LNFM: High-Precision Frequency Stability Measurement System of Atomic Oscillators With Built-In Reference Source

Precise frequency stability measurement is critical for many advanced applications, including satellite navigation and remote communication. Traditional measurement systems rely on external high-precision reference (REF) sources such as hydrogen maser (HM), cesium (Cs) atomic clock which are costly, bulky, and challenging to deploy in remote or mobile scenarios. To address these limitations, this work develops a compact and cost-effective low noise frequency stability measurement system (LNFM). The system uses precise point timing (PPT) technology to dynamically adjust oven-controlled crystal oscillators (OCXO). This approach compensates for long-term frequency drift and enables the OCXO to serve as a stable internal REF with a frequency stability of $1.0 \times 10^{-{14}}$ @10000 s. This capability allows LNFM to function as a standalone device for frequency stability measurement. Besides, to reduce design complexity and cost, LNFM adopts a software-defined radio (SDR)-based dual-mixer time difference (DMTD) method, achieving a noise floor level of $5.3 \times 10^{-14}\,\tau ^{-{1}}$ . Furthermore, this article presents an analog-to-digital converter (ADC) phase distortion model to analyze the theoretical noise floor bound introduced by the ADC in the LNFM system. The proposed system provides an accessible alternative for applications where external REF are impractical. To validate the system, a rubidium (Rb) clock is measured and evaluated. The results show that its Allan deviation (ADEV) measurement precision is comparable to commercial instruments with HM as REF at $\tau \gt 1000~\text {s}$ . This system provides a portable and reliable solution for modern frequency stability measurement.