  We study modulation spectroscopy of the potassium D2 transitions at 766.7 nm.
The vapour pressure, controlled by heating a commercial reference cell, is
optimized using conventional saturated absorption spectroscopy. Subsequent
heterodyne detection yields sub-Doppler frequency discriminants suitable for
stabilizing lasers in experiments with cold atoms. Comparisons are made between
spectra obtained by direct modulation of the probe beam, and those using
modulation transfer from the pump via nonlinear mixing. Finally, suggestions
are made for further optimization of the signals.
