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基于自混频效应的检波式太赫兹无线收发系统链路如图2所示,在自由空间中的传输距离为0.95 m。该系统主要由基带信号产生和接收模块、微波信号源、双边带混频器、340 GHz倍频链路、340 GHz AlGaN/GaN HEMT太赫兹探测器组成。该收发系统采用4ASK幅度调制及解调体制。基带信号产生0~1 V的4电平脉冲幅度调制 (Pulse Amplitude Modulation, PAM)信号,微波信号源产生频率为10.625 GHz、功率为10 dBm的正弦信号,基带信号和微波信号分别输入至双边带混频器的IF端和LO端。调制后的信号经RF端输出后,由级联倍频链路进行32倍频,最后通过喇叭天线发射至自由空间。利用一对离轴抛物面镜(OAP)实现发射-接收准直光路,到达接收机前端硅透镜的太赫兹波功率大于−20 dBm。采用AlGaN/GaN HEMT太赫兹探测器进行检波,电流前置放大器负责放大探测器的响应电流,低通滤波器抑制带外噪声。信号经放大、滤波后由模数转换模块采集,最终还原出原始信元。采用检波式接收前端,系统功耗较低,收发链路简单。检波式接收机的输出信号信噪比SNR与接收信号功率Pin的关系为:
Figure 2. Block diagram of homodyne transmitter-transceiver chain based on AlGaN/GaN HEMT terahertz detector
式中:NEP+10lgB1/2为接收机在带宽为B时的总噪声;SNR为解调后信号的信噪比;NEP为接收机的等效噪声功率(信噪比为1时单位带宽内的接收功率)。采用不同带宽的低通滤波器(LPF),调节接收机带宽分别为20 MHz、50 MHz和100 MHz。比较不同接收机带宽下接收信号信噪比与接收功率的关系,如图3所示。分别对三种接收带宽下的数据进行线性拟合,得到信噪比为0 dB时的接收功率,并通过公式(1)换算出对应的NEP,结果如表1所示。直接检波式接收机的NEP几何平均值−64.6 dBm/Hz1/2。
Figure 3. SNR of the homodyne receiver as a function of the received power with different receiver bandwidths
Different receiver bandwidths/MHz Receiver power when SNR=0 dB/dBm NEP (homodyne receiver)/dBm·Hz−1/2 20 −27.9 −64.4 50 −26.0 −64.5 100 −25.0 −65.0 Table 1. Extrapolated received power with SNR=0 dB and the NEP for homodyne receiver with different receiver bandwidths
发射端倍频链路由82.5~87.5 GHz六倍频器、W波段功率放大器、165~175 GHz二倍频器和330~350 GHz二倍频器组成,经过多级倍频将微波信号倍频至太赫兹频段。接收端信号经探测器检波、电流放大器放大后(反相),与发射端基带信号保持同步,便于后续的信号处理。基带信号采用带宽为10 MHz的4PAM随机信号。当进行高阶调制时,倍频链路的强非线性导致接收端信号失真。如图4(a)所示,倍频链路强非线性导致接收信号幅度分布不均,四种均匀电平经调制、倍频、探测后,输出信号只有三种电平。通过对基带信号进行预失真补偿处理,实现了发射端倍频链路的线性化。图4(b)为补偿后发射端基带和接收端基带波形图的对比,发射-接收链路保留了良好的线性度。
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图5为基于AlGaN/GaN HEMT太赫兹外差混频效应的无线通信收发系统,该收发系统同样采用4ASK幅度调制及解调体制,传输距离为0.95 m。其中本振太赫兹波频率为339.94 GHz,经OAP3、分束器反射、OAP2后,通过硅透镜耦合至探测器。对于链路的发射端,频率为12.594 GHz、功率为10 dBm的微波信号源被基带信号调制后,经27倍频到340 GHz。经OAP1准直、分束器透射、OAP2汇聚和硅透镜耦合至探测器。探测器接收到的功率值在−25~−10 dBm范围内可调。探测器输出的中频信号频率为两路信号的差频60 MHz,中频功率随发射端太赫兹波功率线性变化。中频信号经过Bias-Tee、电流前置放大器和带通滤波器输出,最后由对数检波器输出至基带信号采集模块。Bias-Tee的作用是隔离了由本振太赫兹波在探测器中产生的直流分量。带通滤波器有效地消除了基带信号带宽外的噪声,提高了信号质量。
本振太赫兹波链路采用了与第2节相同的32倍频链路,输出功率可达毫瓦量级,可显著提高接收信号的信噪比。太赫兹波调制发射链路采用VDI公司的27倍频链路,包括9倍频有源倍频器、340 GHz 频段3倍频器和340 GHz衰减器组成。分束器采用高阻硅片制成,透射信号与反射信号功率比约为1∶1。
外差混频式接收机的输出信号信噪比SNR与接收信号功率Pin关系为:
采用不同带宽的带通滤波器(BPF),使接收机带宽分别为20 MHz和40 MHz。图6为本振太赫兹信号功率为0 dBm时接收机信噪比随接收功率变化的曲线。对测试结果进行线性拟合外推得到信噪比为0 dB时的接收功率,通过公式(2)计算得到外差接收机的NEP,结果如表2所示。当带宽为20 MHz时NEP约为−114.79 dBm/Hz。带宽增加到40 MHz时,NEP增加了约3.37 dB,主要原因是总噪声的增加和滤波器衰减系数存在误差。
Figure 6. SNR of the heterodyne receiver as a function of the received power with different signal bandwidths
Different receiver bandwidths/MHz Receiver power when SNR=0 dB/dBm NEP (heterodyne receiver)/dBm·Hz−1 20 −41.68 −114.79 40 −35.22 −111.42 Table 2. Extrapolated received power with SNR=0 dB and the NEP for heterodyne receiver with different receiver bandwidths
图7为本振太赫兹信号功率为0 dBm、调制太赫兹信号功率为−11 dBm时,发射信号和探测器输出差频信号波形图。即使调制太赫兹信号功率很低,由于本振太赫兹波功率较强,输出中频信号的幅度能够很好地保留基带信号的幅度信息。
340 GHz wireless communication receiving front-ends based on AlGaN/GaN HEMT terahertz detectors
doi: 10.3788/IRLA20210202
- Received Date: 2021-04-12
- Rev Recd Date: 2021-05-06
- Publish Date: 2021-05-21
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Key words:
- gallium nitride /
- terahertz wireless communication /
- terahertz detector /
- homodyne /
- heterodyne
Abstract: Based on the self-mixing and heterodyne mixing effects of antenna-coupled AlGaN/GaN HEMT terahertz detectors, two receiving front-ends, namely homodyne receiver and heterodyne receiver, were designed and tested in 340 GHz frequency band, respectively. The equivalent noise powers of two receivers were calculated through measuring the signal-noise ratio (SNR) of the signal and the received power. The experimental result indicates that the responsivity of detectors is about 20 mA/W for homodyne receiver, the noise equivalent power is about −64.6 dBm/Hz1/2 for homodyne receiver and −114.79 dBm/Hz for heterodyne receiver, respectively. With the same carrier power and the signal bandwidth, the SNR of the heterodyne receiver is greater than that of the homodyne receiver when the local terahertz source power is greater than −7 dBm. The heterodyne receiver exhibits excellent demodulation characteristics: a local terahertz source power above 0 dBm boosts the SNR by more than 10 dB compared with the homodyne receiver.