Abstract: The low phase-noise and high frequency-stability 10 MHz hydrogen clock signal (HCS) is the indispensable microwave frequency reference for physics experiments of precision measurement. After transmitted over a certain distance, the phase noise and short-term stability of the HCS will be downgraded due to the disturbance of the external electromagnetic noise and vibration. A scheme to filter the terminal noise of the HCS after transmission is introduced, which reduces the phase noise and improves the short-term stability of the HCS. The scheme is based on phase-locking of an oven-controlled crystal oscillator (OCXO) to the HCS, the phase noise and frequency stability of the reference signal at shorter than 1 s is defined by the OCXO, while the long-term stability follows the HCS. After the filtering scheme, the power of the HCS is amplified from -4.4 dBm to 5 dBm, the terminal phase noise background of the HCS is reduced about 10 dB, and all disturbance higher than 1 Hz are eliminated. The system can dramatically purify the HCS without changing the laboratory hydrogen clock and the HCS transmitting network, and then the accuracy of precision measurement experiment is improved. 

Key words: physics experiments of precision measurement, terminal noise filtering, phase locking, hydrogen clock signal, frequency stability, phase noise, oven-controlled crystal oscillator