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Most voltage controlled oscillators require an extra circuit to make them play with proper scaling at higher frequencies.  The reason for this is: Within a sawtooth VCO core is the capacitor that is charged to build the waveform's ramp, and then instantaneously discharged by the reset circuit to start the next cycle.  In theory, this process scales to any frequency.  However, a real capacitor has a non-zero internal resistance, so it cannot really discarge instantaneously; the process takes a small but finite amount of time.  At low frequencies this is of no consequence.  However, at higher frequencies the reset process starts to take up a larger percentage of the cycle time, which means that the total time for one cycle is slightly longer than it is supposed to be.  The result is that, as the control voltage increases, the VCO starts to play flat. 

A high freqeuncy compensation circuit makes up for this.  It basically adds a small amount to the control voltage when it exceeds a certain range.  The usual standard for a VCO is that a properly compensated one should be capable of playing with proper scaling over a five-octave range.

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