Comparison of fully distributed and periodically loaded nonlinear transmission lines

Two different approaches to realizing nonlinear transmission lines (NLTLs) are investigated in detail. In the first approach, the nonlinearity is continuously distributed along the line; in the second, the line is periodically loaded (PL) with discrete nonlinear elements. Measured heterostructure-barrier varactor (HBV) characteristics are used as the nonlinearities in both pulse-compression and harmonic-generation (20-60-GHz tripler) simulations. We point out that the choice of simulation step size is critical in the case of fully distributed (FD) NLTLs, and should be made sufficiently small that no numerical Bragg cutoff frequency appears. For the frequency tripler considered in this paper, simulations show that with PL (PL) NLTLs, 21% efficiency at 210-mW output power and 30% bandwidth can be obtained, whereas only 4.8% efficiency is possible using FD NLTLs. For pulse compression, we find that when properly matched, the FD NLTLs can deliver pulses that are five times sharper than can be obtained with the PL NLTLs. Measured results for an HBV-based PL NLTL frequency multiplier are reported that agree with our simulations, in particular, the 30% bandwidth. The confirmation of the role of the Bragg cutoff frequency in preventing the generation and propagation of undesired harmonics (this improving the conversion efficiency) is obtained from experimental results carried out from hybrid Schottky diodes NLTL measurements.