Voltage controlled oscillators (VCOs) are oscillators where a defined input signal is controlling the output frequency. They are often used in a closed servo loop, using the phased locked loop (PLL) design as shown in the block diagram. In some cases, they are operated in an open control loop with no feedback for correction.
The setting accuracy of the frequency is depending on various factors including the linearity of the VCO, the capabilities of the phase detector and the frequency divider. As an analog voltage is controlling the output frequency of the system, a linear translation in the VCO from the input voltage to the output frequency is what designers are aiming for. This is even more important in the case of an open control situation. Frequency versus voltage plots show the principle of the VCO behavior. Additional insight to the provided linearity is provided by tuning slope versus voltage graphs. Obviously a clean, low-noise DC source is required to remove external influences while characterizing the VCO.
The other important characteristic of a VCO is its phase noise level. Classically, phase noise is measured at a certain target frequency for which the VCO needs to be tuned in. When looking across the frequency range, the phase noise level increases as the frequency increases which is an expected behavior. Nevertheless, in highly integrated designs interference can happen leading to unexpected high phase noise at certain frequencies. Looking at the phase noise versus voltage helps to find such unwanted effects.
The R&S®FSPN – unrivalled sensitivity meets high speed
Next generation technologies like radar and satellite communication with high data rates call for innovative testing solutions to develop and manufacture RF components. With the launch of the R&S®FSPN, Rohde & Schwarz is adding a new member to the phase noise analyzer portfolio, enabling increased efficiency and reproducibility of phase noise measurements and VCO characterization from lab to production. Watch the video and find out more!
Webinar: Voltage controlled oscillator (VCO) verification
This Webinar is intended for engineers involved with Voltage Controlled Oscillators (VCO) both designing and integrating into systems.
