In modern high-frequency electronic systems such as communications, radar, and precision instruments, the phase noise of the signal source is one of the key indicators that determine the performance of the system. The deterioration of phase noise will lead to higher communication BER, lower radar resolution, and even affect the accuracy of instrument measurement. While direct digital frequency synthesizer (DDS) is widely used as a frequency source, the phase noise characteristics of its output signal are closely related to the quality of its input clock. In this paper, the TQSRO-C50 Sapphire Resonator Oscillator (SRO) of Skydome Electronics is used as the DDS clock source. The TQSRO-C50 is an oscillator that outputs a 10GHz point frequency, and it is used as the clock input of the DDS after dividing the frequency of the 10GHz into 5GHz, and the test results show that the phase noise of any frequency point of the DDS output is very low, and even affects the accuracy of the instrument. The test results show that the phase noise at any frequency point of the DDS output is significantly improved, which significantly improves the system performance.

I. The importance of phase noise and the clock dependence of DDS

Direct Digital Synthesis (Direct Digital Synthesis, referred to as DDS) technology using digital DA conversion to obtain the desired analog signal, can be in a wide frequency range for fine frequency adjustment. The signal source designed by this method can work in the modulation state, and the output level can be adjusted and various waveforms can be output, which is widely used. Phase noise is a core parameter to measure the spectral purity of a signal source, which is essentially a short-term random fluctuation of the signal frequency. For DDS, the phase noise of its output signal is mainly determined by two factors:

1, DDS chip internal noise: including digital quantization noise, DAC conversion errors, etc.;

2, the phase noise of the external reference clock:

The phase noise of the clock source will be passed through the DDS phase-locked loop (PLL) or directly to the output, becoming the “ceiling” of the system noise.

Experiments show that in the case of DDS output frequency close to the clock frequency, the phase noise of the output signal is almost the same as the clock source. Therefore, the selection of low phase noise clock source is the most direct means to optimize the performance of DDS.

Second, the technical advantages of the sapphire oscillator

Sapphire oscillator is a high-performance oscillator based on Sapphire Dielectric Resonator (SDR), whose core advantage lies in ultra-low phase noise and excellent frequency stability, which is reflected in the following aspects:

1. High Q Sapphire Resonator Cavity

Sapphire crystals (Al₂O₃) have a Q value (quality factor) of 100,000, which is much higher than that of ordinary dielectric resonators. The high Q value means that the energy loss in the resonator cavity is extremely low, which effectively suppresses thermal noise and random phase jitter, resulting in a steeper spectrum and lower near-end phase noise (e.g., as low as -148 dBc/Hz at a 10kHz offset) in the frequency domain.

2. Low Thermal Drift Characteristics

Sapphire has an extremely low coefficient of thermal expansion (~5×10-⁶/°C), which, in combination with thermostatic control circuits (OCXO or thermostatic control technology), provides frequency temperature stability down to the order of 0.1 ppm (parts per million). This feature ensures almost no degradation of phase noise performance over a wide temperature range.

3. Anti-interference and long-term stability

The TQSRO-C50 is fully sealed, insensitive to electromagnetic interference (EMI) and mechanical vibration, and has a long-term aging rate as low as 0.5ppb/day, making it suitable for long-term reliable operation in harsh environments.

TQSRO-C50 is the first commercialized sapphire oscillator shelf product in China developed by Changsha Tianqiong Electronic Technology Co., Ltd. and is more than 40% smaller than similar foreign products, which can be widely used in a variety of high-precision signal generation fields.

Figure 1 shows a comparison of the phase-to-noise ratio between the freely oscillating TQSRO-C50 and the output signal of a traditional dielectric oscillator, it can be seen that the phase noise of the TQSRO-C50 is lower in the deviation from the carrier waveform from 1kHz to 1MHz, and its phase noise at 10kHz~100kHz is lower than that of the carrier waveform by 20dBc, which is an obvious advantage. The phase noise within 2kHz can be further reduced. Tests show that <-126dBc/Hz@1kHz phase noise can be realized by locking, which meets the demand for near-end phase noise.

Third, the sapphire oscillator + DDS: performance enhancement of the actual test verification

To a certain high-performance support 6GSPS update rate of the direct digital synthesizer DDS chip, for example, the DDS chip supports up to 7.5GHz RF signal synthesis. E8257D and TQSRO-C50 are used as clock sources to test the phase noise at any frequency point of its output. Since the input clock frequency of this DDS chip cannot exceed 5GHz, the 10GHz output from the TQSRO-C50 is divided into 5GHz and input to the DDS chip. However, due to the noise of the frequency divider, the phase noise at the far end deteriorates by about 15 dB after the frequency division.

Since the input clock frequency of the DDS chip cannot exceed 6 GHz, the output of the TQSRO-C50 is bisected to obtain 5 GHz, and then the 5 GHz signal is input to the DDS chip. The FPGA controls the DDS chip to generate 1.6GHz, 3.2GHz and 4.8GHz signals respectively, and the test results are shown as the blue line in Figure 2. The signal source E8257D is also used to generate a 5GHz signal as the DDS clock signal, and the output results are shown in the green line in Figure 2.

(a) 1.6GHz signal

(b) 3.2GHz signal

(c) 4.8GHz signal

 Fig. 2 DDS output multi-frequency signals

Table 1 Comparison of test data of multi-frequency point signal output by DDS

The data show that the use of the sapphire oscillator reduces the phase noise of the DDS output signal by more than 10dB in the range of 10kHz~100kHz off the carrier, significantly improving the spectral purity of the signal. The performance index can be further improved if the phase noise deterioration caused by crossover frequency can be improved. In radar systems, this improvement can increase the target detection distance by more than 20%; in 5G communications, it can effectively reduce neighboring channel interference and improve throughput.

Typical application scenarios

Phased array radar system: low phase noise clock ensures the accuracy of beam formation and improves multi-target resolution.

Satellite communication uplink: reduce carrier leakage, improve modulation accuracy and spectral efficiency.

Quantum Computing Control Signal Sources: Meet the demanding needs of superconducting quantum bits for very low noise clocks.

High-precision test instruments: such as vector network analyzers (VNAs) to improve dynamic range and measurement repeatability.

V. Conclusion

Sapphire oscillators, with their revolutionary low phase noise characteristics, provide a “pure” clock reference for DDS systems, making them an ideal choice for high-frequency, high-precision applications. With the rapid development of 6G communication, terahertz technology and quantum information, the requirements for signal source performance will continue to rise, and the innovative iteration of sapphire oscillator technology will undoubtedly lay a solid foundation for the next generation of electronic systems.