As I indicated in the previous post, this discussion begins examining signal characteristics from an electromagnetic compatibility (EMC) perspective. But first, a few words to describe EMC. This is a relationship issue. We want all the equipment items to get along in peace and in harmony, but if one of them is a big source and the other is a wimpy receiver, they may not play together very well. The EMC community describes this arrangement as a source and a sink or an emitter (E) and a receptor (R), and this E-R pair occurs in any system or collection of equipments.
The emitter energy can be either intentional or unintentional depending on whether the emitter needs the energy for its operation or not. Here are examples of the three cases:
(1) If the E-R pair comprises an RF communications link, then the emitter energy being coupled would be intentional and desired.
(2) If the emissions from the above communications link were causing upset to some unrelated equipment, then the intentional energy being coupled would be undesired. Now, the intentional RF source has become RF interference.
(3) If the emitter was a computer and the receptor was part of the above communications link, then the coupled energy would be unintentional and undesired. Another source of interference! There are no ordinary cases where the coupling of unintentional energy is desired, but there are some in which it is exploited.
Signals come in two flavors, broadband (BB) and narrowband (NB), and, as the name implies, their spectral occupancy is significantly different. Theoretically a narrowband signal exists at only one frequency and thus has zero bandwidth. As a result, any circuit with a non-zero bandwidth will capture all of the NB energy, and increasing its bandwidth does not increase the energy that is captured. The closest we can come to that is a pure sine wave (also called CW). Unfortunately, in the real world it’s difficult to find a pure sine wave. Plus, it’s also difficult to convey a lot of information with it (other than On or Off), so it’s usually modulated, which increases its spectral occupancy.
In the early days of radio the CW (carrier) was modulated by switching it on and off using a telegraph key and radio telegraphy became known as CW. This is not the same as a continuous sine wave. Regardless, modulating a 30 MHz carrier with Morse code at 50 words per minute or even modulating a 150 MHz signal with speech (300 – 3300 Hz) does not spread the signal very much, so most non-digital radio equipment is considered narrowband.
Broadband signals occupy lots of spectrum. This also makes the interference from a broadband signal much more difficult to control. Theoretically a BB signal has a flat amplitude in the frequency range from DC to daylight. In our case a BB signal only needs to be reasonably flat over the bandwidth of the circuit that detects it. Unlike the narrowband signal, increasing the BB bandwidth will capture more of the broadband energy. How much more is determined by whether the BB signal is random or coherent?
-Ron Brewer
