Space
Why the EMXO is set to become the preferred type of crystal oscillator for spaceflight applications
By Hoklay Pak, senior design engineer, Microchip Technology C
rystal oscillators are among the most important electronic components used in spaceflight applications and, like any other component sent into space,
they need to be capable of operating within a broad range of challenging environments. Not only must crystal oscillators be able to withstand the intense levels of vibration and shock that take place during a rocket’s launch but they also need to be as compact as possible, consume minimal amounts of electrical power, operate error-free and efficiently over widely fluctuating temperatures, and deliver their rated performance reliably for up to 15 years. In addition, crystal oscillators have to provide a highly precise and stable frequency reference.
A key factor in such oscillators is the quartz crystal, a precision piezoelectric component that has been produced to extremely high tolerances. The crystal can be adjusted to vibrate at designated frequencies and is capable of maintaining high levels of stability due to its high Q value. However, a problem with these crystals is that their frequencies can vary because they can be very sensitive to even miniscule changes in temperature. To cope with this issue, the temperature- compensated crystal oscillator (TCXO) was created, incorporating a temperature- sensitive reactance circuit in its oscillation loop. Even then, the problem could still exist in some of the most demanding applications – such as space missions.
Improve stability
It became possible to improve stability more than tenfold by putting the crystal into a small oven, turning it into an oven-controlled crystal oscillator (OCXO). However, even this approach had its issues,
16 October 2022
The contamination-free environment also makes it easier to use an open crystal blank rather than a larger packaged type, leading to even more reductions in size and weight. This means that the internal mass of an EMXO can be smaller than that of a typical OCXO, leading to lower power consumption and less volume for the oven to heat. Since an EMXO is evacuated and has much less thermal mass than an OCXO, its warm-up time is much shorter. In addition, the crystal blank is integrated into the hybrid package, contributing further to size reduction which can make an EMXO less than half the size of a typical OCXO.
Figure 1: Key performance characteristics of Microchip Technology’s EX-219
not least the fact that the oven used to create a typical OCXO is a large and heavy unit that consumes high levels of power. This can present a significant problem in spaceflight and other applications where it is essential to keep size and weight to an absolute minimum.
The ultimate solution, then, is an evacuated miniature crystal oscillator (EMXO) which performs as well as or better than an OCXO but is half the size and consumes much less power. An EMXO is a small, light, rugged and hermetically sealed package which is especially ideal for meeting the demands of spaceflight. Microchip Technology’s EX-219 is a good example of advanced EMXO design and Figure 1 shows its key performance characteristics, such as stability, power
Components in Electronics
consumption and environmental conditions. Typical spaceflight applications for the EX-219 include RF transmitter/receiver, GPS guidance and reference oscillator.
Contamination-free environment It took many years to develop such EMXOs but it was well worth the effort when you consider the benefits that they bring. For example, while an OCXO uses low thermal conductivity insulation to minimise power consumption, an EMXO uses vacuum as the insulation method, producing a contamination-free vacuum level of 10-6 torr and reducing insulation weight to virtually nothing. Also, there is zero contamination from weld splashes, dust or vapour and this extremely high level of vacuum hardly diminishes over time.
The circuit in an EMXO consists of oven and output assemblies mounted on substrates. A stress-compensated, doubly- rotated crystal (SC/IT-cut) is used to obtain good phase noise, slower ageing rate and lower g-sensitivity (the crystal has a four- point mounting structure for ruggedness and low g-sensitivity). Synthetic swept quartz is used to achieve higher radiation tolerance and the thermally-insulated structure maintains a nearly constant temperature over its operating temperature range. Figure 2 shows how the oven substrate assembly is mounted on thermal insulated standoffs to minimise heat loss while the output assembly is mounted on the header platform.
Low leak rate
Another key advantage of using an EMXO in spaceflight applications is its leak rate, which is so low (1x10-12 atm·cc/s helium) it is outside the range of equipment used to measure it. Because the EMXO enclosure is sealed using cold welding that creates a metallurgical bond between the metal surfaces without adding heat during the sealing process, it is impossible for an
www.cieonline.co.uk
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62
