Clock generation

The FMECA severity results for the clock generation functional block are shown in Table 4.15. This functional block is not responsible for any catastrophic failure to external systems. However, expected degradation could lead to functional failures and further degradation of the systems.

Table 4.15: FMECA severity analysis on the clock generation functional block.

ID Failure mode Failure causes Failure effects SN CLK.1 HW Failure SELs or high current

states permanent loss of system

functionality 3

CLK.2 HW Failure TIDs, long-term

degra-dation permanent loss of system

functionality 3

CLK.3 HW Failure SEFIs, recoverable state temporary loss of system

functionality 2

Clocking is required for different parts of the GSDR, such as for the BBP and RFIC.

Clocking sources are usually to be defined as temperature-stable and should hold their output specification over a long period. The experiences of radiation effects on crystal

Chapter 4. A novel approach to a highly integrated and radiation-tolerant solution 73 oscillators are quite rare and only a few researches relevant to this issue are available [123–126]. Two acknowledged issues around crystal-based oscillators are an accumu-lated (TID) degradation of stability and a drift of the oscillator frequency [125, 126].

Radiation tests have shown that oscillators may withstand very high doses for space (≥1 Mrad(SiO2)) with a relatively low degradation. Transient effects causing a de-structive damage are not known. Some studies have mentioned that pulsed radiation effects may cause a loss of oscillation which is recoverable by power-cycling the device [123, 124]. Based on the expected effects and their potential failures due to radiation, a space-qualification is not mandatory but at least an automotive-grade level should be considered.

Devices that require a clock signal, such as the BBP or RFIC, allow a broad range of in-put frequency and can be re-calibrated or tuned to the radiation-induced offset oscillator frequency if required. However, since a significant frequency drift is not expected at a dose level of several 10s to 100s krad(SiO2) and most near-Earth mission will not achieve those values in their lifetime, such effects are of minor concern. Oscillators are available in different types and qualification-level. Based on the requirements of the previously se-lected devices for the BBP and RFIC, three technologies are desirable: (1) temperature controlled crystal oscillator (TCXO), (2) voltage controlled crystal oscillator (VCXO) and (3) oven controlled crystal oscillator (OCXO), as shown in Table 4.16.

Table 4.16: Technology assessment for potential oscillator devices.

Device Techno. Level Review Complex. Perform. Costs Data

TCXO n.a. All n.a. + + ++

-VCXO n.a. All n.a. -+ - +

-OCXO n.a. All n.a. - -+ - -

-The most important features of the desired oscillator technology are the frequency sta-bility, jitter and phase-noise performance. VCXOs are generally less stable compared to TCXO and OCXO. The best stability is provided by OCXOs but they require a stable environmental temperature and are thus not recommended for the GSDR application.

However, the power consumption of the latter is relatively high. TCXOs are highly stable over a broad range of temperatures and are thus the best technology for the BBP and RFIC. Due to the fact that the severity number is fairly low (2 to 3) and that it is known furthermore that specifically TID is not so critical, a criticality determination has not been made and the TCXO selected has not been further investigated or tested.

However, at least an automotive-grade device is desirable for satisfying the required manufacturing review.

Since two AD9361s are used for the RFIC functional block, a clock-buffer is also required for the TCXO signal distribution. For such components, RadHard and space-qualified

Chapter 4. A novel approach to a highly integrated and radiation-tolerant solution 74 devices are available but these often fail to meet the performance required for the AD9361 in terms of signal type (low-voltage CMOS), jitter or phase noise. Expected failures of such devices are listed in Table 4.17.

Table 4.17: FMECA severity analysis on the clock generation functional block.

ID Failure mode Failure causes Failure effects SN BUFF.1 HW Failure SELs or high current

states permanent loss of system

functionality 3

BUFF.2 HW Failure TIDs, long-term

degra-dation permanent loss of system

functionality 3

BUFF.3 HW Failure SEFIs, recoverable state temporary loss of system

functionality 2

Critical failures can be SELs and high current states leading to a destructive effect on the devices. Furthermore, TID may lead to a degradation of the device’s performance (e.g. noise enhancement or extended jitter) and finally to a total loss of function if the exposed radiation exceed the limits. Minor publications referring to radiation test results are available for such devices. However, test data in [127] have shown a very robust response in terms of neutron-induced SEEs and TID. Vendors offer radiation-tolerant and RadHard solutions, but with limited performance compared to COTS devices.

Table 4.18: Device assessment for potential clock generation devices.

Device Techno. Level Review Complex. Perform. Costs Data

CDCLVC-1310 BiCMOS Indust. -+ -+ ++ ++ -+

ADCLK-846 180 nm

CMOS Indust. + -+ -+ + ++

CDCLVP-111 BiCMOS Indust. -+ -+ -+ + ++

CDCLVP-111-SP BiCMOS Space ++ - - - - n.a.

Table 4.18 shows the device assessment for potential clock generation and distribution devices. For the clock-buffer device, a non-space-qualified solution is desired and addi-tional investigations specifically with respect to radiation effects have been considered during the GSDR system development to establish the best RF performances for the RFIC. Further investigations have been undertaken for the CDCLVC1310, which has already been evaluated by [127]. In cases where the selected devices fails under radiation or the performances degrades heavily, a fallback option to a radiation-hardened solution has been implemented in the system design by a dual-footprint on the printed circuit board (PCB).

Chapter 4. A novel approach to a highly integrated and radiation-tolerant solution 75 The selected device has been recently tested under proton irradiation and γ-rays and results confirm their robustness for TID and SEEs. Such results are not, however, within the scope of this thesis but are intended to be published soon. A further determination of criticality is mandatory.