I am developing a new product with the EFR32MG13 wireless MCU for an automotive application in which the device will operate in harsh environmental conditions (high temperature). These questions concern the operation of the HFXO using either a quartz crystal or an external clock (MEMS oscillator).
1) Can either a MEMS oscillator or a quartz crystal be used with the EFR32MG13 HFXO?
2) What recommendations do you have concerning operation of the HFXO at high temperatures?
3) What are the requirements for phase noise /phase jitter of the 38.4MHz HFXTAL input to ensure robust BLE communication?
1) Both MEMS oscillators as well as crystals that meet our datasheet specifications will likely work in this application. The mode of operation for the EFR32MG13 HFXO will be different depending on your choice. For instance, for a crystal oscillator will require that the HFXO be in XTAL mode (CMU_HFXOCTRL.MODE = 0), whereas the MEMS oscillator with CMOS output will require the HFXO to be in EXTCLK mode (MCU_HFXOCTRL.MODE = 1).
Please note that we have a list of recommended crystals in application note AN0016.1: Oscillator Design Considerations (see page 16). In addition, this section lists recommended TCXOs, which will have improved temperature stability. Any of these devices are recommended for wireless applications.
2) Oscillator temperature sensitivity can be a major consideration in applications experiencing a wide range of operating temperatures. We have had some exposure to applications of the EFR32 devices in high temperature environments and with issues involving temperature compensation. It is possible implement a scenario in which you can monitor the temperature of a crystal and adjust the CTUNE value (on-chip load capacitance) to maintain a stable frequency as temperature changes.
An alternative to implementing a temperature compensation scheme involving firmware monitoring of the crystal temperature and subsequent oscillator tuning is to use a temperature compensated crystal oscillator (TCXO).
3) Any phase noise that is present in the HFXO is multiplied up by the RF PLL and therefore becomes phase noise at RF. This has implications for transmit modulation performance and receive blocking performance. We do not currently have data to specify what phase noise tolerance is acceptable at the HFXO input in order to still maintain good BLE performance. Thus, there is a risk associated with using a high phase noise oscillator.
Based on some internal work with external clocks for other wireless applications we have concluded that MEMS oscillators do not have good enough phase noise. Crystals are regularly used in automotive applications, and it would be fair to say that a TCXO is probably the safest bet for an external oscillator. We do use these for some wireless applications where tight temperature tolerance is required.
I recommend using one of the crystals or TCXOs listed under section "7. Recommended Crystals" on page 15 of AN0016.1 (see specifically Table 7.1 and 7.2), as these have been tested by us in the past.
As one last note, we recommend against using crystals with a low peak power (which is characteristic of small crystals) since the device drives the crystal resonator with significant power to reduce phase noise.