Chill out with MEMS TCXOs at high temperatures : Page 2 of 4

June 09, 2020 //By Gary Giust, of SiTime Corp.
Chill out with MEMS TCXOs at high temperatures
Gary Giust, of SiTime Corp. (Santa Clara, Calif.), explains why a MEMS-based timing device fulfilling the role of a temperature-compensated crystal oscillator (TCXO) is a good choice for high-temperature applications that demand reliability.

During their lifetimes, these radios may be exposed to extremes of temperature that are outside the rated range. For example, unusually high air temperatures will raise the temperature inside the radio chassis, increasing the semiconductor junction temperature in the radio’s electronic system. If this exceeds the electronic system’s maximum rated temperature, then the radio may fail.

With more radios used in 5G networks, there are more potential points of failure. If a portion of the network is down, even temporarily, the loss of service could not only result in fines for the carrier but loss of service could be mission-critical for some customers.

To guarantee the reliability of 5G networks and other applications of electronic systems mounted outdoors, the proper choice of components is vital.

Quartz limitations

One of the disadvantages of using quartz oscillators is that they are not designed to operate outside their rated ambient temperature range. Typically, increased temperatures result in unacceptable frequency drift and accelerated crystal aging, which reduces the oscillator's MTBF.

It is also difficult to find a precision quartz oscillator that operates above 85°C. A quartz wafer may be cut using different angles to vary the temperature characteristics, but this is a complex task. It reduces the overall yield and can be extremely expensive.

Another phenomenon to bear in mind when considering a crystal oscillator is that an increase in temperature can produce an increase in resistance, or activity dip. Activity dips can stop the oscillator working altogether at a specific temperature. Steps can be taken to design around this to some degree, but operation cannot be guaranteed beyond the specified operating temperature range.

Running an oven-compensated crystal oscillator (OCXO) above its maximum rated temperature can cause the oven to shut off, which will change its stability by many orders of magnitude.

The OCXO may also draw more current to keep the oven operating at lower temperatures, which may raise the oscillator’s power consumption to an unacceptable level.

Next: MEMS TCXO benefits

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