# Characterization and Modeling of DHBT in InP/GaAsSb Technology for the Design and Fabrication of a Ka Band MMIC Oscillator

Authors: S. Laurent; J. C. Nallatamby; M. Prigent; M. Riet; V. Nodjiadjim
Journal: Active and Passive Electronic Components
 (2012)

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## Abstract

This paper presents the design of an MMIC oscillator operating at a 38 GHz frequency. This circuit was fabricated by the III–V Lab with the new InP/GaAsSb Double Heterojunction Bipolar Transistor (DHBT) submicronic technology (We=700 nm). The transistor used in the circuit has a 15 μm long two-finger emitter. This paper describes the complete nonlinear modeling of this DHBT, including the cyclostationary modeling of its low frequency (LF) noise sources. The specific interest of the methodology used to design this oscillator resides in being able to choose a nonlinear operating condition of the transistor from an analysis in amplifier mode. The oscillator simulation and measurement results are compared. A 38 GHz oscillation frequency with 8.6 dBm output power and a phase noise of −80 dBc/Hz at 100 KHz offset from carrier have been measured.


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## Body

## 1. Introduction

The developments in modern electronics (analog, digital, or mixed), whatever their intended applications (telecommunications, spectroscopy and astrophysics, plasma analysis, medical imaging, etc.) now concern applications operating from RF frequency spectrum up to the optical frequencies. In order to develop solid-state circuits operating in the millimeter wave range, new technological processes are emerging for manufacturing semiconductors operating in these domains. To achieve the performances required by such applications, a suitable solid-state technology must be available. III–V Lab provides a new InP/GaAsSb DHBT technology [1], which contains antimony in the base of the transistors and permits submicronic emitter sizes. The structure has been optimized to get an Fmax greater than 300 GHz and an FT equal to 200 GHz. In this context, the work presented in this paper demonstrates the feasibility of designing and manufacturing a low phase noise MMIC frequency source with this new technology. To provide the oscillator designer with the most relevant devices, various multifinger transistors have been measured, a number ranging from 2 to 8 have been compared and reported in [2]. For the higher-frequency applications, 2-finger devices were found to be best suited and selected for this design. In Section 2, we present the complete characterization and nonlinear modeling of the transistor, including its thermal behavior. Section 3 deals with the cyclostationary LF noise source modeling. Section 4 is devoted to the description of the oscillator design according to the new proposed methodology including the drawing of the MMIC layout. In Section 5, all the measurements are detailed and compared with the results predicted by the simulation. The last section concludes the article.


