A 0.02 Ppb/Step Wide Range DCXO Based on Time-Average-Frequency: Demonstration on FPGA

Frequency source is one of the foundational elements in electronics. It is used to derive clock signal that establishes the sense of time inside electronic system. An important type of frequency source is the one whose frequency can be adjusted through external control, such as VCXO and DCXO. This type of frequency-adjustable source is useful for many applications such as Doppler radar, frequency modulation, data transmission, telemetry, and telecommunication among others. Frequency adjustability and frequency stability are two requirements that contradict each other. The solution is usually a compromise among factors of performance, power, cost and size. Time-Average-Frequency Direct Period Synthesis (TAF-DPS) is an emerging frequency synthesis technique that provides a new perspective on solving this problem. In this work, a TAF-DPS based DCXO architecture is presented. The architecture is proven with an implementation on FPGA. Its frequency granularity is achieved at 0.02 ppb/step, monotonicity is mathematically guaranteed, tuning range is virtually unlimited. The linearity within a 1% range of 26 MHz is measured as 0.005%.

I. Introduction

Clock is the most important electrical signal. It is used to coordinate events inside electronic world. Clock is a key factor in time synchronization process which is the foundation for myriad applications from military, metrology, industrial, consumer, communication networks, automotive, to power grid, banking, scientific project and etc. In applications, clock signal originates from reference frequency standards that require high quality factor (Q factor) resonating element, such as micro-machined or piezoelectric crystal that acts as an electromechanical resonating module. Crystal in oscillator circuit behaves like a very high Q-tuned network, permitting vibrations at its resonant frequency.