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Vol. 94

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2021-09-14 PIER B Vol. 94, 1-18, 2021. doi:10.2528/PIERB21071209

Negative Group Delay Prototype Filter Based on Cascaded Second Order Stages Implemented with Sallen-Key Topology

Miodrag Kandic and Greg E. Bridges

A Negative Group Delay (NGD) filter prototype design based on cascaded identical 2nd-order baseband stages is presented. The prototype design achieves an NGD-bandwidth product that in the upper asymptotic limit for a distributed design is a function of out-of-band gain in decibels raised to the power 3/4. This is an improvement of previous cascaded first-order designs that have an NGD-bandwidth functional dependency of out-of-band gain in decibels to the power of 1/2. The bandwidth is taken as the 3 dB amplitude response bandwidth. The corresponding NGD design upshifted to a non-zero center frequency, i.e. a Band-Stop Filter (BSF) design, is shown to be possible to implement with Sallen-Key topology, and an example is presented for a 500 MHz center frequency and a 100 MHz (20%) 3 dB bandwidth. The filter shows a 4.05 ns negative group delay with a 1.28 ns in-band variation and a 3-dB amplitude response over the bandwidth of 100 MHz, achieving an NGD-bandwidth product of 0.405. An in-band distortion metric is presented, which can be evaluated for any specified time-domain input waveform. It is shown that the bandwidth, order of filter and desired distortion for a particular input waveform are interrelated. Therefore, the proposed in-band distortion metric constitutes another trade-off quantity to be checked for any type of NGD design.