High Temperature VCO Based on GaN Devices for ... - IEEE Xplore

High Temperature VCO Based on GaN Devices for Downhole Communications Tianming Feng, Jebreel M. Salem, and Dong Sam Ha Multifunctional Integrated Circuits and System (MICS) Group Bradley Department of Electrical and Computer Engineering Virginia Tech, Blacksburg, Virginia, 24061, USA E-mail: {fengtm, jebmms10, ha}@vt.edu Abstract— This paper presents a high temperature voltagecontrolled oscillator (VCO) for downhole communications. The proposed VCO adopts the common-source Colpitts oscillator configuration. The VCO is prototyped using 0.25 μm GaN RF transistors due to the high junction temperature and high unity gain frequency. Two GaN varactors are used to achieve the tuning frequency range of 40 MHz up to 230 oC. The measurements at 230 oC show that the VCO has an output power of 17.5 dBm with ±0.5 dB variations over the tuning range from 320.8 to 360.2 MHz. The measured phase noise is -121 dBc/Hz at 100 kHz offset from 343.3 MHz carrier. The maximum power consumption of the VCO is 122.5 mW. Keywords— High temperature VCO; high tempreture oscillator; GaN VCO; GaN transistor; downhole communications

I.

INTRODUCTION

Decreasing reserves of easily accessible natural resources motivates the oil and gas industry to drill deeper to reach previously unexploited wells. However, the downhole environments are becoming harsher, reaching higher temperatures and pressures which necessitate more robust and higher speed electronics to reliably operate in these environments. The main problem for downhole electronics is the temperature limitation, being that the pressures are handled mechanically. Despite wells being classified beyond 210°C the current drilling temperatures do not exceed 200°C [1]. This is due to the fact that the current electronics, which are made from Silicon (Si), used in these systems can only operate up to 150°C before being recovered from the well due to high leakage current. By increasing the ambient temperature capability, longer logging times and deeper drilling is possible. Cooling and conventional heat extraction techniques are impractical to use in downhole due to weight contribution, power consumption, and added complexity. In addition, the current telemetry systems use low frequency circuits that can achieve data rates of only approximately 2 Mb/s at temperatures < 200°C [2], which still does not meet the demand for rapid grown data rates due to higher resolution sensors, faster data logging speeds, and additional tools available for a downhole system. A radio frequency (RF) cable modem provides higher speed compare with the current systems. An essential component in a RF cable modem is the VCO that generates RF carriers. There has been several oscillator designs [3]-[6] and very few VCO designs [7] operating at high temperature reported in the open literature. In [3] a high

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temperature performance of SiC MESFET based oscillator is presented. In this work, a cross-coupled oscillator is designed to operate with 50 Ÿ load and generate signal with frequency of 515 MHz at 125 oC. Then, as temperature increases the load and bias have to be adjusted to generate a signal. Signal with frequencies of 610 MHz and 453 MHz are generated at 200 °C and 475 °C, respectively using different loads and biasing points. Ponchak et al. reports in [4] a demonstration of clap oscillator operating at 470 °C with oscillation frequency of 90 MHz. Also, a clap oscillator operating at 200 °C with oscillation frequency of 1 GHz is presented in [5]. In addition, [6] reports GaN-based Lamb-wave oscillator on Si operating from room temperature (RT) up to 250oC with oscillation frequency of 58 MHz. All abovementioned designs are for single frequency oscillators and suffer from large temperature variations. In [7], a VCO with tuning range from 250 to 350 MHz is designed to operate up to 200 oC. This design was implemented in a 0.5μm Silicon-on-Sapphire CMOS Si technology and as temperature increases, the leakage current becomes a critical issue. This paper presents the first high temperature GaN HEMT VCO operating in UHF band at ambient temperatures from 25 to 230 o C. This paper is organized as follows. Section II describes design challenges for the high temperature VCO. Section III introduces the proposed VCO. Section IV shows the measurement results at RT and high temperature operation. Section V concludes this work. II.

