dBcHz

dBc/Hz, short for decibels relative to the carrier per hertz, is a unit used to express the spectral density of phase noise in a carrier signal. It represents the ratio, in decibels, of the noise power within a 1 Hz bandwidth at a given offset frequency from the carrier to the carrier power itself. The quantity is commonly denoted L(f) and is read as “X dBc/Hz at offset f.”

This unit is widely used in RF, microwave, and timing applications to characterize oscillators, frequency references, PLLs, and synthesizers. It provides a concise way to describe how much noise surrounds the carrier at different offset frequencies, independent of the carrier’s absolute power. Values are typically negative, with more negative numbers indicating cleaner signals.

Measurement is performed with a phase-noise analyzer or a spectrum analyzer equipped with a phase detector or mixer. The carrier is used as a reference, and the noise power in a 1 Hz bandwidth at each offset frequency is compared to the carrier power, then converted to decibels. Because measurement systems have finite noise floors, accurate results often require careful calibration, averaging, and a reference with lower intrinsic noise.

Interpretation and usage notes: lower dBc/Hz values (more negative) imply less phase noise at the specified offset. The useful information is the noise density as a function of offset, which can be integrated over a bandwidth to estimate time-domain jitter. dBc/Hz is a standardized way to compare oscillators and clocks across devices and applications.