The THS6222 amplifies the signal coming from the THVD8000 RS-485 OOK transmitter. This signal is AC coupled into the DC bus, where all the power from the solar panels is injected, by means of an RF transformer.

Figure 2-5 illustrates a schematic of the signal front end. The signs “+” and “-“ are the series connections into the DC bus.

The resistors R13 and R17 are necessary for the out-of-band suppression to keep the data within a certain power level. Next to R13 and R17, are the required biasing resistors R10, R19, R11, and R20, as a signal can be sent out differentially; however, received quasi-single ended. These biasing resistors keep offset to a minimum between the A and B lines. Schottky diodes and TVS are put in place as PLC modules typically operate in harsh environments and suppress unwanted surges. A bandwidth-limiting filter comprised of L1 and C9 are included, as well as a snubber circuit R15 and C10 to avoid high-frequency switching noise being sent over.

As power lines voltage in this application is quite high, an appropriate coupling design has to be made. With higher current, thicker cables are necessary for the transformer if a DC current of > 10 A is expected. This makes a small footprint PCB practically impossible, if this DC current is not decoupled first since such a large transformer is required. CA1 and CB1 are there to reject the DC current, and allow the AC currents pass by which alleviates this problem.

A band-pass filter is implemented for received signals. Since this design supports 4 different carrier frequencies, a filter for each of them was designed.

Figure 2-6 shows the discrete filter for 4 different carrier frequencies.

After the discrete filter, another TMUX1204 device is used to connect the correct filter to the THVD8000, according to which carrier frequency was selected. The TMUX1204 has a leakage source off current of 75 nA, and a drain off leakage current of 200 nA (see the electrical characteristics (V_DD = 5 V ±10%) section of the TMUX1204 5-V, 4:1, General Purpose Analog Multiplexer data sheet. This makes the device an excellent choice in our application as a very minimal leakage current increases overall efficiency of the system. The analog MUX routes the signal back to the THVD8000, where the devices switch from a transmit state to the receive state, demodulates the signal, and feeds the signal back to the MCU, per UART.

To control the TMUX1204, two-selection pins FILTER_B0, FILTER_B1 and one enable pin FILTER_EN are provided. Accordingly to the combination of those 3 pins it is possible to select the correct demodulating frequency.