Approximately linear phase IIR digital filters are designed here as the sum of two allpass transfer functions, one of them is an exactly linear phase FIR filter or simply a delay element, while the other one is an IIR allpass. The IIR allpass parameters are obtained by using an optimization technique, in which the stability of the transfer function is guaranteed through special choice of the optimization parameters. The method has the advantage that it can be easily applied to switched capacitor filters. A new simple implementation method is given for this case.

A simple SC delay line using a three-phase clock is described. The new circuit uses a reduced number of op amps. A circuit for correcting the amplitude deviation arising from the sample-and-hold effect is used. Unlike previous circuits this circuit does not affect the group-delay of the delay line. An example for a 10 µs delay line in the frequency range 0 to 250 kHz is given.