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合成孔径雷达(synthetic aperture radar, SAR)已在战场态势感知、国土普查、环境监测以及灾害评估等领域得到了广泛应用,其中高分辨率SAR是当前研究的热点[1-3]。因平面有源相控阵具备天线增益高、波束灵活可控及宽角扫描等技术特点,是实现高分辨率SAR的主流技术方案[4]。由于大规模平面有源相控阵存在成本高、结构复杂及散热难的缺点,可通过对天线阵面有源馈电网络中的T/R组件进行稀疏布局来改善。然而,简单的稀疏布阵会导致天线波束栅瓣的产生和增益的下降,这时可在稀疏子阵列内采用相位扰动技术以降低天线波束的栅瓣电平并提高增益[5]。为对稀疏阵列信号进行相位扰动,可在天线与TR组件之间的馈电网络中安装扰动移相器。根据相控阵系统的需求以及安装位置,扰动移相器必须具备高功率承受能力、小型化、低差损、低功耗的特性[6]。
射频信号的移相方式包括改变信号传输的相位常数、信号传输线的长度以及信号传输网络的传输特性[7-12]。文献[7]中铁氧体移相器通过改变外加电场来改变信号的传播相位常数的方式以实现射频信号的相移。但该类移相器的体积较大,不利于系统集成。对于文献[8]中提出的基于PIN开关线型移相器则是利用开关切换改变信号的传输线长度以实现移相的目的。由于PIN管串联在传输路径中,该类移相器的功率承受能力较低,插损大,直流功耗高。为实现大功率、低插损移相器,文献[9-10]提出了基于PIN管的支节加载移相器。通过控制PIN管的通断可以改变射频信号的传输网络特性,但在切换移相状态时响应速度较慢,PIN二级管的驱动电流致使直流功耗较大。相较于传统的PIN管,GaN FET作为功率器件,具有高击穿电压、低阈值电压、低的栅极电荷Qg,其开关频率高,导通电阻小,能够承受大功率信号[12-14]。小导通电阻能使GaN-FET型移相器实现低功耗特性。由于GaN-FET属于电压驱动型晶体管,采用GaN FET设计移相器能够有效降低大规模相控阵系统的功耗、体积,并具有承受大功率的特性。因此,GaN-FET型移相器受到学者的关注并得以发展[11]。
本文提出了一款基于GaN-FET支节加载大功率移相器,由2 bit控制位对移相状态进行切换。通过改变GaN FET的栅极电压实现GaN-FET开关的导通和关断,从而改变信号的传输网络相移特性。结合系统需求,本文设计的移相器工作在9.2~9.8 GHz,能够实现30°/60°两种移相状态。与传统的大功率移相器相比,本文的GaN-FET型移相器还具备超低功耗、快速切换、低插损的优势,并且该移相器可通过利用MMIC技术进一步小型化,以期满足小型化电路系统的应用需求。
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为了实现基于GaN-FET开关的低功耗、大功率移相器,图1给出了两支节加载移相器的电路结构,并以此作为基本单元进行扩展为多支节加载移相器。两支节加载移相器是由特性阻抗为Z0的微带传输线、特性阻抗为Z1并联高阻抗微带支节以及末端加载的GaN-FET组成。通过控制GaN-FET开关的导通或者关断来改变整个电路结构的传输网络特性,从而实现对射频信号相位的改变。
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由于GaN-FET在导通状态时呈低阻状态(等效为小于Ron = 2 Ω的电阻),而在截止状态时呈高阻状态(可等效为小于COFF = 0.1 pF的电容)的特性[13],因此可用线性等效模型简化分析支节加载移相器的传输网络特性,获得满足工作带宽的相移量,GaN-FET单支节加载如图2所示。
文献[15]已对两支节加载线移相器进行了详细分析,但结果过于复杂,实际设计过程中只能通过查找图形曲线的方式得到粗略的结果。为简化两支节加载移相器的设计,本文对末端加载GaN-FET单支节加载基本电路单元进行了分析并给出了相应的设计公式。利用公式便可得到多支节加载移相器的初始尺寸,后续便可结合高频仿真软件HFSS进行建模优化获得满足指标要求的移相器。
根据图2所示的单支节加载GaN-FET移相器电路结构并结合ABCD矩阵与S参数的转换关系,其传输网络的S参数矩阵中的S21可表示为[16]:
$$ {S_{21}} = \frac{2}{{\left( {2 + {Z_0}Y} \right)\left( {{\text{cos}}\left( {2\beta L} \right) + {\rm{j}}{\text{sin}}\left( {2\beta L} \right)} \right)}} $$ (1) 式中,Z0为微带传输线的特性阻抗;L为微带传输线的长度;Y为从并联高阻抗微带传输线向GaN-FET看过去的输入导纳;β为微波信号的传播常数。当GaN-FET处于截止状态时,从并联高阻抗微带传输线看过去的输入导纳近似为:
$$ Y = {\rm{j}}{Y_1}\tan \left( {\beta {L_{\text{1}}}} \right) $$ (2) 式中,Y1=1/Z1为并联高阻抗微带传输线的特性导纳。此时传输网络的传输参数为:
$$ S_{21}' = \frac{{{\text{2cos}}\left( {2\beta L} \right) - j2{\text{sin}}\left( {2\beta L} \right)}}{{2 + {\rm{j}}{Z_0}{{{Y}}_1}{\text{cot}}\left( {\beta {L_1}} \right)}} $$ (3) 当GaN-FET处于导通状态时,从并联高阻抗微波传输线看过去的输入导纳Y为:
$$ Y = - {\rm{j}}{Y_1}{\text{cot}}\left( {\beta {L_1}} \right) $$ (4) 此时传输网络的传输参数为:
$$ S_{21}^{''} = \frac{{{\text{2cos}}\left( {2\beta L} \right) - {\text{j}}2{\text{sin}}\left( {2\beta L} \right)}}{{2 - {\text{j}}{Z_0}{Y_1}{\text{cot}}\left( {\beta {L_1}} \right)}} $$ (5) 此处定义IL为单支节加载移相器的插损,Δθ为相移量,ΔM为寄生调幅。则由式(1)~式(5)可得单支节加载移相器的插损、相移量以及寄生调幅为:
$$ {\text{IL}} = 20{\text{log}}\frac{1}{{\sqrt {1 + {{\left( {\dfrac{{{Z_0}}}{{2{Z_1}}}{\text{tan}}\left( {\beta {L_1}} \right)} \right)}^2}} }} $$ (6) $$ {{\Delta }}\theta = {\text{arcot}}\left\{ {\frac{{{Z_{\text{0}}}}}{{2{Z_{\text{1}}}}}{\text{cot}}\left( {\beta {L_{\text{1}}}} \right)} \right\} + {\text{arctan}}\left\{ {\frac{{{Z_{\text{0}}}}}{{2{Z_{\text{1}}}}}\tan \left( {\beta {L_{\text{1}}}} \right)} \right\} $$ (7) $$ {{\Delta }}M = 20\log \sqrt {\frac{{{\text{1}} + {{\left\{ {\dfrac{{{Z_{\text{0}}}}}{{2{Z_{\text{1}}}}}{\text{cot(}}\beta {L_{\text{1}}}{\text{)}}} \right\}}^{\text{2}}}}}{{{{\left( {\dfrac{{{Z_{\text{0}}}}}{{{\text{2}}{Z_{\text{1}}}}}} \right)}^{\text{2}}} + {{\left\{ {{\text{cot}}\left( {\beta {L_{\text{1}}}} \right)} \right\}}^{\text{2}}}}}} $$ (8) 由式(8)可知,取βL1=π/4时,ΔM=0,此时移相器的插损IL、相移量Δθ分别为:
$$ {\text{I}}{{\text{L}}_{(\beta {L_1} = \tfrac{{\text{π}}}{4})}} = 20\log \frac{1}{{\sqrt {1 + {{\left( {\dfrac{{{Z_0}}}{{2Z{}_1}}} \right)}^2}} }} $$ (9) $$ \Delta {\theta _{(\beta {L_1} = \tfrac{{\text{π}}}{4})}} = 2{\text{arcot}}\left( {\frac{{{Z_0}}}{{2{Z_1}}}} \right) $$ (10) 由式(9)和式(10)可知,当βL1=π/4时,移相器插损和相移量由微带主传输线特性阻抗Z0和并联高阻抗支节加载线的特性阻抗Z1决定。
综上所述,根据移相器的工作频段可以计算出并联高阻抗微带传输线的长度L1,由单支节加载移相器的插损和相移量表达式,可综合出给定插损和相移量的支节加载传输线的数量和并联高阻抗支节加载线的特性阻抗Z1。
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图6为采用薄膜技术制造的大功率移相器实物,所用的GaN-FET开关为裸芯,利用导电银浆粘接在陶瓷基片上。所有的陶瓷基片包括移相器电路、GaN-FET控制电路也利用导电银浆粘接于腔体底部。
大功率移相器的GaN-FET开关控制信号V1和V2采用0 V/−28 V,其中−28 V偏置电流小于6 μA。GaN-FET开关在−28 V时处于截止状态,在0 V时处于导通状态。因此,当V1 = 0 V、V2 = −28 V时,移相器可实现30°的移相状态;当V1 = V2 = 0 V时,移相器可实现60°的移相状态。图7、图8给出了大功率移相器在9.2~9.8 GHz范围内经调试后的测试结果。由图7可知在30°移相状态时,移相器的相移精度变化范围为−32±1.5°,未进行移相时的初始插损小于−0.75 dB,移相寄生调幅小于0.25 dB。在60°移相状态时,移相器的相移精度变化范围为73±3°,未进行移相时的初始插损小于−0.9 dB,移相寄生调幅小于0.2 dB。虽然所设计的移相器与给定的技术指标存在一定的偏差,但不影响在SAR系统中的使用。
