Experimental Time-Domain Study for Bandpass Negative Group Delay Analysis With Lill-Shape Microstrip Circuit

This paper focuses on the time-domain analysis of bandpass (BP) negative group delay (NGD) function. The innovative NGD investigation is based on the time-domain experimentation of an innovative topology of “lill”-shape passive microstrip circuit. The design principle of the proof of concept (POC) constituted by particular microstrip shapes is described. The NGD circuit is inspired from a recent fully distributed “li”-topology. Before the time-domain investigation, the BP NGD specifications of the circuit under study are academically defined. As practical application of the basic definition, a frequency domain validation of “lill”-circuit is presented in the first section of the paper. The POC circuit is specified by a -8 ns NGD value at 2.31 GHz NGD center frequency and a 27 MHz NGD bandwidth. The “lill”-circuit exhibits an attenuation loss of about -6.2 dB at the NGD center frequency. Then, the two-port black box model of “lill”-NGD-circuit represented by touchstone data of the measured S-parameters is exploited for the transient simulation. The measured group delay (GD) illustrates that the tested “lill”-circuit operates as a BP function regarding the NGD with NGD equal to -8.1 ns at the NGD center frequency. The time-domain demonstration of the BP NGD function was performed using a gaussian pulse modulating sine carrier. The innovative experimental setup with the possibility to plot simultaneously well synchronized input and output signals is explained. The BP NGD time-domain response is understood from commercial tool simulation using the touchstone S-parameters of the measured “lill”-circuit by using a Gaussian up-converted pulse having a 27 MHz frequency bandwidth. It was observed that the output signals are delayed when the sine carrier is out of NGD-band. However, the output signal envelope is in advanced of about -8 ns when the carrier is tuned to be approximately equal to the 2.31 GHz NGD center frequency. To confirm the time-domain typical behavior of BP NGD response, an input pulse signal having Gaussian waveform were considered during the test. However, the input signal spectrum must be determined in function of the NGD bandwidth. After tests, measured output signal envelopes presenting leading edge, trailing edge and peak in time-advance compared to the input ones are observed experimentally. The results of the present feasibility study open a potential microwave communication application of BP NGD function notably for systems operating with ISM and IEEE 802.11 standards.