# A Power-Line Communication System Governed by Loop Resonance for Photovoltaic Plant Monitoring
## 3 Results  
Transmission tests have been performed over the setup described before within our 11-module PV plant, with two communication circuits as the one shown in Figure 1 connected in one of the PV modules and in the combiner box respectively. In the rest of the PV modules, a simple circuit is replicated and attached to each to simulate the total impedance in the reception mode of the communications electronics, with the corresponding anti-saturation coils connected, in order to get the behavior of the entire loop as close as possible to the real setup. The combiner box bypass capacitor, was adjusted in such a way that after connecting the transceivers and impedance simulating circuits, the loop resonance frequency of the signal path was 1 MHz. All the measurements have been performed with the inverter on and the plant working (with a DC string current around 5 Amps) in order to test the anti-saturation coil performance. A test signal composed of 1 MHz carrier bursts was injected in the loop by one of the transceivers in TX mode. Two different Bursts were alternated in time seven and three carrier cycles long respectively, representing a digital modulation of alternating 0,s and 1.s. The dead time between bursts is programmed for a fixed symbol duration of 13 us, same for \"0\" and \"1\". The signal received in the circuit on reception mode was measured with an oscilloscope and is shown in Figure 3a. The ability of the push-pull TX stage of featuring a high output impedance by opening both n and p MOSFETs allows for the shortening of the damped resonance oscillation after the end of each burst, avoiding prolongation of the symbol transmitted and the consequent Inter Symbol Interference (ISI), as seen in Figure 3a. Figure 3b shows the output of the RX comparator for the same frame of RX signal shown in Figure 3a. These signal bursts switches from zero to 5 Volts and reproduces accurately the burst duration of the input signal. Our test modulation scheme based on the length of the burst, despite being ineffective from a practical point of view, allows for checking both the amplitude and time responses of our hardware to the communication signal. First, input level threshold, influence of growing noise levels, and RX dynamic range can be observed from the presence/absence of the comparator output burst for different levels of TX signal, and second, the time duration of the received bursts allows the characterization of the ISI, a quite important parameter to evaluate the maximum communication speed achievable by the system. Regarding this last parameter, in our tests a maximum of one extra pulse at the end of the burst has been observed (as the one shown in the second burst of Figure 3b) at the output of the RX comparator, indicating a low ISI. On the other side, an enormous dynamic range has been observed for the RX amplitude (from 0.8 Volts to 25 Volts). These results suggest the possibility of exploring digital modulation schemes that will lead to propagation speeds over 200 kbps with a 1 MHz carrier (depending on the spectral efficiency of the modulation), or even higher if the carrier frequency is increased. Figure 3c shows the recovered digital frame with alternating 1,s and 0,s after the firmware demodulation of the output comparator signal Figure 3b. Note that symbol duration is the same for \"1\" and \"0\" (13 us), but at the stage where the signal in Figure 3c is taken the demodulated digital value is available before for \"1\" then for \"0\", and this is the reason why the pulses with level \"1\" are longer than the ones with level \"0\". When the digital frame is extracted out the bit duration becomes equal for 1 and 0. As a summary, Table 1 shows the prototype communication parameters, and a picture of the real circuit can be seen in Figure 4. The design here described, has been integrated into the same printed circuit board (PCB) in the I-V tracer designed in our previous work [4], and for this reason not all the components are used or installed in the PCB.