Many applications require the design of custom analog filters (Reference 1). The application for this design required simple and low-cost I/O filters for PLC (power-line communications), where low power consumption is a crucial factor. Figure 1 shows the filters, which use passive components because of the requirement for low power consumption. The PLC system needs an input and an output filter. They are 100-Hz to 20-kHz passband filters; the communication frequency is 5 kHz. The difference between the two filters lies in the input impedance. The input filter must present a 2.2-kΩ impedance, and the output filter must have a 30 Ω impedance. The circuit also needs a solid-state relay, the PVT412 from International Rectifier to isolate the output filter. When the circuit is active, the relay connects the output filter to the line. A microcontroller controls the relay to implement the signal-transmission and -reception protocols. ACTIVE_TRANS, RELAY, and Tx are the microcontroller pins that control transmission, and Rx is the pin that controls reception. Figure 2 shows waveforms before (Figure 2a) and after (Figure 2b) insertion of the input filter. Figure 3 shows waveforms before (Figure 3a) and after (Figure 3b) insertion of the output filter.
|Figure 1.||A power-line-communications system needs input and output filters
to eliminate interference.
|Figure 2.||Without an input filter, a great deal of hash accompanies
the input signal (a); the addition of the filter considerably
cleans up the signal (b).
|Figure 3.||Interference and noise are evident in the output signal (a);
the addition of the output filter (b) markedly reduces the noise.
- Lacanette, Kerry, “A basic introduction to filters: active, passive, and switched-capacitor,” Application Note AN-779, National Semiconductor, 2010.