## 2. Transistor Modeling

In order to design oscillators with improved characteristics, we need accurate models of the passive and active elements of these circuits. The models of passive elements of this process fabricated on coplanar waveguide technology (CPW) have been developed by the MC2 Technology Company. The DHBTπ model is based on the Gummel and Poon model [3] improved in [4]. It is presented in Figure 1 with its associated thermal circuit formed by two-pole RC cells. The intrinsic part of the convective equivalent circuit is described by four diodes and one voltage-controlled current source: Dec and Dcc control the current source with, respectively, a direct gain βf and a reverse gain βr, Dfe, and Dfc model leakage currents. The base collector charge qbc results from the sum of the junction charge with “punchthrough effect” charge, and the depletion charge. The base emitter charge qbe results from the sum of three charges: the junction charge, the depletion charge due to the collector transit time, and the Kirk effect charge. These charges depend on the Vce and Vbe voltages. The extraction of parameters describing the non linear transistor model is performed from pulsed I–V, pulsed [S]-parameters measurements. More details on this currently conventional operation are given in [5, 6]. Note that this paper is a detailed version of [7] in which we will explain all the measurement and extraction procedure for the modeling and the design methodology of the oscillator.Figure 1
DHBT modelπ-topology with a two-pole RC thermal subcircuit.To determine the thermal circuit elements, the new method proposed here is a good trade-off between speed and accuracy. This method involves the determination of the impedance and thermal time constant from the measurement of the transistor input impedance at low frequencies. Indeed, the thermal resistance can be estimated from the input impedanceZBE by (2):
(1)ZBE=VBEIB=h11ISO+ϕ⋅ZTH(ω)⋅h21⋅(VCE0-R2⋅IC0),
with(2)ϕ=∂VBE∂T|IB=CTE⁡,
where VBE and IB are HBT Base-Emitter voltage and base current, h11ISO is the h11 isothermal parameter, ZTH is the thermal impedance, VCE0 and IC0 are the bias Collector-Emitter voltage and Collector current, R2 is the load resistance on the collector (see Figure 2).Figure 2
Measurements used to determine theh21 parameter.So, according to this equation, we must determine theϕ, h21, and h11ISO parameters to deduce the thermal impedance ZTH. The ϕ parameter is determined using the Ib measurement as a function of Vbe for a constant current Ic at different temperatures in DC mode. The methodology is detailed in [8]. The h21 parameter is determined using the following four quantities shown in Figure 2 where Zin is the input impedance, Zout is the output impedance, Zin0 is the input impedance when the transistor output is short circuited at LF, and Gd is the transistor voltage gain. Thus, it is possible to extract the h21 parameter from these four measures by using the hybrid equivalent circuit of the transistor, and with (3) [9]:
(3)h21=Zin⋅Gd⋅R1Zout(Zin0-R1),
where R1 is the base resistance (see Figure 2). The h11ISO parameter is determined by measuring the transistor input impedance with R2=Vce0/Ic0. In this case, the measured impedance is defined by ZBE=h11ISO. At last, the input impedance is measured with a resistor R2 different from the previous value, but at the same bias point (Figure 3). Knowing the parameters of the equation, we can deduce the thermal impedance Zth. To implement this self-heating effect in a simulator, we include a low-pass equivalent electrical circuit with two RC parallel cells in series whose impedance fits the measured Zth. Thus, it is possible to obtain an equivalence between voltage/temperature and current/power as shown in Figure 3.Figure 3
Determination of thermal impedance.To conclude this first modeling step, we compare the convective model simulation with measurements. For example, Figure4 shows the comparison of the Ic(Vce) network, the current gain β as a function of Vce and Vbe versus Vce for an Ib current range from 0.5 mA to 3 mA in steps of 0.5 mA at a temperature equal to 25∘C.Comparison between model (red) and I(V) measurement (blue cross) withIb current ranging 0.5 to 3.5 mA and a Vce voltage ranging 0 to 3.5 V for a temperature equal to 25∘C.
(a)(b)(c)The last modeling step is the determination of the nonlinear chargesqbc and qbe by means of the pulsed [S] parameters measurements. The final modeling verification consists in comparing the complete electrothermal nonlinear model simulation with measurement. Figure 5 shows the comparison between simulation and measurement of the [S] parameters, the maximum available gain (MAG) and h21 parameter versus frequency for a Ib current equal to 2 mA and Vce voltage equal to 1, 2, and 2.5 V. Thus, we get a good agreement between the two results.Comparison between model simulation results in red and measurements (blue cross) for the MAG,S11, S12, S22, S21, and h21 parameters with Ib=2 mA and Vce=1-2-2.5 V.
(a)(b)(c)


