Question 3-81L2

Radio Frequency Interference (RFI) occurs when unwanted radio signals disrupt electronic devices. Here, the radio system's transmissions are interfering with the elevator's tachometer. **B) Add a .01 μF capacitor across the motor/tachometer leads.** This is a common and effective RFI mitigation technique. A small capacitor, such as .01 μF, acts as an RF bypass filter. It provides a low-impedance path for high-frequency radio signals (the RFI) to bypass the sensitive tachometer circuitry, shunting them to ground. This effectively filters out the interference without affecting the DC or low-frequency AC signals required for the motor and tachometer's normal operation. **A) Replace the tachometer of the elevator.** This is often an expensive and ineffective first step. Unless the original tachometer was faulty, a new one would likely experience the same interference as the RFI source and pathway remain unaddressed. **C) Add a 200 μF capacity across the motor/tachometer leads.** A 200 μF capacitor is a very large value typically used for power supply smoothing. It would likely short out or significantly alter the low-frequency signals necessary for the motor/tachometer's operation, causing malfunction rather than just filtering RFI. **D) Add an isolating resistor in series with the motor leads.** A series resistor would introduce voltage drop and power loss, significantly impacting the motor's operation and efficiency. It is not an effective or appropriate solution for filtering RFI in this context.