## 2 Method of Detection  
Because of the fact that a PCL system emits no signal, it is assumed that the C-PCL method bases on using of special sounding signal in order of search for a PCL. The PCL system may be contained in a shielded case; therefore, the only way a signal may penetrate the system is through the antenna aperture (receiver's input). The PCL antenna (set of antennas) is matched to receive a specialized frequency band. Most frequently it is the DVB-T band, Ultra High Frequency (UHF)/Very High Frequency (VHF) (for FM radio) or GSM. The basis of the proposed method is the detection of the nonlinear circuit, which is an LNA receiver. For the purpose of further analysis, it is assumed that the PCL system is dedicated to receive the DVB-T signals in a broad band. The PCL receiver may be presented in a general form as an antenna followed by a low-noise amplifier stage and further common amplifying stages. This receiving path ends with a downconverter or direct analog-to-digital converter (ADC) circuitry (Figure 2). Figure 1: General scheme of a Passive Coherent Location (PCL) radar. Because of the inherent one-direction characteristic of an amplifier, it may be assumed that for the purposes of PCL detection only the first stage of LNA may be exploited. The issue now is simplified to the case of a wideband antenna followed by an LNA, i.e., a low-noise transistor at the first stage of a low-noise amplifying cascade, as presented in Figure 3.  
It was assumed that the receiver antenna is wideband and the passive system bandwidth covers a number of telecommunication channels, e.g., DVB-T broadcast band from 170 to 860 MHz. Any signal of frequency within the PCL input band may be received and enters through the antenna into an LNA circuit. A signal of frequency out of this band cannot be received and cannot penetrate a PCL structure by other means, e.g., imperfect shielding of the hardware housing. Therefore, the only way to excite the PCL front-end by a sounding signal is to use an intentional signal at a frequency within the PCL working band. However, the use of a dedicated sounding signal of a frequency from the PCL band means that this signal is received and there is no signal coming back from the PCL. The only way to obtain a kind of useful response in order to detect the PCL presence is to use a harmonic (nonlinear) radar approach [13; 14; 15]. Theoretically, it may be possible to obtain the harmonic response of an LNA by transmitting the sounding signal of a given frequency f\({}_{0}\) and measuring the response signal at 2f\({}_{0}\) or 3f\({}_{0}\) frequencies. This approach presents a serious drawback. Successful implementation of this scheme requires that the second or third harmonic of the sounding signal falls within the PCL receiver's working band. For narrowband PCLs, this may be not fulfilled. Taking the above into consideration, the authors proposed here to apply the intermodulation approach. Then, the C-PCL system emits two sounding signals at frequencies f\({}_{1}\) and f\({}_{2}\) and receives the third-order nonlinear response at frequencies 2f\({}_{2}\) - f\({}_{1}\) or/and 2f\({}_{1}\) - f\({}_{2}\). Such signals are further referred to as an intermodulation (IMD). The assumption is that the frequencies f\({}_{1}\) and f\({}_{2}\) are closely spaced in order to ensure that both of them fit the PCL's input band. Additionally, the resulting intermodulation signals frequencies appear close to f\({}_{1}\) and f\({}_{2}\) and, therefore, also fit inside the PCL input band. Figure 3: PCL front-end structure assumed for further analysis. Figure 2: General structure of a PCL. One may consider the use of second-order nonlinearity because of the fact that the transistor inside an LNA presents both--odd and even kinds of nonlinearity. Although the second order nonlinearity exhibits a higher frequency conversion efficiency, the frequency components in the resulting spectrum are difficult to exploit in the case of a harmonic sensor. For one-tone excitation, the only useful signal is the second harmonic.