## 3. Modeling of Transistor Low-Frequency Noise Sources

An accurate prediction of oscillator phase noise requires a cyclostationary model of the LF noise sources of the transistor [10]. A transistor model with internal LF noise sources [11] with cyclostationary properties has been developed for a GaInP/GaAs HBT technology. This model is only based on low-frequency measurements around DC bias. The noise sources variations versus the DC collector current are taken into account to elaborate cyclostationary noise model. We propose to reuse this topology (Figure 6) for this InP/GaAsSb technology with the new extraction method presented here.Figure 6
LF equivalent circuit of the transistor with noise sources.This LF model of the transistor (the nonlinear charges are not represented here) is based on the original representation of Ebers and Moll (Model T), called injection model, which is based on current-controlled current sources. He directly comes from the equations of physics and models the injection currents of the transistor.α is the large-signal forward mode current gain of a common-base transistor. More details on its equivalence to the Gummel-Poon model are given in [12]. Moreover, the extraction of noise sources from measurement is easier with this model. The diode Dbe represents the base-emitter junction, Dfe represents the leakage current of the base-emitter junction. The transistor effect is modeled by the controlled current source α·Ie. The current sources Icshot and Ibshot are the shot noise sources, Ic and Inc represent the LF noise sources (1/f and GR). The thermal noise sources of access resistors are negligible. The noise source Ic is generated within the base-emitter junction and it is associated to the Deb diode. It participates in the transistor effect through the controlled current source α·Ie. The origin of the Inc source comes from the periphery of the base-emitter junction and can be associated with the leakage diode Dfe. This representation with only two LF sources results from many measurements and simulations of oscillators which have shown that this transistor model is sufficient to accurately simulate oscillator phase noise. The purpose of the modeling is to extract the sources Ic and Inc from noise measurement. In order to do that, we suppose that the transistor low-level model with its noise sources can be represented by the conventional schema of Figure 7(a) with a Norton equivalent current source at each access of the transistor. Noise measurements consist in successively measuring these two noise sources Iecc and Iscc. To do that, two low value resistances Rin and Rout, properly chosen (Rin≪Zin0 and Rout≪Zout0), are successively connected to each access in order to short-circuit the corresponding source [13]. Moreover, to achieve this condition, a huge capacitance is successively connected to the transistor access (in parallel with Rin or Rout of Figure 7(b) and 7(c)) where we do not measure the noise. The voltage thus generated is then amplified by the voltage amplifier and its output is connected to the FFT spectrum analyzer in order to measure the noise voltage spectral density.Equivalent circuit of a noisy transistor with current noise sources.
(a)(b)(c)The schema used to obtain the noise sources of the transistor accesses is presented in Figure8.Figure 8
Equivalent schema for the extraction of short-circuit noise current spectral density at transistor accesses.This schema is the same at each access of the transistor. The amplifier is represented by its noise voltage sourceeat and by its input impedance Zat. The voltage Vs is amplified with a gain G to be measured on the FFT analyzer. The [ABCD] matrix represents all the passive elements considered in the measurement (bias tees, lines, etc.). R can be either Rin or Rout, IR is the thermal current noise source associated to R, Idut can be either Iecc or Iscc, and Zdut can be either Zin0 (with the collector short-circuited) or Zout0 (with the base short-circuited). Thus, applying Kirchoff’s laws, we obtain (4):
(4)Vs=N2⋅Idut+N3⋅eat+N4⋅IRN1,Now, we must determine the noise voltage spectral densities at each access:(5)SVs=|N2N1|2SIdut+|N3N1|2Seat+|N4N1|2SIR,
with
(6)N1=Zat⋅R⋅D+Zat⋅R⋅Zdut⋅C+Zpolar⋅Zdut⋅A+R⋅B+Zat⋅Zdut⋅A+Zat⋅B,N2=Zat⋅Zdut⋅R⋅(A⋅D-B⋅C),N3=Zat⋅(-R⋅D-R⋅Zdut⋅C-Zdut⋅A-B),N4=Zat⋅(Zdut⋅R⋅A+R⋅B).Now, we can define the short-circuit noise current spectral density, which is expressed by the following equation:(7)SIdut=|N1N2|2SVs-|N4N2|2SIR-|N3N2|2Seat.From the previous equation applied to each access of the transistor, we thereby obtain the expressions of the noise current spectral densitiesSIecc or SIscc. The last step of this noise modeling consists in extracting the two internal noise sources spectral densities SIc and SInc in Figure 6 with (8):
(8)SInc=-R12RD2⋅β2⋅SIscc+R12RD2⋅SIecc-SIbshot-Re2⋅β2-R32RD2⋅β2⋅SIcshot,SIc=-R22⋅R12RD2⋅Rbe2⋅β2⋅SIscc-R42⋅R12RD2⋅Rbe2⋅SIecc+R42⋅Re2⋅β2-R22⋅R32RD2⋅Rbe2⋅β2⋅SIcshot-Rb2Rbe2⋅SIrbb-Re2Rbe2⋅SIrbe,
with
(9)R1=(β+1)(Rbe+Re)+Rb,R2=(β+1)⋅Rbe+β⋅Re,R3=(β+1)⋅Rbe+Re+Rb,R4=Re+Rb,RD2=[(β+1)Rbe+βRe]2-(Re+Rb)2.In order to limit the LF noise measurement time while obtaining a relatively accurate model, we will only measure the noise sources in the normal, active region where the transistor operates in oscillation condition: the area of the transistor optimum load cycle for low-phase noise operation is defined in [14], by minimizing the LF noise sources conversion. Moreover, some noise measurements have shown that in the area where the base-collector junction is negatively biased, the noise level is independent of Vce. Thus, the measurements will only consist in noise source measurements versus collector current, for a given Vce close to the transistor bias point for low-phase noise oscillator operation. Following this procedure, Figure 9 shows the spectral densities of the short circuit noise current at transistor access for collector currents Ic0=6.7,13.5,54,108mA.Spectral densities plot of the short-circuit noise current at transistor access for collector currentsIc0=6.7,13.5,54,108mA.
(a)(b)Then, from (8), the spectral densities SIc and SInc are computed and plotted versus Ic0 in log-log plot (Figure 10) for four frequencies 100 Hz, 1 KHz, 10 KHz, and 100 KHz, which correspond to the frequencies at which oscillators are generally qualified. Note that, from the point of view of the phase noise simulation, these frequencies are independent.Plot ofSic and SInc noise current spectral densities.
(a)(b)The final step of noise modeling is to determine the spectral density equation ofSIc and SInc obtained by fitting the curves of (Figure 10) with an equation of the form:
(10)SIx=Kx⋅Icax,
where Ic is the collector current, x is taken for nc or c.The values ofKx and ax are given in Table 1.Table 1
Kx and  ax values of the internal LF noise sources for each frequency.
SIcSIncFrequencyaIcKIcKIncaInc100 Hz2.36.607e− 112.553.63e− 131 KHz2.337.9433e −122.391.585e −1410 KHz2.287.08e −132.133.981e− 15100 KHz2.365.012e− 141.971.585e −17In summary, we extracted the LF noise model of the HBT with internal noise sources. This model was implemented in ADS [15] and will enable us to simulate the phase noise of the oscillator while taking into account the cyclostationary property of noise sources.


