What modulation method does HART use?

• A. Amplitude Modulation
• B. Phase Shift Keying
• C. Pulse Width Modulation
• D. Frequency Shift Keying

Answer: (D)

HART sends its digital information by modulating a carrier on the existing 4-20 mA loop using Frequency Shift Keying. This means the transmitter alternates between two distinct frequencies to represent binary data while the analog loop current remains within the 4-20 mA range. The receiver detects which frequency is present to recover the digital bits, all without changing the current's amplitude. This approach is robust in noisy industrial environments and lets digital communication ride on top of the standard analog signal.

Other modulation schemes would alter the current’s amplitude, timing, or phase in ways that could distort the measured process variable or complicate the separation of the digital data from the analog signal, which is why HART uses frequency-based shifting instead.