Design of wideband tunable dispersive delay using cascaded all pass networks

In this paper, a tunable dispersive delay line using all pass network (APN) in UHF-Band has been designed using lumped elements. Two stages of second order APN has been cascaded, out of which one stage is fixed, where as the other stage is tuned for its group delay response. Tunability is achieved by replacing capacitors with varactor diodes. This approach gives a fractional bandwidth greater than 30% along with a tunable negative group delay slope of [8-3] ns/GHz over the frequency range of [650-850] MHz. The input and output matching is better than 10 dB and insertion loss less than 1.8 dB. Time domain measurements are carried out to validate the tunability in pulse stretching and compression. Such networks can be used for group delay engineering based applications such as frequency discriminators, spectrum sensing and reflection as well as transmission type group delay based RFID tags.