The term "quadrature" means in this connection that the oscillator outputs two trapezoid waves with 90 degrees phase shift.
The term "Thru-Zero" means that even "negative" frequencies are generated, but this a bit a misleading term as negative frequencies do not really exist.
"Negative" means in this connection simply that the TRASIN/TRACOS waves will stop when the linear control voltage reaches 0V and continue with the opposite direction as the linear control voltage becomes negative and vice versa.
The module has two control sections: linear and a exponential.
The exponential section consists of the XTune control, the 1V/Oct input and the XFM input with the corresponding attenuator XFM. The exponential control voltage is the sum of these three voltages.
The linear section consists of the LFrq control and the LFM input with the corresponding attenuator LFM.
The linear control voltage is the sum of these two voltages.
A dual color LED is used to display the polarity of the linear control voltage (blue = positive, red = negative).
The pitch of the sine/cosine outputs is determined by the control voltages of both sections.
Linear modulation by another oscillator using the thru zero feature generates audio spectra than cannot be obtained from an oscillator without the thru zero function.
The reason is that a "normal" VCO will simply stop as the linear control voltage becomes zero or negative. But a thru zero VCO will start again with "negative" frequencies as the the linear control voltage becomes negative.