APN1012: VCO designs for wireless handset and CATV set-top applications
Author: Skyworks Solutions Inc.

VCOs have come to the forefront of RF designs together with the first PLL circuits. In the era before the PLL, oscillators were mostly free running, and only in rare cases were varactors used for modulation or temperature compensation. Nowadays, we rarely see free running oscillators, instead they have become varactor-controlled oscillators. This is because most RF applications require band coverage, which can be realized through the PLL circuit requiring two sources of RF power. The reference source frequency is often a VCXO or TCXO, while the other frequency is controlled by the PLL phase detector.