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%.