## 4 Test Setup for Experimental Measurements
### Radar System Specifications  
A portable briefcase-sized 10 GHz continuous wave (CW) microwave radar system was used in the measurements reported in this paper. The use of 10 GHz is a good compromise between sensitivity on the one hand and reliability and cost on the other hand. The wavelength is 3 cm. This frequency is high enough to capture fluctuations of about one-eighth of the wavelength (0.375 cm) compared to lower frequencies. The frequency is also low enough to avoid scattering or propagation losses in an operational environment and has less expensive components compared to higher frequencies. Figure 2: Fishbone diagram for radar micro-Doppler signals from stationary engine-running vehicles. Separate antennas were used for transmit and receive functions. Radar system control and data acquisition were achieved using a laptop computer. Details about the system block diagram, relevant component specifications, and photographs of the system are provided below [27]. The experimental micro-Doppler radar system is a continuous wave (CW) device operating at a frequency of 10 GHz. Figure 3 shows the system's block diagram, while Figures 4 and 5 show photographs of the overall system and a zoomed in view indicating the microwave components, respectively. At the core of the system is a voltage-controlled oscillator (VCO) that delivers a nominal output power of +10 dBm at a center frequency near 10 GHz, depending on the applied tuning voltage. The VCO operates on a 12 V supply and draws 150 mA of current. Its tuning voltage is set at 5 V with negligible current draw. The VCO output is directed to the input of a directional coupler (DC), which has an insertion loss of 0.5 dB and a coupling factor of 10 dB. The signal at the through port, reduced to 9.5 dBm (10 dBm \(-\) 0.5 dB), is routed to the local oscillator (LO) port of an 1/Q mixer--sufficient to meet the mixer's minimum LO drive requirement of +10 dBm. Meanwhile, the coupled port output, at 0 dBm (10 dBm \(-\) 10 dB), is sent to a power amplifier. The power amplifier offers a gain of 26 dB and has a 1 dB output compression point at +24 dBm. Given a 0 dBm power input, the amplifier is expected to produce an output near +25 dBm, likely operating in the compression region. It is equipped with a heat sink and powered by a 12 V supply drawing 400 mA. The output from the power amplifier is directed to the transmit antenna, which has a minimum gain of 15 dB and comes equipped with a mounting bracket. The receive antenna is identical to the transmit antenna, offering the same 15 dB gain and mounting hardware. The signal received by the antenna is routed to the first low-noise amplifier (LNA 1). This amplifier provides a gain of 20 dB and has a noise figure of 1.7 dB. Its 1 dB output compression point is +13 dBm, which is sufficiently high to prevent saturation, as the incoming signal power is relatively low. The LNA operates on a 5 V supply and draws 50 mA of current. The output from the first low-noise amplifier (LNA 1) is routed to a second low-noise amplifier (LNA 2), which provides an additional gain of 26 dB with a noise figure of 2 dB. LNA 2 has a 1 dB output compression point of +12 dBm, which is sufficiently high to avoid saturation, even after the signal has been amplified by LNA 1, due to the inherently low received power. LNA 2 operates on a 5 V supply and draws 75 mA of current. The amplified signal from LNA 2 is fed into one of the RF input ports (either RF1 or RF2) of the 1/Q mixer, while the unused RF port is terminated with a 50 ohm load. The mixer introduces a conversion loss of 7 dB. The resulting in-phase (I) and quadrature (Q) outputs are passed through the first stage of low-pass filtering (LPF 1), followed by a second stage (LPF 2). LPF 1 has a cutoff frequency of 3.4 GHz and serves primarily to suppress any residual high-frequency signals near 10 GHz that may leak through. At baseband frequencies associated with micro-Doppler signals, its insertion loss is minimal--less than 0.01 dB.