In English-language papers, I often see "dc" (as in direct current) in lowercase, but "RF" (as in radio frequency) in uppercase. For example, this paper includes both usages at the bottom of page 618:
This means that the ac (RF) waveform applied to […]
Some more papers using "dc".
- http://instructor.physics.lsa.umich.edu/adv-labs/Mass_Spectrometer/MassSpecQMS.pdf
- http://www.vacuumcursus.nl/casussen/jcequad.pdf
- http://ir.gig.ac.cn:8080/bitstream/344008/13774/1/08018.pdf
Oddly, this one uses lowercase "dc" and "rf":
Where does this convention stem from?
Best Answer
The American Institute of Physics Style Guide (4th edition) has a table of common abbreviations. (http://kmh-lanl.hansonhub.com/AIP_Style_4thed.pdf)
Alternating-current, direct-current and radio-frequency are all abbreviated in lower-case in this guide.
I'll make one comment on all three of these abbreviations and their use.
These terms are often used in sentences where their expansion would not make any sense. Whether this is acceptable use or not is a matter for debate, as these abbreviations carry meaning which goes beyond their original (narrow) definition.
In particular the concept of a time-varying or constant signal is often described/explained using 'ac' or 'dc' despite the signal concerned having nothing to do with electrical current (alternating or direct). For example you often hear the term 'dc' being applied to the first term (representing the mean) in a discrete Fourier transformation (e.g. FFT) of some (arbitrary) signal.
The term 'rf' (radio-frequency) is used to describe the physical phenomenon (wireless electromagnetic radiation) rather than the frequency of that phenomenon. e.g. you might have an 'rf communications link' which would mean some kind of wireless radio link as opposed to a communication link that uses a cable (despite both potentially using the same frequencies).