Friday, May 16, 2014

CW vs. DW

Let's break out the acronyms today: CW and DW. Unless you are a ham or a radio engineer, these two short acronyms are meaningless. They don't stand for Creative Writing and Data Window. These are continuous wave and damped wave and as you might suspect... they are related.


CW stands for Continuous Wave sometimes written as "continuous waveform."  This describes an electromagnetic wave with both a constant amplitude and a constant frequency.  Rather, that is the text book definition. CW is also the name used to an early form of radio transmission. In CW the carrier wave itself is switched on and off. With this method data is encoded by varied durations of  on and off time intervals. This is a classic implementation of Morse Code.  To further complicate the history of CW these were also called "undamped waves."  Interestingly it has a very modern use as random keyed modulated CW. It is a common source of radio jamming against both radio facsimile and CW transmissions.


DW stands for  damped wave. In DW the amplitude of oscillation decreases over time dropping eventually to zero.  This term also refers to another early method of radio transmission. DW was generated by spark gap transmitters: make/break circuits. These created a series of damped electromagnetic waves. Here information was carried on this signal by telegraphy, turning the transmitter on and off (on-off keying) to send messages in Morse code. Damped waves were the first practical means of radio communication, used during the wireless telegraphy era which ended around 1920. In radio engineering it is now generally referred to as "Class B" emission. However, such transmissions have a wide bandwidth and generate electrical "noise" (electromagnetic interference) which interferes with other radio transmissions.

In nature, amplitude rolls off.  Only in Matlab can we see a hypothetical square wave. So what's the difference again? While CW sounds like it's just perfect Lego blocks of RF it's not.  Both waves still have peaks and valleys, but DW has attack and decay. If you look at the two images it's a lot more clear. DW hits and then diminishes like an acoustic wave, CW has more or less equal amplitude until it drops to zero.