In this second lecture, the XOProf will discuss the processes used to optimize crystal oscillator performance in the face of potentially damaging levels of radiation in space applications.
Radiation Effects on the Quartz Crystal
There are two separate things to consider, first, the effects of radiation on quartz crystal resonators, and then the effects of radiation on all other components in the oscillator circuit. In this section we will address the former. Concerning the quartz crystal itself, the good news is that, unlike many electronic components, the quartz crystal resonator will not die, it will not cease to operate due to radiation under any reasonably foreseeable circumstances. The only thing that will destroy the crystal completely is if it is mechanically shocked at a high enough level to actually break the crystal, which is possible, but not under any normal space flight conditions.
What can happen is that certain kinds of radiation and amounts of radiation can change the frequency of the quartz resonator. The amount of frequency change is normally not excessive, but varies according to many variables, including the frequency of the crystal, the quality factor (Q) of the crystal, the type of cut of the crystal, the type of radiation and the amount of radiation. The frequency change comes primarily from very small levels of impurities embedded within the silicon dioxide quartz crystal lattice. These various possible impurities can be dislodged and moved within the quartz, resulting in small frequency changes.
Quartz for electronic purposes is grown synthetically under very high temperature and pressure conditions. Vast improvements have been made in the growing of such quartz, to the extent that frequency changes due to radiation are much reduced. Beyond that, quartz for use in resonators intended for space go through a special process called “sweeping”.
“Sweeping” quartz bars involves subjecting them to a very high unidirectional DC-electrostatic field of 1000 V/cm (400 V/inch), while simultaneously exposing them to a very high temperature of about 500°C and monitoring the current flow. This “sweeping” causes many impurities to migrate through the quartz bars to the edges and those edges, that are then cut off with a saw, leaving the pure quartz bars known as “swept quartz”. The purified quartz bar has improved radiation insensitivity. Most space level specifications for crystals (almost all) therefore specify that swept quartz must be used.
Using such high reliability swept quartz, the resonators used in the most precise crystal oscillators, typically SC Cut crystals in OCXOs (Oven Controlled Crystal Oscillators) will typically only change frequency by a few or several PPB (parts per billion) in most radiation environments. When well characterized and understood, these kind of frequency changes can be dealt with. The typical crystal resonators used in less precise XOs (simple crystal oscillators) will typically change frequency by a few or several PPM (parts per million), and because the tolerances on these XOs is usually about ±50 PPM, this can also be dealt with. In fact, the improvements in growth of high-quality quartz bars has been so good, many companies are using non-swept quartz in XOs for use in some of the less demanding space environments.
In part three of this series, I will survey the environments that make up the “spacescape” and discuss what each environment requires in terms of radiation tolerance.
