Jianguo Hu

A Practical Model of Quartz Crystal Microbalance in Actual Applications

A practical model of quartz crystal microbalance (QCM) is presented, which considers both the Gaussian distribution characteristic of mass sensitivity and the influence of electrodes on the mass sensitivity. The equivalent mass sensitivity of 5 MHz and 10 MHz AT-cut QCMs with different sized electrodes were calculated according to this practical model. The equivalent mass sensitivity of this practical model is different from the Sauerbrey’s mass sensitivity, and the error between them increases sharply as the electrode radius decreases. A series of experiments which plate rigid gold film onto QCMs were carried out and the experimental results proved this practical model is more valid and correct rather than the classical Sauerbrey equation. The practical model based on the equivalent mass sensitivity is convenient and accurate in actual measurements.

The Resistance–Amplitude–Frequency Effect of In–Liquid Quartz Crystal Microbalance

Due to the influence of liquid load, the equivalent resistance of in-liquid quartz crystal microbalance (QCM) increases sharply, and the quality factor and resonant frequency decreases. We found that the change in the resonant frequency of in-liquid QCM consisted of two parts: besides the frequency changes due to the mass and viscous load (which could be equivalent to motional inductance), the second part of frequency change was caused by the increase of motional resistance. The theoretical calculation and simulation proved that the increases of QCM motional resistance may indeed cause the decreases of resonant frequency, and revealed that the existence of static capacitance was the root cause of this frequency change. The second part of frequency change (due to the increases of motional resistance) was difficult to measure accurately, and may cause great error for in-liquid QCM applications. A technical method to reduce the interference caused by this effect is presented. The study contributes to the accurate determination of the frequency and amplitude change of in-liquid QCM caused by liquid load, which is significant for the QCM applications in the liquid phase.

Femtosecond-level timing fluctuation suppression in atmospheric frequency transfer with passive phase conjunction correction

We demonstrate femtosecond-level timing fluctuation suppression in indoor atmospheric comb-based frequency transfer with a passive phase conjunction correction technique. Timing fluctuations and Allan deviations are both measured to characterize the excess frequency instability incurred during the frequency transfer process. By transferring a 2 GHz microwave over a 52-m long free-space link in 5000 s, the total root-mean-square (RMS) timing fluctuation was measured to be about 280 fs with a fractional frequency instability on the order of 3 × 10-13 at 1 s and 6 × 10-17 at 1000 s. This atmospheric comb-based frequency transfer with passive phase conjunction correction can be used to build an atomic clock-based free-space frequency transmission link because its instability is less than that of a commercial Cs or H-master clock.

Femtosecond atmospheric frequency transfer using diode laser with electronic phase compensation

We demonstrated an atmospheric radio-frequency transfer over a 50 m outdoor free-space link using a compact diode laser with an electronic phase compensation technique. With transferring a 1 GHz microwave signal in 5000 seconds, the total root-mean-square (RMS) timing fluctuation was measured to be about 560 fs, with fractional frequency instability on the order of 4 × 10−13 at 1 s, and order of 4 × 10−16 at 1000 s. This atmospheric frequency transfer scheme can be used to disseminate a commercial Rb or Cs clock via a free-space transmission link.

Limiting factor for the performance of high stability quartz crystal oscillators – The residual heat of crystal resonators

The high stability crystal oscillator has been playing the heart of modern electronic communication equipment for decades, because of its excellent performance and reasonable price. It has three most significant indexes, namely, the aging, the phase noise and the frequency temperature stability. We found that when investigating the frequency temperature stability of high stability quartz crystal oscillator, with the increase of the oven control accuracy, oscillators frequency temperature characteristic is deteriorating instead of improving. This is considered as a result of the impact of the residual heat effect in crystal resonator. In this case, by reducing the drive level properly, the frequency temperature stability increases accordingly, achieving about 1ppb in the temperature range of −20∼+70°C, which explains the validity of the residual heat effect. Furthermore, after reducing the drive level appropriately, the abatement of oscillators aging and the improvement of close-to-carrier phase noises performance signifies that the residual heat effect of crystal resonators may be the limiting factors for improving the performance of high stability quartz crystal oscillator.

Simulation of the Ramsey cavity response

Ceramic Sealing Window (CSW) and Frequency Tuning Rod (FTR) are two important components of the Ramsey cavity. Here, a complete electromagnetic model of the Ramsey cavity is built with Finite-element tool HFSS, and the effects of CSW and FTR on the cavity response are simulated and analyzed in detail. To achieve high coupling efficiency, the estimated parameter ranges of CSW and FTR are investigated. The results obtained here offer a helpful guide for Ramsey cavity design in Cs and Rb beam clocks.

A novel design of voltage controlled temperature compensated crystal oscillator

A novel design of voltage controlled temperature compensated crystal oscillator (VCTCXO) is presented. This design will consider the current temperature compensation and the voltage-controlled voltage of regulation of the central frequency as a whole. A variable capacitance diode is adopted to fit the function of two variables, which avoids the problem that the temperature of the crystal oscillator is decreased after the mutual interference between the load capacitance of the two varactor diodes and the change of the central frequency. On this basis, by the debug and work switch on the user control voltage of the output frequency control and microcontroller output voltage-controlled voltage switch, the voltage-controlled temperature compensation crystal oscillator can adapt to the users output frequency control in the control voltage situation. This new approach provides a novel design of VCTCXO. The results show that this design achieve desired frequency temperature compensation effect.

Direct loop gain and bandwidth measurement of phase-locked loop

A simple and robust technique for directly measuring the loop gain and bandwidth of a phase-locked loop (PLL) is proposed. This technique can be used for the real-time measurement of the real loop gain in a closed PLL without breaking its locking state. The agreement of the measured loop gain and theoretical calculations proves the validity of the proposed measurement technique. This technique with a simple configuration can be easily expanded to other phase-locking systems whose loop gain and bandwidth should be measured precisely.

Using QCM for field measurement of liquid viscosities in a novel mass-sensitivity-base method

In this paper, a novel mass-sensitivity-based method for field measurement of liquid viscosities using a quartz crystal microbalance (QCM) is presented. And a model which yields a quantitative description of the influence of the liquid properties on the oscillation frequency is established as well. The viscosity of liquids (which loaded on a QCMs center) could be quickly and conveniently obtained using this model by measuring the frequency shift of QCM and the radius of liquid drop. Two groups of verified experiments are performed using two different QCMs, and the experimental data show that the presented method is efficient.