## 4. Oscillator Design

The main objective of this design is to reduce the oscillator-phase noise while retaining a significant output power (few dbm). The desired oscillation frequency is 38 GHz and the MMIC uses the CPW technology. These two goals can be controlled by the intrinsic load cycle form of the transistor in oscillation [14]. This section describes the original design methodology used to reach these objectives by controlling the nonlinear behavior of the transistor when it operates as an oscillator circuit, by means of conventional amplifier nonlinear simulation. Its specific interest is the simulation rapidity and the easy control of the load cycle shaping. Then, by computing the load impedances at transistor input and output and the corresponding voltages, we are able in oscillation to obtain a nonlinear state as close as possible to that obtained in amplifier simulation. For this purpose, the three following steps are necessary.


### 4.1. Transistor Analysis in Large Signal Amplifier Mode

The schema of the transistor with its bias circuits (in CPW technology) and load impedance (synthesized with an inductor and a resistor in parallel) for nonlinear amplifier simulation is presented in Figure11.Figure 11
Circuit for nonlinear amplifier simulation.After some tuning of the bias point and load impedance, the gain, the output, and added powers versus the input power are plotted in Figure12 and the load cycle in Figure 13.Figure 12
Amplifier simulation results.Figure 13
Simulation of amplifier load line.Thus, we obtain the values given in Table2 for the following bias points Ib0=1.5mA,Vce0=2.2V,Ic0=35mA, where Pe and Ps represent the transistor input and output powers, C the gain compression. Note that to reach a sufficient output power and compression gain (to ensure oscillation after manufacturing despite technological spread), the load cycle must be close to the area where the base-collector junction conducts. From this simulation, we compute the input and output impedances Zin and Zout and voltage gain G=Vout/Vin. These data will serve to design the feedback circuit in order to keep this nonlinear state in oscillation as explained in the next section.Table 2
Load pull simulation results.
Pe(mW)Ps(mW)Pajmax(mw)Gain (dB)C(dB)11.5627.117.68.24.3


### 4.2. Determination of the Feedback Circuit

This step involves the determination of the feedback loop designed with transmission line and of the output circuit (matching circuit) whose schema is given in Figure14. In this schema, the transistor input impedance Zin is synthesized with an RC circuit.Figure 14
Feedback loop circuit.The goal of the linear optimization of the line widths and lengths is to obtain the same output impedanceZout and voltage gain G=Vout/Vin as in the previous section and by taking into account the technology and geometric constraints. Thus, the first draft of the oscillator circuit is given in Figure 15.Figure 15
First oscillator circuit.


