Fig. 1. Schematic of passive hydrogen maser

Fig. 2. Operating principle of passive hydrogen maser

Fig. 3. Transition spectrum curve of hydrogen atoms

Fig. 4. Resonance curve of microwave cavity

Fig. 5. Frequency discrimination spectrum curve of hydrogen atoms

Fig. 6. Frequency discrimination spectrum curve of microwave cavity

Fig. 7. Frequency discrimination spectrum curve of hydrogen atoms from error signal

Fig. 8. Frequency discrimination spectrum curve of microwave cavity from error signal

Fig. 9. Curve of J0(m)×J1(m)

Fig. 10. Amplitude of error signals with different modulation indexes

Fig. 11. Stability of output signal at different modulation depths. (a) Modulation depth of 10 kHz; (b) modulation depth of 15 kHz; (c) modulation depth of 20 kHz