相位均方根方差θΔ,RMS和幅度均方根误差AΔ,RMS通常被用来衡量移相器的性能优劣,其表达式分别为[10]:
$$ {\theta _{\Delta ,{\text{RMS}}}} = \sqrt {\frac{1}{{N - 1}}\sum\limits_{i = 2}^N {{{\left| {{\theta _{\Delta i}}} \right|}^2}} } $$ (11) $$ {A_{\Delta ,{\text{RMS}}}} = \sqrt {\frac{1}{N}\sum\limits_{i = 1}^N {{{\left| {{A_{\Delta i}}} \right|}^2}} } $$ (12) 式中,N为移相状态数量;θΔi为第i种移相状态的实际相移量与理想相移量的差值;AΔi为第i种移相状态的插损与该种移相状态的插损均值的差值。由图7、8和式(11)、式(12)可计算出θΔ,RMS和AΔ,RMS,如图9所示。该移相器在整个频率范围内的相位均方根方差为8°~10.8°,幅度均方根误差小于0.45 dB。表1给出了所提出移相器与已有移相器的指标对比。由表中可以看出,所提出的GaN-FET移相器具有最优的功耗、插损、功率容量性能。
Design of GaN-FET Phase Shifter with High Power Handling Capability and Ultra-Low DC Power Consumption
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摘要: 提出了一种基于支节加载GaN-FET的大功率、低插损、超低功耗移相器。该移相器的核心电路由多节高阻抗微带支节线以及支节末端加载的GaN-FET管芯构成。通过对单支节加载GaN-FET移相器电路模型的详细分析,得到了移相器的插损和相移表达式,并将其作为基本单元进行拓展可得到满足所需工作带宽的多支节加载GaN-FET移相器。由于采用了高阻抗微带线支节,该移相器具有大功率处理能力。结合理论分析,对多支节加载GaN-FET移相器进行了设计、加工和测试。测试结果表明,所加工的移相器在9.2~9.8 GHz范围内,通过控制GaN FET的关断实现了30°和60°两种相移状态。同时,移相器功率承受能力大于10 W,插损优于1 dB,控制电流小于6 μA。Abstract: This paper proposes a stub-loaded gallium nitride field-effect transistor (GaN-FET) phase shifter with ultra-low DC power consumption and high power handling capability. The phase shifter consists of multiple micro-strip stubs with high impedance and GaN-FET loaded. By analyzing the circuit model of the single stub-loaded GaN-FET phase shifter in detail, the formulas for insertion loss and phase shift are obtained. Then, as the basic element, the GaN-FET phase shifter using multiple stubs is developed, which can operate at the predefined frequency band. Due to the employment of the high-Z micro-strip stub, the proposed phase shifter can handle high input power. Following the design theory, the GaN-FET phase shifter using multiple stubs is designed, fabricated and measured. The tested results show that the phase shifter achieves the phase shift of 30° and 60° by controlling the GaN FETs within the frequency range of 9.2 GHz to 9.8 GHz. In addition, the power handling capability is over 10 W, insertion loss less than 1 dB and DC current for control less than 6 μA.
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Key words:
- FET /
- GaN /
- SAR imaging /
- stub-loaded phase shifter
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