We now introduce yet another variant of the decibel called dBc. dBc is decibels relative to the carrier. dBc/Hz is decibels relative to the carrier per Hertz. These units are used to describe in decibels how far down signals and noise are relative to a known signal.
In a sense, this acts for signals in the frequency spectrum very much like dBi does for antenna patterns.
The most typical use of these terms is to describe spurious signals and noise compared to a desired transmit signal. Although we desire our radar transmit signal to be extremely pure in frequency, it is not physically possible to be perfect.
The figure shows a pure signal with noise and spurious signals added in. The desired signal is much higher than the noise and spurious signals. Spurious signals will show up on a spectral plot such as this as lines. By looking at the power ratio between the desired signal and the spurious signal, we can define a measure for how far down the spurious is next to the signal. This measure is dBc.
Noise is a little tougher. With noise, the key is recognition that noise has a meaningful energy level when calculated within a certain bandwidth. The bandwidth is shown in red. If we determine the energy in a nominal 1 Hz filter, and then compare this energy to the desired signal, the term we use is dBc/Hz.
The decibel calculator can be useful for these problems. Use the dB power ratio section for your calculations. All these questions relate to a signal viewed spectrally as above. Assume that the levels shown for the noise are all for 1 Hz bandwidths.