Proceedings of the 5th European Microwave Integrated Circuits Conference
Power detectors and envelope detectors in mHEMT MMIC-technology for millimeterwave applications Herbert Zirath*#1, Zhongxia (Simon) He*2 #
Ericsson AB
1
[email protected]
*
Chalmers University of Technologies, Microwave Electronics Laboratory – MC2,Göteborg, 41296, Sweden 2
[email protected]
Abstract— Two types of microwave and millimeterwave power detectors realized in a commercial mHEMT MMIC-process is presented for the first time. The detectors are based on either a gate Schottky diode or active mHEMT. Possible application are microwave/millimeterwave power detectors, and multi Gb/s high speed demodulators for OOK, BPSK, QPSK etc. Index-Terms—MMIC microwave microwave envelope detector
power
detector,
TL
MMIC
Port Input
I. INTRODUCTION Power detectors are needed in many microwave and millimeterwave systems for power level detection [1-3], demodulators in communications systems [4-6], and in radiometers [7,8]. Power detectors can be implemented in bipolar [3,6,7], field effect transistor processes [9,10] and Schottky-diode technologies [4,8,9]. In this work we have implemented power detectors in a commercial 0.15m mHEMT-process offered by WIN Semiconductors. Both Schottky based detectors and active detectors have been designed and experimentally verified for use in multifunction MMICs for power detection in millimeterwave power amplifiers for output power sensing and levelling, and envelope detectors for OOK, BPSK and QPSK demodulators. To the best knowledge of the authors, power detectors suitable for MMIC-integration has not previously been reported for a HEMT-process. The examples in this work are applicable to all HEMT-processes, i e GaAs-mHEMT, GaAs-pHEMT, and InP HEMT. II. SCHOTTKY DIODE DETECTOR The simplified schematic representation of the Schottky detector is shown in Figure 1 and the layout in Figure 2. The RF is applied to the diode through a 170 fF MIM coupling capacitor, an open stub is used for minimizing the reflection coefficient at 60 GHz. Bias current can be supplied to the diode through a 900 ohm resistor. This resistor forms a simple low pass filter with capacitor C2 (220 fF), which is connected to ground. The RF-input and the detected signal are connected to CPW-probe pads for characterization. The Schottky diode consist of two parallelled gatefingers, each 20 um wide. The diode is modeled by the foundry model, based on the EEHEMT-model. The output voltage was simulated as a function of input power at different frequencies and bias currents, and optimized for frequencies above 50 GHz.
978-2-87487-017-0 © 2010 EuMA
C C1
R R1
C C2
Port Output
diode
Fig. 1 Schematic diagram of Schottky diode based power detector
Fig. 2 Photo of Schottky diode based power detector
In Figure 3, the measured output voltage is depicted for a constant biascurrent of 1 uA. The bias current is supplied through the output port by an external current source. At no input power the output voltage is of the order -710mV which is the voltage drop across the diode and R1. A sensitivity of 500 V/W is obtained at 60 GHz at -20 dBm. The incremental detector voltage, defined as the voltage difference with and without a certain RF-power, is plotted in Fig 4. The RFreflection coefficient is plotted in Figure 5, above 50 GHz it is better than -10 dB. This corresponds well with the simulations.
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27-28 September 2010, Paris, France
Port Gate-bias
Port Drain-bias
C C3
R R4
R R3 R R1
Port Input
C C1
C C2
Port Output
R R2 FET1
Fig. 6 Schematic diagram of active power detector Fig. 3 Measured detector output voltage at 1 uA biascurrent as a function of frequency.
Fig. 7 Layout of active power detector
Fig. 4 Measured incremental detector output voltage at 1 and 10 uA bias current as a function of input power in dBm and frequency. 0
-2
dB(S(1,1))
-4
-6
-8
-10
-12
-14 0
10
20
30
40
50
60
70
freq, GHz
Fig. 5 Measured reflection coefficient for the RF-port at 1 uA biascurrent
III. MHEMT DETECTOR The simplified schematic representation of the active power detector is shown in Figure 6, and the layout in Figure 7.
