VCXO makes inexpensive dual-clock reference

摘要

THIS DESIGN IDEA describes an inexpensive circuit to generate two extremely high-quality, crystal-clock-reference-signals, one of which is a PWM-controlled VCXO (voltage-controlled crystal oscillator) clock signal (Figure 1). The design also includes circuitry to statically switch and hold the VCXO at its nominal fixed frequency of operation (equivalent to 50% PWM) without requiring any external PWM stimulus. Most digital audio/video microprocessor-based systems today require several independent clocks with low jitter and the potential adjustability a VCXO provides. The described circuit replaces two expensive monolithic VCXO and crystal oscillators at a fraction of their cost and provides much higher quality output signals than the monolithic solutions can generate, especially at the control limits of the VCXO (±100% deviation). The circuit generates signals with higher stability, much lower jitter, lower operating voltage (3.3 versus 5V) and a wider VCXO pull range than comparable monolithic approaches at less than one-third of their cost.