DESIGN CHALLENGES

The key design challenge for the proposed VCO is the temperature limitations of the active and passive commercial-ofthe-shelf (COTS) components. Wide bandgap materials such as GaN and SiC have extremely low leakage current and offer superior performance at high temperature [8]. However, COTS Silicon Carbide (SiC) are mainly for low frequency applications. GaN, on the other hand, offers a reliable combination of high temperature and high frequency capabilities [1]. It also has been shown to be thermally robust and can have very low noise figure. The high temperature capability of GaN HEMT power transistor and high operating frequency make them an excellent candidate for VCO designs for high temperature applications. The thermal limitations of the package must also be considered. The thermal effect can be understood clearly from this relationship: PMAX= (TJ – TAmbient )/ RșJA. The designer can balance the requirements of the ambient temperature (TAmbient), total junction-to-ambient thermal resistance (RșJA), power limitation (PMAX), and necessary junction temperature (TJ) as

B. VCO Deign Fig. 1 shows the proposed VCO design. It is a common source Colpitts oscillator with two GaN varactors. This topology is chosen mainly due to its simplicity. A complex circuit is more vulnerable to the temperature variations, which deteriorates reliability at high temperature. The biasing condition is chosen so that the thermal effect of the package is minimized and the loop gain of the VCO is large enough to maintain the oscillation at wide temperature range. The transistor is biased at VDD = 2.5 V and Vgs=-2.5V and dissipates a maximum current of ID = 51 mA at RT. Fig. 1. The common source colpitts oscillator adopted for our VCO design. Components in red represent parastics.

needed. However, the physical limitations of the device (TJMAX and RșJA) are often the limiting factor. Using the thermal limit for safe operation (TJMAX) in place of TJ allows for the relaxation of the other parameters at the cost of reduced lifetime. Also, typical passive components exhibit large variance over significant temperature changes, particularly inductors. Temperature variations of inductance and Q of inductors and varactors are characterized and considered in the design process to ensure the operation of VCO at high temperature. Other passive devices such as RF chokes (RFCs) and DC blocking capacitors are noncritical and could be used freely. The bonding material is also a primary concern. Efforts to ensure reliable connections at high temperature operation are paramount; therefore, careful selection of solders and epoxies is necessary. There exists a wide variety of materials that can meet this requirement. III.

PROPOSED VCO DESIGN

The main objective is to design a VCO that can cover tuning range from 328 to 350.5 MHz and provide reliable performance at high temperature. To achieve these objectives, first, the main devices are selected and characterized then the circuit design is proposed and implemented. A. Device Selection and Charactrization The active device used in this design is a commercial Qorvo T2G6000528-Q3 packaged with 0.25μ GaN on SiC HEMT [9]. The chosen transistor has a maximum junction temperature TJMAX of 275°C and reasonable thermal resistance RșJC of 12.4°C/W. The main application for this device is high power amplifiers, and datasheet provided by the manufacturer has the characterization at Vds=28 V and Id =200 mA at RT and 85oC. For the VCO design, the required operating point and power are much smaller than that provided in the datasheet. Also, the design is required to operate at temperatures up to 230 °C. Therefore, new device characterizations are performed for the range of RT to 230 °C. The passive devices are also characterized at high temperature and radio frequencies. S-parameter models are created and used in the design process. IPDiA 1μF capacitors that operate up to 250°C are selected for our design. These capacitors are used as blocking caps. Coilcraft 1 uH cored inductors that operate up to 300°C are used as RF chocks.

The loop gain has to be greater than one at all operating temperatures to satisfy the oscillation condition. To ensure the loop gain is equal or greater than unity, the transconductance, gm, has to be: ‰௠ ൒

