Operation of Silicon Germanium Voltage-Controlled Oscillatorsat Cryogenic Temperatures

April 2005
NASA Electronic Parts and Packaging Program
Operation of Silicon Germanium Voltage-Controlled Oscillators

at Cryogenic Temperatures

Richard Patterson, NASA Glenn Research Center

Ahmad Hammoud, QSS Group, Inc. / NASA GRC

Malik Elbuluk, University of Akron
Scope
Electronic circuits and systems designed for use in NASA space exploration missions are required to operate efficiently and reliably in harsh environments. Cryogenic temperature, which constitutes one of such stresses, will be encountered in space exploration missions, such as Mars and Saturn environments. Therefore, electronics designed for such applications must be able to withstand exposure to cryogenic temperatures and to perform properly, for extended period of time, under temperature swings, i.e. thermal cycling. Silicon germanium-based devices, which rely on band gap engineering, show a great promise for operation at cryogenic temperatures. Very little data, however, exist on the performance of such devices and circuits under cryogenic temperatures. In this work, the performance of a silicon germanium (SiGe) voltage-controlled oscillator was evaluated under low temperature and thermal cycling. The investigations were carried out to establish a baseline on the functionality and to determine suitability of this device for use in space exploration missions at cryogenic temperatures. The findings will be disseminated to mission planners and circuit designers so that proper selection of electronic parts can be made, and risk assessment can be established for such devices for use in space missions.

Test Procedure
The devices investigated in this work comprised of Maxim Integrated Products, MAX2622 monolithic voltage-controlled oscillator (VCO). This device utilizes silicon germanium technology, and combines an integrated oscillator and output buffer in a miniature 8-pin package [1]. The center frequency of oscillation and frequency span are factory preset. The frequency can be also controlled by an external voltage applied to the TUNE pin of the chip. The built-in buffer amplifier provides good isolation from load variations and boosts the output power [1]. Two of these VCO integrated circuit chips were examined for operation between -195 °C and +85 °C. Performance characterization was obtained in terms of output frequency and supply current at specific test temperatures. Dependence of the central frequency on the tune voltage (V_tune) was also determined. An Agilent E4448A Spectrum Analyzer was used to measure the frequency and to capture the output waveforms. A temperature rate of change of 10 °C per minute was used, and a soak time of at least 20 minutes was allowed at every test temperature. Cold-restart capability, i.e. power switched on while the device was at a temperature of
-195 °C, was also investigated. In addition, the effects of thermal cycling under a wide temperature range on the operation of these parts were determined. The devices were exposed to a total of 12 cycles between -195 °C and +85 °C at a temperature rate of 10 °C/minute. Following the thermal cycling, measurements were then performed at the test temperatures of +20, -195, +85, and again at +20 °C. Some of the manufacturer’s specifications for this SiGe VCO are shown in Table I [1].

Table I. Specifications of MAX2622 Oscillator [1].

Parameter	Symbol	MAX2622EUA
Supply Voltage (V)	Vcc	2.7 to 5.5
Supply Current (mA)	Is	8
Oscillating Frequency Range (MHz)	f	855 to 881
Output Power (dBm)	Po	-3
Power Dissipation (mW)	PD	457
Operating Temperature (°C)	T(oper)	-40 to +85
Package		8-pin µMAX
Lot Number		NY7DE3015A

Test Results
Although two units of this oscillator were evaluated, data pertaining to only one device is presented due to the similarity in their results.
Temperature Effects
The oscillator output frequency at various tune voltages is shown as a function of temperature in Figure 1. For any given test temperature, the output frequency increases with increase in the external control tune voltage. It can be also seen that the frequency increases, almost linearly, as the temperature is decreased from +85 °C to -150 °C. This trend is observed at all levels of the applied tune voltage. As the temperature is further decreased beyond -150 °C, i.e. to -195 °C, the oscillating frequency experiences insignificant increase. These temperature-induced effects on the frequency of this voltage-controlled oscillator are depicted in Figure 2, at various levels of tune voltage. These results tend to agree quite well with those reported by the device manufacturer’s at +25, +85, and -40 °C temperatures [1].
A typical waveform of the oscillator frequency as recorded by the spectrum analyzer at +20 °C is shown in Figure 3. Those obtained at the extreme temperatures of +85 °C and -195 °C are shown in Figures 4 and 5, respectively. Once again, the shift in frequency of this voltage-controlled oscillator is evident with change in test temperature. These waveforms were recorded with an applied external voltage of 1.5 V to the tune voltage pin of the device.

