Today’s mobile communications systems demand higher communication quality, higher data rates, higher frequency operation, and more channels per unit bandwidth. As much of this equipment is portable, low power consumption and small size are also required. All of these constraints combine to make the whole design including component selection and evaluation quite challenging. This paper will describe the evaluation of the PLL and VCO and relate those evaluations to information that will allow the circuit designer to optimize the whole oscillator design including the loop filter.
One portion of this design that is very critical with regard to all of the requirements mentioned above is the synthesized oscillator. Typical synthesized oscillators combine a Voltage Controlled Oscillator (VCO) with a Phase-Locked Loop IC (PLL), frequency reference (e.g. Crystal / TCXO) and a loop filter. The VCO is used to generate the RF output frequency.
The PLL (which is of the “analog type”; i.e. different from a pure digital PLL) is used to stabilize and control the frequency. The loop filter design must integrate all of the components to establish, among other things, a tradeoff between noise and transient response.