need arises to control the phase shift or delay of an audio signal. The all-pass active filter shown performs this function. The transfer function has the form T(S)=Vout (S)/Vin(S)=1-sRC/Vc/(1+sRC/Vc) where Vc is the control voltage and Vc > 0. As a result, this filter has unity voltage gain for all frequencies. Vc can be used to vary the phase over the theoretical range from -180°≤φ≤0. In the circuit, the left op amp and analog multiplied combination produce an inverting, first-order, low-pass filter whose pole frequency is set by the control voltage, according to ωp=VC/RC. An MC1495 analog multiplier has an adjustable scale factor, which, in this case, is set to 0.1 for convenience. Choosing a center frequency of fo at the geometric mean of the audio range, or fo= √20×20KΩ=633 Hz, centers the phase shift at -90°. For VC=1Vdc, using the relationship fo-VCωo/2π=VC/2πRC sets the RC time constant to 252μs. Selecting R 10 kΩ makes C 25 nF.



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