### 4.3. Oscillator Simulation

This section describes all the necessary simulations to ensure good performances of the oscillator (stability, phase noise, etc.). The first simulation is an open-loop simulation [22]. This simulation allows us to determine the starting oscillation frequency and to verify that no other parasitic oscillations exist. In order to open the oscillation loop, a new circuit is drawn (Figure 16): the voltage-controlled current source of the transistor fundamental effect is isolated from the other elements of the transistor model and is now controlled by an external generator Vext. All the other circuit elements are included in the feedback two-port circuit.Figure 16
The open loop operation.Then the open loop gain is defined by(11)GOL(ω)=Vbe(ω)Vext(ω).The conditions to obtain oscillation starting are given by (12):(12)|G(jω0)⋅H(jω0)|>1,arg(G(jω0)⋅H(jω0))=0,∂arg(G(jω)⋅H(jω))∂ω|ω0<0,Figure17(a) shows this simulated open-loop gain for the oscillator circuit of Figure 15. A parasitic oscillation appears at frequency 5.67 GHz. An RC series circuit has been added in parallel with the bias circuit to eliminate these oscillations. The new open loop gain is shown in Figure 17(b).Magnitude and phase of open loop gain.
(a)(b)We can note only one starting oscillation frequency at 37.08 GHz. This open-loop analysis also allows to compute the oscillator loaded quality factor with expression (13) [23]:(13)QLoscill(ω0)=ω02|darg(G̃ol(jω))dω||ω0.The quality factor obtained by this simulation is equal to 33. Now, we perform a Harmonic Balance simulation of the oscillator. The time domain waveform ofIc current and Vce voltage are shown in Figure 18.Figure 18
Intrinsic time domain waveform ofIc current and Vce voltage.The intrinsic load line cycle is shown in Figure19.Figure 19
Oscillator intrinsic load line.Finally, the oscillator electrical performances are detailed in Table3.Table 3
Simulated oscillator preformances.
Pe(mW)Ps(mW)Pajmax(mw)Gain (dB)C(dB)13.950.37.35.64.3The simulation of the oscillator phase noise (PN) is shown Figure20.Figure 20
Phase noise simulation result.Table4 presents the contribution of elementary noise sources of the internal transistor at the 100 KHz frequency.Table 4
Contribution of elementary noise sources to total phase noise at 100 kHz from the carrier.
Total PN noise−76.94 dBc/HzPN due to sourceSIc−76.94 dBc/HzPN due to sourceSInc−106.3 dBc/HzPN due to shot noise source at collector−119.8 dBc/HzThe final layout, taking into account all the technological constraints, including stabilizing circuits and a 3 dB attenuator at the output to reduce pulling effect, is shown Figure21. The circuit was fabricated by III–V Lab.Figure 21
Oscillator layout (2070×1635μm2).


## 4.1. Transistor Analysis in Large Signal Amplifier Mode

The schema of the transistor with its bias circuits (in CPW technology) and load impedance (synthesized with an inductor and a resistor in parallel) for nonlinear amplifier simulation is presented in Figure11.Figure 11
Circuit for nonlinear amplifier simulation.After some tuning of the bias point and load impedance, the gain, the output, and added powers versus the input power are plotted in Figure12 and the load cycle in Figure 13.Figure 12
Amplifier simulation results.Figure 13
Simulation of amplifier load line.Thus, we obtain the values given in Table2 for the following bias points Ib0=1.5mA,Vce0=2.2V,Ic0=35mA, where Pe and Ps represent the transistor input and output powers, C the gain compression. Note that to reach a sufficient output power and compression gain (to ensure oscillation after manufacturing despite technological spread), the load cycle must be close to the area where the base-collector junction conducts. From this simulation, we compute the input and output impedances Zin and Zout and voltage gain G=Vout/Vin. These data will serve to design the feedback circuit in order to keep this nonlinear state in oscillation as explained in the next section.Table 2
Load pull simulation results.
Pe(mW)Ps(mW)Pajmax(mw)Gain (dB)C(dB)11.5627.117.68.24.3


