What is frequency transmission?

• A. A signal where amplitude is proportional to the measured value.
• B. A signal where frequency is proportional to the measured value.
• C. A signal where the voltage remains constant.
• D. A signal where the phase shifts with time.

Answer: (B)

Frequency transmission conveys the measured value by changing the carrier’s frequency to reflect that value. The amplitude stays the same as the signal conveys the information through frequency, not by how strong the signal is. This is different from amplitude-based signaling, where the measured value would be shown by changes in amplitude, or from phase-based methods where the phase carries the information. In HART, while the loop current carries the analog value, digital data can be sent by a high-frequency carrier whose frequency encodes the information, so describing the signal as one where frequency is proportional to the measured value is the correct interpretation.