Microcomputer Compensated Crystal Oscillators (MCXOs) Deliver High Temperature Stability

Microcomputer compensated crystal oscillators or MCXOs, are the ideal choice in many applications, including military and commercial avionics, ground-based electronics, as well as undersea oil exploration. These smaller, lighter, lower-power devices can often replace bulkier and power-consuming oven-controlled crystal oscillators (OCXOs), while also providing comparable stability over a wide range of operating temperatures. This article describes the topology of MCXOs and describes the technological advances that are enabling MCXO deployment in a wide range of space applications.

Crystal Oscillators

All crystal oscillators are based on the very stable frequency vibrations of a piezoelectric quartz crystal resonator. Usually, the crystals and their associated circuitry are carefully designed and crafted so that the quartz crystal will vibrate only at the desired resonant frequency. A stand-alone crystal oscillator can hold frequency stability of better than ±50 parts-per-million (ppm) over the wide military temperature range of -55 to +125 degrees C, which is good enough for most electronics applications.

If a more tightly controlled stability over temperature is needed, a temperature-compensated crystal oscillator, or TCXO, adds compensating circuitry to correct for the crystal frequency’s temperature variation and can thus achieve about ±1 ppm. 

If even more stability is needed, an oven-controlled crystal oscillator places the crystal inside a very precise proportionally controlled oven, which can achieve about three orders of magnitude better frequency stability over temperature.  While highly stable, OCXOs come at the expense of a lot more size, weight and power consumption. A typical OCXO draws at least a few watts of power, while the power consumption of XOs (simple crystal oscillators) and TCXOs is measured in milliwatts. On the plus side, in addition to high-temperature stability, OCXOs typically have higher performance for other important oscillator parameters including phase noise, jitter, and long-term stability (aging).

The Benefits of MCXOs

The driving purpose of the MCXO is to achieve the performance of the OCXO, but with much lower power consumption and much faster warm-up, i.e., the time it takes an oscillator to reach its required stability after turn-on. To perform this feat, MCXOs operate the quartz crystal resonator at two different frequencies at the same time. That’s where microcomputer compensation comes into play.

The microcomputer is programmed to manipulate the frequency data at each temperature, so the crystal oscillator becomes a self-sensing thermometer providing highly accurate temperature data at any given moment.  

One of the primary reasons for the superiority of the MCXO’s temperature compensation is that the self-thermometry of the quartz crystal resonator eliminates the need for a separate thermometer. 

Every TCXO and OCXO requires a separate temperature sensor to precisely monitor the temperature of the quartz crystal resonator. In the case of the OCXO, one must know the crystal temperature in order to continually correct that temperature to the desired oven temperature. In the case of the TCXO, knowing the crystal’s temperature allows the compensation circuitry to calculate the exact correction needed due to frequency-temperature variations. The difficulty is that the temperature sensor cannot be mounted on the actual crystal resonator, because of mass loading and contamination effects, but instead must be mounted on the outside of the crystal’s hermetically sealed package. Due to thermal time lag the thermometer will never actually be at the exact crystal resonator’s temperature. 

One Crystal - Two Frequencies 

A key point of any crystal’s design is to make it prefer to oscillate in one particular mode. But with the MCXO crystal, the crystal is designed to oscillate in two modes at the same time, one being the fundamental mode of an SC cut crystal – a very special doubly rotated cut with respect to the hexagonal quartz crystal axes that give the crystals excellent temperature stability. The second mode of vibration is on the crystal’s third overtone.

Interestingly, the third overtone frequency is not exactly three times that of the fundamental but very close - 2.999 – and this ratio varies minutely with temperature. As this ratio varies, it provides a precise indicator of the exact temperature of the crystal at any moment. As part of the manufacturing process, this information is carefully characterized and stored in the MCXO’s microcomputer and used in real-time to calculate the exact temperature based on the ratio of the two frequencies at any moment. (Figure 1)

Figure 1. An MCXO can be accurately temperature compensated by characterizing the frequencies of the fundamental and 3^rd overtone over a wide temperature range.

The result is that the MCXO can be made to give approximately the same performance as a good OCXO but with power less than 100 milliwatts compared to an OCXO’s three to five watts. The typical warm-up time of an OCXO is 10-plus minutes versus less than one minute for the MCXO. In other words, the MCXO can offer more than a full order of magnitude lower power consumption and faster warm-up time than the OCXO. This, for some applications, is revolutionary. 

Developing Space-Qualified MCXOs

Crystal oscillators deployed in space must be capable of withstanding many types and levels of radiation exposure based on their orbital location. When developing MCXOs in the early 2000s, the availability of radiation-hardened, space-qualified digital components was very expensive, which meant space-qualified MCXO space-level products would sell for hundreds of thousands of dollars each.

At the dawn of the era of mega satellite constellations, known as low-earth orbit (LEO), or New Space, it became possible to find microcontrollers and other digital devices that were rad tolerant and up-screenable. The use of these digital components went into the QT2020 MCXO (Figure 2), released in 2021, which has now been fully qualified for use in LEO New Space applications.

Figure 2. The small, light (21” x 1.0” x 0.33”) low-power-consumption QT2020 MCXO.

The QT2020 MCXO was designed with the objective of use in satellites and other space applications, using only rad-tolerant components. The product series is available at 10, 20, 30, 40, 50 60, 80 or 100 MHz, with stability as low as ±10 ppb in a 2-inch by 1-inch by 0.33-inch package. And it offers the high performance of an OCXO but with less than 90 mW power consumption.

The QT2020 MCXO is now a standard product that can be procured without difficulty and at a reasonable cost. Prices vary based on the stability and other options. For instance, a full totally RAD-hard version can be developed if the application will support a higher price. 

The QT2020 MCXO has been tested for TID up to 50 kRADs without experiencing problems, and the current consumption level was “rock steady” as the radiation dosage went up – giving optimism that single-event results will be good. Now, single-event testing is being arranged.  

Figure 3 shows a simplified block diagram of the QT2020 MCXO. The signals from the dual mode oscillator get mixed down to generate beat frequency after being normalized by a frequency divider. The beat frequency is a difference between two oscillator modes and represents the crystal temperature. It feeds the microcontroller counter to generate a digital temperature reading “N1.” Data for N1 is collected and stored in microcontroller memory. For each N1, a polynomial calculation provides a correction coefficient of “N2.” A 10MHz VCXO provides the signal to one of the microcontroller counters to get compared to the Fo signal. N2 correction is applied here. A digital-to-analog converter applies a control voltage to the VCXO to keep it at the target frequency.

Figure 3. Simplified block diagram of the QT2020 MCXO.

Conclusion

The QT2020 MCXO fills a niche with much better stability than the best TCXO and offers the equivalent stability and noise to typical OCXOs. It also has extremely low power consumption, small size, fast warm-up, and is fully certified and rated up to 50 kRad TID. Time will tell, but this rad-hardened MCXO promises to be a revolutionary, enabling technology. Already, many satellite manufacturers are placing orders, evaluating the product, and planning to fly the QT2020 MCXO in exciting New Space applications.