## 4.2. Determination of the Feedback Circuit

This step involves the determination of the feedback loop designed with transmission line and of the output circuit (matching circuit) whose schema is given in Figure14. In this schema, the transistor input impedance Zin is synthesized with an RC circuit.Figure 14
Feedback loop circuit.The goal of the linear optimization of the line widths and lengths is to obtain the same output impedanceZout and voltage gain G=Vout/Vin as in the previous section and by taking into account the technology and geometric constraints. Thus, the first draft of the oscillator circuit is given in Figure 15.Figure 15
First oscillator circuit.


## 4.3. Oscillator Simulation

This section describes all the necessary simulations to ensure good performances of the oscillator (stability, phase noise, etc.). The first simulation is an open-loop simulation [22]. This simulation allows us to determine the starting oscillation frequency and to verify that no other parasitic oscillations exist. In order to open the oscillation loop, a new circuit is drawn (Figure 16): the voltage-controlled current source of the transistor fundamental effect is isolated from the other elements of the transistor model and is now controlled by an external generator Vext. All the other circuit elements are included in the feedback two-port circuit.Figure 16
The open loop operation.Then the open loop gain is defined by(11)GOL(ω)=Vbe(ω)Vext(ω).The conditions to obtain oscillation starting are given by (12):(12)|G(jω0)⋅H(jω0)|>1,arg(G(jω0)⋅H(jω0))=0,∂arg(G(jω)⋅H(jω))∂ω|ω0<0,Figure17(a) shows this simulated open-loop gain for the oscillator circuit of Figure 15. A parasitic oscillation appears at frequency 5.67 GHz. An RC series circuit has been added in parallel with the bias circuit to eliminate these oscillations. The new open loop gain is shown in Figure 17(b).Magnitude and phase of open loop gain.
(a)(b)We can note only one starting oscillation frequency at 37.08 GHz. This open-loop analysis also allows to compute the oscillator loaded quality factor with expression (13) [23]:(13)QLoscill(ω0)=ω02|darg(G̃ol(jω))dω||ω0.The quality factor obtained by this simulation is equal to 33. Now, we perform a Harmonic Balance simulation of the oscillator. The time domain waveform ofIc current and Vce voltage are shown in Figure 18.Figure 18
Intrinsic time domain waveform ofIc current and Vce voltage.The intrinsic load line cycle is shown in Figure19.Figure 19
Oscillator intrinsic load line.Finally, the oscillator electrical performances are detailed in Table3.Table 3
Simulated oscillator preformances.
Pe(mW)Ps(mW)Pajmax(mw)Gain (dB)C(dB)13.950.37.35.64.3The simulation of the oscillator phase noise (PN) is shown Figure20.Figure 20
Phase noise simulation result.Table4 presents the contribution of elementary noise sources of the internal transistor at the 100 KHz frequency.Table 4
Contribution of elementary noise sources to total phase noise at 100 kHz from the carrier.
Total PN noise−76.94 dBc/HzPN due to sourceSIc−76.94 dBc/HzPN due to sourceSInc−106.3 dBc/HzPN due to shot noise source at collector−119.8 dBc/HzThe final layout, taking into account all the technological constraints, including stabilizing circuits and a 3 dB attenuator at the output to reduce pulling effect, is shown Figure21. The circuit was fabricated by III–V Lab.Figure 21
Oscillator layout (2070×1635μm2).


## 5. Oscillator Measurement

### 5.1. Power and Oscillation Frequency Measurement

To test the robustness of the oscillation, measurements have been performed on 15 oscillators of the wafer for various DCIc currents. Figure 22 presents the oscillation frequency and the output power versus Ic0 current for different Vce0 voltages and a Vbe0 voltage equal to 0.8 V.Measurement result of oscillation frequency and output power versusIc current for Vbe0 voltage equal to 0.8 V.
(a)(b)The output power increases by 5 dB over the variation range ofIc0, while the oscillation frequency shows a variation of 1 GHz over the same range. This shows that the oscillation frequency remains stable for an Ic0 current variation between 35 and 60 mA.


