In a receiver, a frequency-synthesis circuit generates a stepped-frequency signal having a frequency which can be varied in steps. Depolymerization and deoxygenation, the desired lignin convention steps to yield aromatic hydrocarbons, are carried by contacting a mixture of lignin and a solvent with hydrogen in the presence of a catalyst. The sensor assembly has two ohmically interconnected stretch sensors each having a first layer bearing a variable resistance element, whose resistance changes with displacement of the compressible portion, and a second layer for providing support for the first layer.