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Efficient Design and Analysis of Airborne Radomes
Precision crystal oscillators, which are at the heart of many high level electronic systems, are often the most susceptible component in the system to acceleration, vibration, shock and acoustic environments. These dynamic conditions are common in airborne, ground and shipboard applications and significant degradation of the entire system can occur as a result. For these systems, it is imperative for designers to select a crystal oscillator that has been designed to minimize the effects of the crystal’s acceleration sensitivity.
Reducing Acceleration Sensitivity
Three approaches are used to improve the crystal oscillator’s performance when exposed to dynamic conditions: minimize the crystal’s intrinsic acceleration sensitivity, vibration isolate the crystal and apply active compensation (as in Wenzel’s Bootstrap Series). These techniques have been successfully implemented in many cases. The most common approaches using inherent low-g sensitivity crystals and vibration isolation is available in Wenzel’s Citrine Series.
Vibration mounts must be chosen carefully since many materials exhibit substantial stiffness changes over temperature and have direction-dependent isolation. When size is critical, small, omni-directional shock mounts are contained within the oscillator package. For lower resonant frequencies, larger mounts supporting heavier isolated masses are used, increasing the size of the outer chassis.
Citrine Series Crystal Oscillators
The new Citrine Series crystal oscillators were developed specifically to provide a broad product offering for the most demanding applications requiring low phase noise performance in both static and dynamic conditions. The Citrine oscillators can be configured at any fixed frequency between 1 and 650 MHz.
The Citrine Series is available in eight configurations. Both low frequency HF oscillators and VHF oscillators are available with and without a vibration isolation system. And, both the HF and VHF Citrines are available in the “PLUS” series, which adds multipliers, dividers, amplifiers, filters and dual outputs, when the application demands something extra.
The Citrine Series comes in a variety of package configurations that are selected depending on the frequency, options desired, performance specifications and other application considerations. See Table 1 for standard Citrine configurations and performance options.
Citrine Static Phase Noise Performance
Phase noise floor performance for both HF and VHF frequency ranges can be specified for the oscillator as Standard at -165 dBc/Hz or Premium at -176 dBc/Hz at a fixed frequency within the range. The close-to-the-carrier phase noise performance below 100 Hz is primarily a function of the quartz crystal used in the oscillator, and can be selected based on the system’s requirement.
The Golden Citrine option is now available within the VHF frequency range, which offers typical phase noise performance to -183 dBc/Hz at 10 kHz offset and -190 dBc/Hz at and beyond the 100 kHz offset. See Figure 1 for a static phase noise plot of the 100 MHz Golden Citrine crystal oscillator.
Citrine Dynamic Phase Noise Performance
Dynamic phase noise performance of an oscillator can be improved with careful selection of a low-g crystal depending on the application specifications. Although typical low-g crystals have acceleration sensitivity at 5E-10/g per axis, they can be specified at 3E-10/g per axis and as low as 2E-10/g per axis at some frequencies. Specifying the right crystal for each requirement can be a difficult task considering some are better for static performance and others are best for dynamic conditions.
Dynamic phase noise performance can be improved further by using shock mounts. Although vibration isolation may not be a viable solution for applications where vibration is significant below 100 Hz offsets and performance is critical in this region, it works well for minimizing vibration beyond the natural resonant frequency of the isolated unit. In the Citrine Series, isolator resonance can be optimized in the range of 30 to 70 Hz, and performance varies depending on the weight of the isolated unit and vibration profile.
Figure 2 shows the dynamic phase noise measurement of a Premium 100 MHz vibration isolated Citrine, P/N 501-24942 Rev. B, during a vibration profile of 10 Hz to 2 KHz at 0.01
g2/Hz. This oscillator has a natural resonance of ~50 Hz. To mitigate vibration at frequencies lower than ~30 Hz such as in shipboard applications, selecting the best low-g crystal and using a hard-mounted option may be the best solution. Regardless of the application, Wenzel’s Application Engineers study each requirement carefully to configure the best solution possible for each customer.
Citrine Plus Options
Since many applications require high quality signals at frequencies or stabilities that cannot be created by the oscillator alone, several additional circuits are available to satisfy this demand. The Citrine Plus, with a slightly larger housing, has several standard options. These additional circuits are based on our Blue Tops RF Modules product line and include multipliers, dividers, amplifiers, filters and a PLL option, all integrated into the Citrine Plus packaging along with the oscillator to create the desired final frequency or function. The Citrine Plus models can also be provided in a vibration isolated configuration. See Table 1 for additional details about the Citrine Plus options.
The Citrine Series product line offers a wide variety of standard oscillators that provide extremely high performance in high vibration and benign environments. Low phase noise, high stability and customized outputs are available on a standard COTS delivery schedule. Since the series is configurable for many applications, very few compromises are required.
Wenzel Associates Inc.
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