A vacuum-insulated EMXO.
Typical OCXOs are constructed with a quartz crystal embedded in a metal shell that distributes internally generated heat. A power transistor and temperature sensor are bonded to the metal shell to provide an additional source of controlled heat. An operational amplifier drives the power transistor, as shown in Figure 1. Since the oscillator's circuitry can also contribute to temperature drift, the circuit components are heat sunk to the metal shell as well. The metal module is encased in thermal insulation, such as polyurethane foam, forming an oven with an internal temperature that self-regulates with a typical thermal gain of 500. To provide a thermal margin the oven's temperature is typically set 10 [degrees] C hotter than the highest specified ambient temperature. Thus, for -30 [degrees] to +70 [degrees] C ambient temperature range the oven will maintain a +80 [degrees] [+ or -]0.2 [degrees] C internal temperature.
The drawbacks of this OCXO approach are excessive power dissipation and the long warm-up time required to reach thermal stability. More insulation can be added at the expense of increased physical size. The keys to improved performance are the effectiveness of the thermal insulation and the thermal mass of the oven assembly. Figure 2 shows a simple thermal analog of the oven structure. The thermal resistance of a typical OCXO using this construction technique is 40 [degrees] C/W.
The ideal thermal insulator is a vacuum, where the thermal insulating properties of insulation materials improve significantly. The EMXO utilizes this characteristic to improve the thermal resistance of the oscillator assembly to 175 [degrees] C/W. One of the difficulties of using this approach to increase the oscillator's thermal resistance is the need to maintain a vacuum over the useful life of the product. Common hybrid metal packages employ resistance-weld enclosures that cannot ensure a constant vacuum. The EMXO utilizes a cold-weld sealing process similar to the process used to seal high stability crystal packages. Extended life tests have demonstrated no significant increase in the EMXO's power consumption while enclosed in this cold-weld package, indicating no degradation in the vacuum insulation.
Although improved insulation resistance provides some benefit to oscillator warm-up time, the primary improvement factors are the thermal capacity of the oven structure and the quick transfer of heat to the crystal resonator. Rather than using the conventional metal heat shell, the EMXO uses a small 0.75[inches] x 0.75[inches] AlNi substrate to which the crystal is bonded. The temperature-sensitive oscillator circuitry and a voltage regulator are assembled onto the side of the substrate as well, using thick-film hybrid construction. AlNi was chosen as the substrate material due to its good thermal conductivity relative to the more common [Al.sub.2][O.sub.3]. Also, AlNi does not have the toxicity problems associated with BeO.
An SC-cut crystal with a four-point mount is used for the resonator. The SC cut provides better warm-up characteristics due to its inherent stress compensation and near immunity to thermal gradients. The oscillator's output frequency reaches steady state in 90 s, as shown in Figure 3. Frequency drift is typically 1 ppb from two minutes to one hour.
Due to the very high thermal resistance of the oscillator module, particular care was taken to minimize the power dissipation of the oscillator's RF circuit. A Kapton ribbon cable was used to connect the oven module to a second circuit board containing the buffer amplifier and an additional voltage regulation circuit. A Colpitts (emitter follower) oscillator configuration minimized the effects of circuit noise. The low close-in flicker noise (1/f) of the oscillator is particularly important. Short term stability from 0.1 to 10 s is typically less than 2 x [10.sup.-12] at 10 MHz.
TYPICAL EMXO PERFORMANCE
The model CO-805 oscillator is typical of the EMXO series, featuring a 10 MHz output frequency at +7 dBm output power. An external DC control voltage provides a [+ or -]5 x [10.sup.-7] frequency adjustment. Frequency stability varies with the type of crystal specified and can achieve [+ or -]1 x [10.sup.-8] over a -55 [degrees] to +85 [degrees] C temperature range. Aging is 5 x [10.sup.-8]/year, and the Allan variance from 0.1 to 1 s is 5 x [10.sup.-12]. Maximum specified warm-up time from -55 [degrees] C is two minutes to 1 x [10.sup.-7] and four minutes to 1 x [10.sup.-8]. Input voltage is +12 V DC for the EMXO and +11 to +15 V DC for the oven. Input power varies from 1 W at -55 [degrees] C to 0.6 W at +25 [degrees] C. Maximum warm-up power is 8 W. The oscillator is housed in a 1.33[inches] x 1.33[inches] x 1.27[inches] enclosure with solder pins on the bottom surface. Typical weight is 1.5 oz.
EMXOs are suitable for applications with fast warm-up and low power requirements. Typical uses are global positioning system receivers, remote instrumentation and monitoring, and satellite communications equipment. The EMXO can be a significant advantage in space applications due to the simplified setup and testing requirements resulting from the similar vacuum insulation properties in space vs. on the ground. The EMXO is particularly suited to the low earth-orbiting satellites currently in development.
Vectron Laboratories Inc., Norwalk, CT (203) 853-4433.
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|Title Annotation:||evacuated miniature crystal oscillator|
|Date:||Apr 1, 1996|
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