஼మ ோ೟ ஼భ

(1)

where Rt is the total resistive loading of the tuned circuit and it comprises the load resistance, RL, the inductor series resistance, Rs, and transistor output resistance, ro. C1 and C2 are tuning capacitors. To find the loop gain that allow the transistor to oscillate at wide temperatures, gm, ro and Rs are characterized at different temperature ranges. It is found that from the characterization that the gm decreases from 159 mS to 114 mS as temperature increases from RT to 230 oC. The other temperature dependent parameters are ro and Rs since the load resistance, RL, is fixed to =50Ÿ, and assumed temperature independent. It is found that temperature variation of ro and Rs leads Rt to decrease as temperature increases. Rt is 45 Ÿ at RT and 30.1 Ÿ at 230 oC. For C1/C2 equals to any value between 0.3 and 1, eq (1) will be satisfied at all temperatures. To achieve a wide tuning range, two identical varactors are used in replace of C1 and C2. The main motivation for using two varactors is to achieve wide tuning range while minimizing the output power variation over tuning range and with temperature. These varactors are designed using GaN HEMT transistor with their drain and source terminals grounded and gate terminals connected to control voltage through RF chocks. Fig. 2 shows GaN based varactors network. Each varactor provides range from 7.5 pF to 11.2 pF at RT.

Fig. 2. Varactors network based on GaN HEMT transistor, used to provide tuning range of 40 MHz.

However, it is found that as temperature increases, both the capacitance and quality factor, Q, of the varactrors decrease. Fig. 3 shows the measured tuning range for one varactor at different temperatures. At 230 oC, capacitance decreases to 6.8 pF - 10 pF with the same tuning range. These temperature variations affect

A. VCO Prototype and Measuremnt Environments The proposed VCO shown in Fig. 4 is prototyped with Qorvo T2G6000528-Q3 packaged with 0.25μm GaN on SiC HEMT. The blocking capacitor is IPDiA XTSC427 which is an extreme temperature silicon capacitor rating up to 300 oC. Coilcraft AT549RBT extreme temperature 1 uH cored spring coil is used as RF choke. All the components are assembled on Rogers RO4003C PCB.

12 T = 25°C T = 100°C T = 170°C

Capacitance (pF)

11

T = 230°C

10 9 8 7

Output

6 -6

-5

-4

-3 -2 Control Voltage (V)

-1

0

Fig. 3 Measured capacitance tuning range for one varactor at different temperatures.

VCtrl

the oscillation frequency, and the tuning range, and they can be compensated by re-tuning the varactors as temperature increases.

The inductor value is estimated from the oscillation frequency based on the following equation: ݂௢ ൌ

ଵ

(2)

಴ ಴ ଶగට௅ భ మ

಴భ శ಴మ

At frequency of 340.5 MHz, the inductance value is 44.2 nH. Air coil inductor is designed using high temperature copper wire. However, it is found that as temperature increases, the Q of the inductor decreases. The decline of Q for both the inductor and varactor with temperature degrade the phase noise performance at 230 oC. The temperature effect on phase noise of the VCO design is estimated using (3) [10]. ‫ܮ‬ሼο߱ሽ ൌ ͳͲ Ž‘‰ ቈ

ଶி௞் ௉ೞ೔೒

൤ͳ ൅ ቀ

ఠ೚ ଶொοఠ

ଶ

ቁ ൨ ൤ͳ ൅

οఠభȀ೑య ȁοఠȁ

൨቉

(3)

where L{ǻȦ} is SSB noise spectral in units of dBc/Hz. k is Boltzman’s constant, T is temperature in Kelvin, Psig is the power fed to the transistor, Ȧo is the oscillation frequency, and ǻȦ is the offset from Ȧo. Q is the unloaded quality factor of the resonator, F is the noise figure of the oscillator and ǻȦ1/f3 is the noise corner frequency of the transistor. As temperature, T, increases to 230 oC, F increases by 1.5 dB, and Psig decreases by 1.0 dB. Also, Q decreases by 40%. This leads to 6.0 dB increase in the phase noise at 230oC. This increase agrees with the measurements shown later in Fig. 7. Although there is 6dB increase at high temperature the performance is still satisfactory since the predicted phase noise at high temperature is better than -109 dBc/Hz at 100KHz offset. IV.

MEASUREMENT RESULTS

This section presents VCO prototype and the measured performance at temperatures ranging from RT to 230 oC.

VGS

Fig. 4. A photograph of the common source Colpitts VCO prototype.