Figure 1. Oscillator frequency versus temperature for various tune voltages.

Figure 2. Oscillator frequency as a function of tune voltage at different temperatures.


Figure 3. Oscillator output frequency at +20 °C.


Figure 4. Oscillator output frequency at +85 °C.


Figure 5. Oscillator output frequency at -195 °C.

The supply current of the voltage-controlled oscillator is shown as a function of temperature in Figure 6. The curves depicted are for various tune voltages between 0 and 3.0 V. In general, all curves fall within the current band of 8.5 to about 10 mA when the test temperature is varied from +85 °C to -150°C. As temperature is decreased to -195 °C, however, all curves converge to a reduced supply current level of about 7.6 mA. This reduced current at cryogenic temperatures is advantageous as it results in lower power loss by the device.

Figure 6. Supply current versus temperature for various tune voltages.

Cold Re-Start
Cold-restart capability of the MAX2622 SiGe voltage-controlled oscillator chips was investigated by allowing the devices to soak at -195 °C for at least 20 minutes without the application of electrical bias. Power was then applied to the device under test, and measurements were taken on the output frequency and supply current characteristics. Both oscillators were able to successfully cold-start at -195 °C, and the results obtained were the same as those obtained earlier at that temperature.

Effects of Thermal Cycling
The effects of thermal cycling under a wide temperature range on the operation of the MAX2622 voltage-controlled oscillators were investigated by subjecting them to a total of 12 cycles between -195 °C and +85 °C at a temperature rate of 10 °C/minute. Measurements of the output frequency and supply current of the cycled devices were then taken at +20, +85, and -195 °C. A comparison of the output frequency of the oscillator as a function of tune voltage at test temperatures of +20, +85, and -195 °C for pre- and post-cycling conditions are shown in Figures 7 and 8, respectively. It can be clearly seen that the post-cycling frequency/voltage curves at any given temperature were the same as those obtained prior to cycling. Similar results were obtained on the supply current of the oscillator chips. Figures 9 and 10 display the supply current values at the extreme temperatures of +85 °C and -195°C for pre- and post-cycling conditions, respectively. Therefore, the extreme temperature exposure and the thermal cycling did not induce much change in the behavior of these silicon germanium oscillators. This limited thermal cycling also appeared to have no effect on the structural integrity of these parts as no structural deterioration or packaging damage was observed.





Fig 7. Frequency vs voltage (pre-cycling).



Fig 9. Current vs temperature (pre-cycling).

Fig 8. Frequency vs voltage (post-cycling).



Fig 10. Current vs temperature (post-cycling).




Conclusions
Silicon germanium voltage-controlled oscillators, Maxim Integrated Products MAX2622, were evaluated for operation between -195 °C and +85 °C. The effects of thermal cycling under a wide temperature range on the operation of these oscillators and cold-restart capability were also investigated. The two devices tested of this type of oscillator were able to maintain good operation between -195 °C and +85 °C with minimal changes in their characteristics. The limited thermal cycling performed on the oscillators had no effect on their performance, and both devices were able to cold start at -195 °C. These preliminary results indicate that these silicon germanium-based voltage-controlled oscillators could be potentially used in space exploration missions under cryogenic environments. Further testing under long-term cycling is, however, required to fully establish the reliability of these devices and to determine their suitability for extended use in extreme temperature environments.
References
[1]. Maxim Integrated Products, “MAX2622 Monolithic Voltage-Controlled Oscillators”, Data Sheet Document Number 19-1528; Rev. 1; 5/00.
Acknowledgements
This work was performed under the NASA Glenn Research Center GESS Contract # NAS3-00145. Funding was provided by the NASA Electronic Parts and Packaging (NEPP) Program.