Research on Digital Cavity Thermal Control Techniques for Performance Improvements of Hydrogen Masers

Hydrogen Maser is a sort of stable frequency sources which can be served as the primary frequency standard. When investigating the working mechanism of a H-Maser, evidences can be found that its internal resonance cavity has significant influences on the long-term stability performance of the H-Maser. More specifically speaking, the center frequency of the cavity changes when the temperature fluctuation exists, and therefore the output frequency of the H-Maser would change accordingly. The long-term stability performance of the H-Maser would get worse due to the environmental temperature fluctuation. In order to improve the long-term stability of the H-Maser, the improvements on the precision of the cavity thermal control is of importance. This article introduces a high-precision temperature control system based on ADRC's improved PID algorithm. The system uses WRB2405 as a voltage regulator chip, a thermistor as a temperature sensing element, a double-stranded manganese copper wire with an inner diameter of 0.32 mm as a heating wire, and a half-bridge circuit composed of MOSFETs to drive the heating wire for heating. In the regulated circuit, the chip WRB2405 regulated the +24V DC voltage into a +5V DC voltage with a ripple lower than ±1mv, which ensures the accuracy of the subsequent chip power supply. In the temperature measurement circuit, a high-precision reference voltage chip is used ADR4525 supplies power to the unbalanced bridge, and the two pins of the 24-bit ADC measurement chip are connected to both ends of the balanced bridge, so that the A/D conversion result depends only on the change of the thermistor resistance with temperature; In the circuit, a half-bridge circuit composed of two N-channel MOSFET power tubes realizes the control of the heating wire. In the temperature measurement process, the STM32 processor uses median filtering and sliding window filtering successively according to the A/D sampling value, combined with polynomial fitting, and performs real-time calculation to obtain the final temperature measurement result. The experimental results show that the temperature control system can keep the temperature of the hydrogen maser within ±0.001°C when the ambient temperature does not change more than ±1°C.