The prototyped VCO is placed inside a Yamato natural convection drying furnace. Special high temperature cables and connecters are used for the measurements. The output power and phase noise are measured using R&S FSU26 spectrum analyzer. Rigol power supplies are used to set up the required bias. The VCO board is measured in the oven for different temperatures with R&S FSW Signal and Spectrum Analyzer. B. Measurement Results i. Tuning Range The designed VCO performance is firstly measured at RT. The design is biased at Vdd of 2.5 V and gate voltage Vgs of -2.5 V. The control voltage, Vctrl, is swept from -6V to 0V. The output frequency changes from 327.5 MHz to 367.7 MHz with tuning range of 40.2 MHz at RT. Then, temperature of the furnace is elevated gradually up to 230 oC. As temperature increases the output frequency decreases at given control voltage. Fig. 5 shows the output frequency versus control voltage at different temperatures. It can be observed that as temperature increases to 230oC, the output frequency ranges from 320.8 MHz to 360.2 MHz, representing frequency shifts by 8 MHz while maintaining the same tuning range of 40MHz. This frequency shift at high temperatures does not affect the required tuning range for the designed transceiver. 380 T = 25°C T = 100°C T = 170°C T = 230°C

370

Operating Frequency (MHz)

In addition, the effect of the gate-source, Cgs, which is highlighted in red in Fig.1, should be considered. Cgs is 2.2 pF at RT and it is connected in parallel with C2. Therefore, the total C2 is equal to the capacitance value of the varactor plus Cgs. This leads C1/C2 =0.77 at all temperatures.

Varactor Network

VDD

360 350 340 330 320 310 -6

-5

-4

-3 Control Voltage (V)

-2

-1

0

Fig. 5. Measured output frequency versus control voltage at different temepratures.

Also, we observed that drain current changes with the control voltage. At RT, the current changes from 41 mA to 51 mA, and at 230oC, the current changes from 40 mA to 49 mA. ii. Output Power Fig. 6 shows the output power versus control voltage at 25oC, 100 oC, 170 oC and 230 oC. It can be noticed that the output power of the VCO varies from 18.16 to 19 dBm over the control voltage at RT, resulting in 0.83 dB maximum variation at RT. This small variation is due to the fact that two varactros being used and tuned the same time which leads to small variation in the output power. As temperature increases, the output power decreases. The output power of the fundamental frequencies of the VCO varies from 17.11 to 17.96 dBm at 230 oC, indicating a 0.85 dB variation over the tuning range and 1.5 dB variation with temperature. The amplitude drop with temperature is due to the fact that gm decreases as the temperature increases. 20

T T T T

19.5

Output Power (dBm)

19 18.5

= = = =

25°C 100°C 170°C 230°C

18 17.5 17 16.5 16 15.5 15 -6

-5

-4

-3 -2 Control Voltage (V)

-1

0

Fig. 6. Measured output power versus control voltage at different

iii. Phase Noise The phase noise at 100 KHz offset varies from -109 dBc/Hz to-121 dBc/Hz over frequency tuning range. Fig. 7 shows the phase noise of the VCO at the center of the tuning range at frequency of 340.5 MHz for different temperatures. It can be noticed that as the temperature increases the phase noise increases as it is predicted by (3). The phase noises at frequency offset of 100 KHz are -116.4 dBc/Hz and 110 dBc/Hz at 25 oC and 230oC, respectively. Although the phase noise deteriorates by 6.4 dB at 230oC, the performance is satisfactory. Also, it can be noticed that there are two peaks in the phase noise between 1 MHz and 10 MHz frequency offset and these are mainly due to the noise generated by the power supply. Also, the maximum frequency drifts of the VCO has been measured at each frequency. The measured frequency drift is 35 kHz and 25 KHz at 25 oC and 230oC, respectively. The harmonics of the VCO has been measured. The second harmonic ranges from -20.46 dB to -25.44 dB below the output power of the fundamental frequency. -80

T T T T

-90

Phase Noise (dBc/Hz)

-100

= = = =

25°C 100°C 170°C 230°C

-110 -120 -130 -140 -150 -160 -170 10 KHz 10,000

100 KHz 100,000

1 MHz 1,000,000

10 MHz 10,000,000

100MHz 100,000,000

Frequency Offset

Fig. 7. Measured phase noise at the center of tuning range (340.5 MHz) at different temperatures.

