Techogenerator output is sinusoidal or linear?

Answer 1

It depends.

A digital tachogenerator device will produce a series of pulses (that may be both postive and negative in amplitude). The frequency of the pulses correlates to speed; the amplitude of the pulses is defined by the line driver portion of the circuit.

A DC analog tachogenerator produces a positive (or negative, but usually only one of the two) signal where amplitude is proportional to speed.

An AC analog tachogenerator produces a sinusoidal waveform with both positive and negative amplitude. The frequency of the wavefrom is proportional to speed.