A VCO is an oscillator that uses a variable DC voltage to change its frequency. The design of the circuit and its power supply voltage determine its high output voltage and its low output voltage. If you want to change its output level then you need a voltage controlled amplifier (VCA).
Do you know that a variable resistor is commonly used as a volume Control? It is a variable voltage divider. But instead of audio as the input you could use a DC voltage that is changed when you turn the variable resistor. Connect +5V to one end and 0V to the other end. The center pin is the output which goes from 0V to +5V and it can control the frequency of a VCO.
It is easy to change a voltage as a function of time with a resistor connected to a fixed voltage that charges a capacitor. The voltage rising in the capacitor can control the resistance of a transistor or a Mosfet.
