The so-called coupling circuit is a circuit for connecting signals between a low-voltage power line and a carrier signal transmission and a carrier signal receiving circuit, and the signal is interlinked by a coupling circuit. Depending on the type of signal and the circuit environment, proper coupling will play a crucial role in the normal transmission of the signal.
1.1 carrier transmitting end coupling circuit
The signal transmitting end circuit of the system is shown in Fig. 1. The primary coil of the transistor V1, the transformer T1 (set to L2) and the C3 and C4 constitute a single tuning power amplifying circuit.
Figure 1 carrier transmitting end coupling circuit
Here, the resonant transformer T1 has a dual function: on the one hand, the carrier signal is coupled through the transformer; on the other hand, the communication circuit is strongly isolated from the power frequency AC. T1 uses a tapped transformer to reduce the Q value of the parallel resonant network consisting of C3, C4 and T1 primary coils due to the V1 access, and even stop the vibration.
The high-frequency carrier small signal outputted by the pre-stage is amplified by the frequency-selective power amplifier through a single-tuned power amplifier composed of a triode V1 and peripheral components, and the output power of the carrier signal can be improved by about 10 times. The network oscillation frequency can be fine-tuned by adjusting the size of C4. The system selects the carrier center frequency as 120kHz, when the natural oscillation frequency of the network loop 
At 120 kHz, the loop resonant impedance is maximum and the pure resistance is Z=R0=1/g0; the voltage across the loop is also the maximum value U0=IS/g0. The resonant voltage U0 is coupled to the low voltage power line via the secondary winding of the transformer T1, while the transformer T1 isolates the power line from the carrier transmitting circuit. In order to prevent the circuit from self-exciting, the V1 emitter is connected to the resistor R2. This resistor cannot be too large or it will affect the transmission power. This system takes 1Ω.
In order to transmit the carrier signal to the low voltage power line with higher efficiency. It is necessary to filter out noise and spurious signals doped in the carrier signal. Generally, the second harmonic and the third harmonic of the transmitted signal (the second and third harmonics of the system are 240 kHz and 360 kHz, respectively) have relatively large noise pollution to the power grid, so it is necessary to filter and shape the carrier signal. Set a series resonant bandpass filter consisting of L1 and C1 to set the oscillation frequency 
Also 120kHz. When the transmitted carrier signal is in series resonance with the L1 and C1 oscillating networks, the series circuit impedance Z is the smallest and Z=r, because it is connected in series in the circuit, the carrier signal with a frequency of 120 kHz is very easy to pass, and the signal for other frequencies or The noise impedance is large. Its impedance is Z = r + j (2Ï€fL - 12Ï€fC). Where: r is the DC internal resistance of L1. It can be seen that the band pass filter circuit composed of the capacitor C1 and the inductor L1 can filter and shape the transmitted carrier signal. In addition, the capacitor C1 can also reduce the voltage of the 50Hz power frequency AC to reduce the power frequency AC voltage that the transformer T1 is subjected to, and also prevent the transformer T1 from being saturated. Therefore, the choice of C1 should be considered to have a sufficiently high withstand voltage.
1.2 carrier receiving end coupling circuit
The carrier signal transmitted by the transmitter is transmitted to the receiving end via the low voltage power line, and then coupled to the receiver by the coupling circuit. The carrier receiving end coupling circuit is shown in Figure 2.
For the coupling circuit at the receiving end, the passive filter is preferred over the active filter because the active filter produces a white noise comparable to the received signal. The system uses a passive bandpass filter (composed of C11, C12 and L10) and adopts a parallel resonant loop form. The center frequency of the parallel circuit is determined by the values ​​of capacitors C11, C12 and inductor L10, center frequency 　
The design value of this system is 120kHz. It is calculated that when L10=68μH is selected, it corresponds to (C11+C12)=25.86nF, but C11 here must be combined by several capacitors in parallel to obtain this value.
The carrier coupling portion is composed of a transformer T2, a capacitor C10, and an inductor L9. Transformer T2 uses a 1:1 isolation transformer. Capacitor C10 isolates the transformer from the power frequency AC, so that the communication circuit can completely isolate the power frequency signal of the power network. This will prevent low frequency signals from entering the circuit and allowing certain high frequency signals to pass. In case the capacitor C10 loses the ability to filter the 50 Hz power frequency signal due to a short circuit, the interface circuit will be damaged. Therefore, C10 should use X2 type capacitor with short circuit protection. The resistor R10 connected in parallel across C10 is a bleeder resistor that acts to discharge the capacitor C10 when the device is offline, preventing high voltage at the input of the device. This resistor can be slightly larger, taking 1MΩ in this design.
Capacitor C10 and inductor L9 also form a band-pass filter, which can filter other interference noise and out-of-band noise from the power line on the one hand, and ensure impedance matching between the front and rear stages on the other hand to achieve smooth transmission. The purpose of the carrier signal.
1.3 interface protection circuit
All devices on the power line are connected or disconnected, which can cause spikes and permanent damage to the transceiver circuitry. At the same time, the system may be interfered by transient overvoltages such as strong lightning impulses, so it is necessary to use a protection circuit in the interface coupling circuit between the carrier transmitting end and the carrier receiving end and the power line.
In the circuit, TVS is a transient suppression diode that acts as a surge protection to effectively prevent high voltage breakdown of the latter circuit. In addition to the occasional high-voltage pulse-breaking device on the power line, when the device is just connected to the power supply, if the power line is just at the maximum voltage, then the voltage on the capacitor is 0V, then there will be a high voltage of 300V or more directly applied to the transformer. At the end, it will cause a large current to generate a spike at the secondary. The current of this pulse is quite large, up to several tens of amps to hundreds of amps. The pulse can not be eliminated by the general voltage regulator. The response of the varistor is relatively slow, and there are still several tens of volts within 1 μs of the pulse. The voltage is enough to burn out the amplifier circuit. Experiments have shown that the transient force generated by this transient pulse when entering the circuit is quite large. However, although its current is large, its energy is small. The selected transient suppression diode 1.5KE6.8CA has a response time of 5ns and can absorb 200A current, and the transient power can reach 1500W.
Generally, the interference between the live line and the neutral line is differential mode interference; the interference between the live line and the ground line, the neutral line and the ground line is common mode interference. If a bidirectional voltage regulator is used only for the differential mode spike signal and does not contribute to the common mode spike signal, the circuit will be damaged when a common mode spike occurs. The system uses three diodes D7, D8 and D9 to form a star structure, as shown in Figure 2. For differential mode spikes, D7 and D8 form a bidirectional voltage regulator; for common mode spikes, this star structure is equivalent to two bidirectional regulators.
In the circuit of Figure 2, the Zener diodes D10 and D11 form a bidirectional limiting circuit. When the amplitude of the received signal is equal to or greater than the voltage regulator of the Zener diode, the Zener diode clamps the potential of the received signal to the regulated value. Preventing the signal amplitude from being too large and damaging the subsequent decoding integrated circuit. R11 and R12 are isolation resistors of the parallel resonant network, which take 27kΩ.
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