Editing Electronic oscillator (section)

===Frequency limitations===
At high frequencies it becomes difficult to physically implement feedback oscillators because of shortcomings of the components.  Since at high frequencies the tank circuit has very small capacitance and inductance, [[parasitic capacitance]] and [[parasitic inductance]] of component leads and PCB traces become significant.  These may create unwanted feedback paths between the output and input of the active device, creating instability and oscillations at unwanted frequencies ([[parasitic oscillation]]). Parasitic feedback paths inside the active device itself, such as the interelectrode capacitance between output and input, make the device unstable. The [[input impedance]] of the active device falls with frequency, so it may load the feedback network.  As a result, stable feedback oscillators are difficult to build for frequencies above 500&nbsp;MHz, and negative resistance oscillators are usually used for frequencies above this.