Table I shows a comparison of the proposed VCO with stateof-the-art high temperature VCO and oscillators. Although a fair comparison is difficult due to different technologies used and design objectives and constraints, the proposed VCO shows an excellent performance with a wide tuning range and for a wide temperature variation. TABLE I. PERFORMANCE COMPARISON OF HIGH TEMPERATURE VCO [7] [6] [5] Parameter This work Temperature (oC) 25-200 25-250 30-200 25-230 SOS GaN GaN Technology SiC CMOS HEMT HEMT Frequency (MHz) 300 58 1000 350 Output Power (dBm) Output power variation with T (%) Tuning Range (MHz) Power Dissipation (mW) Phase Noise @ 100KHz (dBc)

4.9-2.2

11

21.8

18

-

62.5

19

8.3

100

0

0

40

-

-

1010

127.5

-70

-120

-

-121

V. CONCLUSION A high temperature RF VCO for downhole communication is designed and prototyped using Qorvo T2G6000528-Q3 packaged with 0.25μm GaN on SiC HEMT technology. The VCO delivers power of 19 dBm at RT with 1.0 dB drop at 230 o C to 50 Ÿ. Also, it achieves a tuning range of 40 MHz at a wide temperature range. Minimum phase noise of -121 dBc/Hz is achieved at RT and 114.6 dBc/Hz at at 230 oC at 100KHz offset. Also, the measured second harmonics are 25 dBc below the fundamental frequency. REFERENCES [1]

J. D. Cressle, and H. A. Mantooth, eds. Extreme environment electronics. CRC Press, 2012. [2] T. Tran, W. Sun, J. Zeng, and Boguslaw Wiecek. "High-bitrate downhole telemetry system." In IEEE International Symposium on Power Line Communications and its Applications (ISPLC), 2015, pp. 280-284. [3] Z. D. Schwartz and G. E. Ponchak, “High temperature performance of a SiC MESFET based oscillator,” in IEEE MTT-S Int. Dig., Long Beach, CA, Jun. 12–17, 2005, pp. 1–4. [4] G. E. Ponchak,M. C. Scardelletti, and J. L. Jordan, “30 and 90 MHz oscillators operating through 450 and 470 oC for high temperature wireless sensors,” in Proc. Asia-Pacific Microw. Conf. (APMC), 2010, pp. 1027– 1030. [5] Z. D. Schwartz and G. E. Ponchak, “1-GHz, 200 oC , SiC MESFET Clapp oscillator,” IEEE Microw. Wireless Compon. Lett., vol. 15, pp. 730–732, 2005. [6] X. Lu, Jun Ma, C. P. Yue, and K. M. Lau. "A GaN-Based Lamb-Wave Oscillator on Silicon for High-Temperature Integrated Sensors." IEEE Microw. and Wireless Compon. Lett., vol. 23, no. 6, pp. 318-320, May 2013. [7] A. P. Moor, J. M. Rochelle, C. L. Britton, J. A. Moore, M. S. Emery, and R. L. Schultz. "A voltage-controlled oscillator for a 300 MHz hightemperature transceiver realized in 0.5 ȝm SOS technology." In Proc. of IEEE 44th Int. Midwest Symp. Circuit and Syst., 2001, pp. 614-617. [8] J. M. Salem and D. S. Ha, "A high temperature active GaN-HEMT downconversion mixer for downhole communications," 2016 IEEE International Symposium on Circuits and Systems (ISCAS), Montreal, QC, 2016, pp. 946-949. [9] Qorvo, “10W, 28V DC – 6 GHz, GaN RF Power Transistor,” T2G6000528-Q3 datasheet, Nov. 2014. [10] T. Lee and A. Hajimiri, “Oscillator phase noise: a tutorial,” IEEE J. SolidState Circuits, vol. 35, no. 3, pp. 326–336, Mar. 2000.