VCO supply touts low noise
Ted Henderson, Linear Technology Corp, Milpitas, CA -- EDN, October 28, 1999
Many portable RF products use VCOs to generate the RF carrier frequency. These applications often require low-noise VCO power-supply voltages that are higher than the primary battery voltage. Many designs use a dc/dc converter powering a low-noise linear regulator. This approach has several inherent disadvantages. The dc/dc converter produces noise that the regulator may not reject, resulting in regulator-output noise far greater than the thermal-noise levels. The linear regulator might require a large output-compensation capacitor with stringent ESR requirements. Finally, the board area for both devices and support components can be large. The circuit in Figure 1 operates from an input range of 2.5 to 4.4V and generates a 4.2V low-noise voltage for a 900-MHz VCO.
The circuit is based on IC1, an LTC1682 charge-pump dc/dc converter. The device includes a charge-pump/linear-regulator tandem that's optimized for minimum regulator-output noise. The linear regulator operates with several types of output capacitors, including small, low-value, low-ESR ceramic capacitors. Figure 2 shows the close-in phase noise of the VCO operating in open-loop mode; Figure 3 shows the peak-to-peak noise voltage at the regulator's output. (DI #2430).