### 5.2. Phase Noise Measurement

The XLIM Laboratory has an AEROFLEX PN9500 bench (Figure23) to perform the phase noise measurement of the oscillator. This bench enables us measure the phase noise of the oscillator in the 250 MHz–26.5 GHz bandwidth with down-converter. The oscillation frequency of the circuit being out of range, we developed our own down-converter to reduce the oscillation frequency in the PN9500 bandwidth. Thus, the system using the delay line method is presented Figure 24.Figure 23
Synoptic of PN9500 phase noise bench.Figure 24
Principle of realized phase noise bench.It is composed of a mixer in which an LO signal at frequency 10.75 GHz is generated by a high spectral purity synthesizer PN9276-20 from the PN9500 setup bench. The RF signal comes from the measured oscillator. The IF output is connected to the PN9718 input. Before measuring the oscillator phase noise, we must ensure that the measurement chain will not add noise. To do that, we replace the oscillator by a high spectral purity synthesized source (ANRITSU 68367C). The phase noise of the system measured with this source is represented in Figure25 and gives the noise floor of the system.Figure 25
PN floor of the system.Figure26 shows the plot of the phase noise measurement of the 15 oscillators on the wafer and the simulated phase noise for the bias point Vbe0 = 0.8 V and Vce0 = 2.2 V.Figure 26
Comparison between measurement and simulation of oscillator phase noise.We can note that the phase noise simulation coincides with the average results of measurement. The oscillation frequency measurement for the nominal bias point is close to 37.1 GHz with an output power equal to 5.2 Bm. The phase noise measure is much higher than the system PN floor (>15dB), so we can confirm that the phase noise is that of the oscillator. To compare our oscillator with published oscillators with an oscillation frequency close to 38 GHz, we will use a figure of merit described by (14):(14)FOM=L{foffset}-20log⁡(f0fref)-25log⁡(foffsetreffoffset)-20log⁡(TB%100)-10log⁡(Pout1mW)
where ℒ{foffset} is the oscillator phase noise at a frequency offset from carrier foffset. The oscillation frequency of the various oscillators (f0) is reduced to a reference frequency (fref=37.1GHz) which is the oscillation frequency of the oscillator of this work. The carrier frequency offset is reduced to a distance foffsetref=100KHz. The multiplier factor is equal to 25 because the oscillator phase noise of this work has a slope of −25 dB/decade. The tuning bandwidth TB (in percentage) is also taken into account. The oscillator output power Pout is reduced to a reference of 1 mW. This figure of merit is valid for oscillator with a tuning bandwidth of more than 1%: otherwise, these oscillators are considered as fixed-frequency oscillators. The results are summarized in Table 5. We have chosen a value of phase noise of −80 dBc/Hz which corresponds to an average value of all those measured. We can verify that we obtain an average performance as regarding the figure of merit compared to those published.Table 5
Comparison of oscillator performances.
(a)Technologyf0(GHz)TB (%)ℒ{foffset}(dBc/Hz)Si/SiGe [16]38not−55InGaP/GaAs [17]40.8<1%−95GaAs [18]38not−104AlInAs/InGaAs [19]399−75AlInAs/InGaAs [20]38.1<1%−85InP [21]39.9<1%−84InP (this work)37.1not−80(b)Δfoffset (KHz)Pout (dBm)FOM (dBc/Hz)1002−53.21005.3−90.5100011.9−67.31005−89.51008.4−76.81005−79.61005.18−74.8


## 5.1. Power and Oscillation Frequency Measurement

To test the robustness of the oscillation, measurements have been performed on 15 oscillators of the wafer for various DCIc currents. Figure 22 presents the oscillation frequency and the output power versus Ic0 current for different Vce0 voltages and a Vbe0 voltage equal to 0.8 V.Measurement result of oscillation frequency and output power versusIc current for Vbe0 voltage equal to 0.8 V.
(a)(b)The output power increases by 5 dB over the variation range ofIc0, while the oscillation frequency shows a variation of 1 GHz over the same range. This shows that the oscillation frequency remains stable for an Ic0 current variation between 35 and 60 mA.


