Chill out with MEMS TCXOs at high temperatures
More electronic systems are being adapted for use outdoors, for example information kiosks, but some high-temperature operation electronic systems, such as radios, need careful design considerations before they can operate effectively outdoors.
In these systems, fans are impractical, as they have a high failure rate, or mean time between failures (MTBF), in comparison to the electronic components. Servicing them incurs the cost of sending a repair truck, which is time consuming and could lead to unacceptable downtimes.
To avoid this, high-temperature outdoor units, such as radios, often use convection cooling.
Figure 1: A radio tower is a typical example of a high temperature operation outdoor electronic system. (Picture – Pixbay)
The demands of 5G
5G networks will be deployed to handle the increase in traffic data to connect devices in the Internet of Things (IoT). Fast upload and download speeds, with low latency will increase connectivity, for example in autonomous vehicle systems, as well as enhanced mobile broadband for home and enterprise use.
To increase data rates, the 5G architecture calls for a reduced distance between radios and user terminals, which causes a corresponding rise in the number of cell sites and nodes in the network. The rate of ‘densification’ of radio access networks (RANs) is considerable, with 10x more radios than 4G and 100x more than 3G networks, to meet 5G’s Gbps data rate and low latency specifications.
In metropolitan areas, where there is a high concentration of users, cellular radios will become ubiquitous, but their design criteria remains strict. They must be compact as they will be mounted on telephone poles, lamp posts, the corners of buildings and curb-side municipal power supply cabinets. Locating 5G radios in these locations will subject them to a broad range of environmental conditions. The timing solution employed must have the capability to withstand high temperatures and maintain system functionality.
Next: Lifetimes
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
The physics of a MEMS resonator is very different to that of quartz. Its inherent qualities offer several advantages over quartz TCXOs in outdoor mounted electronic systems.
The first is availability. MEMS precision TCXOs rated up to +105°C ambient temperature are readily available. Figure 2 shows how typical quartz OCXOs’ stability falls sharply above 85°C. However, the TCXO devices remain stable well above their maximum rated operating ambient temperature range of 105°C, extending all the way up to +125°C.
The MEMS TCXO also provides high stability and improved dynamic performance in extreme conditions caused by limited air flow, temperature changes, mechanical shock, vibration, noise from power supplies, and EMI.
Figure 2: Comparison of stability beyond rated temperature of three typical OCXOs versus multiple SiTime Elite TCXOs
MEMS TCXOs also do not experience frequency jumps and activity dips when temperatures rise. Figure 3 shows the “graceful degradation” of MEMS TCXOs operating outside of their maximum temperature range.
Figure 3: The graceful degradation of production-trimmed precision MEMS TCXOs ensures operation during extremes of temperatures.
The graph shows the frequency stability-over-temperature performance of an SiT5356 ±100 ppb MEMS TCXO as it operates from -60°C and up to +125°C; far beyond its rated operating ambient temperature range (-40°C to +105°C).
Stability remains similar – only slightly reduced – enabling the MEMS oscillator to continue to function until a later time when the ambient temperature drops back within the normal operating range.
Finally, because it is 100 percent silicon, a MEMS resonator is inherently higher quality and more reliable than quartz-based devices. These characteristics increase the quality and reliability of the networks in which they are deployed.
Next: Conclusions
As described above, a MEMS-based TCXO has the capability to cover all required deployment temperatures. The ability to gracefully degrade when operating beyond its rated temperature range makes a MEMS TCXO a more reliable, stable device than a quartz TCXO.
It also means that a single design can be used in equipment deployed anywhere in the world. These qualities not only reduce development time, but also streamline logistics and enhance customer support.
Gary Giust, is a senior manager of product markeing with SiTime Corp. (Santa Clara, Calif.)
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