The RF-input is connected to the gate of the 2x20 um gate width mHEMT device through a MIM-capacitor C1 in series with resistor R2 (37 Ohm), the drain quiescent current, IDD is controlled by the gate bias voltage, VGG. The drain is connected through resistor R4 (750 ohm) to VDD. VGG is adjusted to be close to the pinch off voltage. The capacitor C2 (0.66pF) is part of a low-pass filter. The characteristic of the active detector was simulated as a function of frequency and bias conditions and optimized. At VDD=2V and IDD=15 and 300 uA, the incremental output voltage was measured, plotted in Fig. 8 a,b. The sensitivity is higher for the 300 uA bias compared to 15 uA, but the variation with frequency is much larger. The maximum sensitivity at 20 GHz is 2800V/W. In order to measure the linearity of the detector we used a lock-in amplifier based measurement setup according to Fig 9. The measurement system consist of an Agilent 67 GHz PSG E8257D Signal Generator and a Stanford Research SR830, 100 kHz DSP lock-in amplifier. The bias current was set to approximately 200 uA. The result is plotted in Fig 10. Linear operation from lowest power up to -10dBm is obtained. The deviation at some frequencies below -40 dBm is believed to be caused by the measurement setup and not the circuit itself. The reflection coefficient was measured with an Agilent PNA, the result is plotted in Fig. 11. The reflection measurements compares well with simulations.
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Fig. 10 Output incremental voltage as a function of input power and frequency
0
-2
dB(S(1,1))
-4
Fig. 8 Measured incremental detector output voltage at 15 and 300 uA drain current and VDD=2V. The frequency is 10 to 90 GHz in 10 GHz steps.
-6
-8
-10
-12 0
10
20
30
40
50
60
70
freq, GHz
Fig. 11
Fig. 9 Measurement setup for sensitivity and linearity
Measured reflection coefficient for the RF-port at 200 A biascurrent
The dynamic response of the detector makes it useful as an envelope detector for OOK etc. for several Gbps. With an input pulse depicted in Fig 12, an output signal shown in Fig 13 is obtained from simulation. The input signal corresponds to a 0.5 ns long pulse with 3dBm peak power. It is assumed that the output is terminated in 50 ohm and that a filter with a 10 GHz passband and a dc-block is placed between the MMIC and the load. The input frequency is 20 GHz. From Fig. 13, rise and fall-time is less than 50ps is obtained. We have initially verified the functionality of the envelope detector by measurements based on a 5 GS/s oscilloscope. Input carrier frequency is 1 GHz and the datarate 300 Mbit/s. The output of the detector is shown in Fig. 14, showing that the detector is fully functional as an envelope detector.
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Input [V]
1.0
REFERENCES
0.5
[1]
0.0
[2]
-0.5 [3]
-1.0 0.0
0.5
1.0 [4]
time [ns] Fig. 12 Simulated input signal to the detector
[5]
100
Output [V]
0
[6]
-100 [7]
-200 -300 0.0
[8]
0.5
1.0
time [ns]
[9]
Fig. 13 Simulated output signal from the detector [10]
Fig. 14 Measured output of the detector. The lower trace shows the input signal; a modulated RF (1 GHz RF and 300 Mbps), the upper trace shows the detector output signal.
IV. SUMMARY AND DISCUSSION Power detectors and envelope detectors suitable for multi Gbps communication systems have been designed, fabricated and characterized. The detectors are realized in a commercial 0.15 m gatelength mHEMT-MMIC process suitable for multifunction MMICs up to 90 GHz. The Schottky diode based detector obtained a sensitivity of 500V/W (60 GHz) and the active mHEMT detector 2800 V/W (10 GHz). The active detector was tested as an envelope detector up to a bitrate of 300Mbps. The measured datarate is at the moment limited by the measurement setup i.e. due to lack of a fast modulator.
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