## 5.2. Phase Noise Measurement

The XLIM Laboratory has an AEROFLEX PN9500 bench (Figure23) to perform the phase noise measurement of the oscillator. This bench enables us measure the phase noise of the oscillator in the 250 MHz–26.5 GHz bandwidth with down-converter. The oscillation frequency of the circuit being out of range, we developed our own down-converter to reduce the oscillation frequency in the PN9500 bandwidth. Thus, the system using the delay line method is presented Figure 24.Figure 23
Synoptic of PN9500 phase noise bench.Figure 24
Principle of realized phase noise bench.It is composed of a mixer in which an LO signal at frequency 10.75 GHz is generated by a high spectral purity synthesizer PN9276-20 from the PN9500 setup bench. The RF signal comes from the measured oscillator. The IF output is connected to the PN9718 input. Before measuring the oscillator phase noise, we must ensure that the measurement chain will not add noise. To do that, we replace the oscillator by a high spectral purity synthesized source (ANRITSU 68367C). The phase noise of the system measured with this source is represented in Figure25 and gives the noise floor of the system.Figure 25
PN floor of the system.Figure26 shows the plot of the phase noise measurement of the 15 oscillators on the wafer and the simulated phase noise for the bias point Vbe0 = 0.8 V and Vce0 = 2.2 V.Figure 26
Comparison between measurement and simulation of oscillator phase noise.We can note that the phase noise simulation coincides with the average results of measurement. The oscillation frequency measurement for the nominal bias point is close to 37.1 GHz with an output power equal to 5.2 Bm. The phase noise measure is much higher than the system PN floor (>15dB), so we can confirm that the phase noise is that of the oscillator. To compare our oscillator with published oscillators with an oscillation frequency close to 38 GHz, we will use a figure of merit described by (14):(14)FOM=L{foffset}-20log⁡(f0fref)-25log⁡(foffsetreffoffset)-20log⁡(TB%100)-10log⁡(Pout1mW)
where ℒ{foffset} is the oscillator phase noise at a frequency offset from carrier foffset. The oscillation frequency of the various oscillators (f0) is reduced to a reference frequency (fref=37.1GHz) which is the oscillation frequency of the oscillator of this work. The carrier frequency offset is reduced to a distance foffsetref=100KHz. The multiplier factor is equal to 25 because the oscillator phase noise of this work has a slope of −25 dB/decade. The tuning bandwidth TB (in percentage) is also taken into account. The oscillator output power Pout is reduced to a reference of 1 mW. This figure of merit is valid for oscillator with a tuning bandwidth of more than 1%: otherwise, these oscillators are considered as fixed-frequency oscillators. The results are summarized in Table 5. We have chosen a value of phase noise of −80 dBc/Hz which corresponds to an average value of all those measured. We can verify that we obtain an average performance as regarding the figure of merit compared to those published.Table 5
Comparison of oscillator performances.
(a)Technologyf0(GHz)TB (%)ℒ{foffset}(dBc/Hz)Si/SiGe [16]38not−55InGaP/GaAs [17]40.8<1%−95GaAs [18]38not−104AlInAs/InGaAs [19]399−75AlInAs/InGaAs [20]38.1<1%−85InP [21]39.9<1%−84InP (this work)37.1not−80(b)Δfoffset (KHz)Pout (dBm)FOM (dBc/Hz)1002−53.21005.3−90.5100011.9−67.31005−89.51008.4−76.81005−79.61005.18−74.8


## 6. Conclusion

In conclusion, this work enables us to build an MMIC Ka-band oscillator with good performances. An electrothermal nonlinear model of the transistor was proposed, including cyclostationary noise sources to accurately predict its phase noise. The design method used allowed us to define a nonlinear operation of the transistor to limit the contributions of low-frequency noise sources responsible for the phase noise. This amplifier nonlinear state has been kept in oscillation operation. This evaluation of the III–V Lab process has given encouraging results concerning the feasibility of efficient Ka-band circuits.



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*Source: 796973-2012-04-02.xml*