System Noise & Interference Analysis
within RF Communications
by Russ Pittman

The coordination of frequency spectrum has become a daunting task as
more and more density techniques are applied allowing more conservative
utilization of bandwidth. Emission sources are carefully regulated by the
FCC to achieve organizational sources that provide carriers and carrier
applicants with a baseline of potential interfering signals. With today's
multiplexing, compression, and streaming techniques one stray signal can
wreak havoc on thousands and even millions of paths providing
mission-critical information. Perhaps the most critical phase of any
communications project is to determine the potential for RF interference.
This interference can be present within the system structure or exterior
such as point to point sources of radiation incidental to the carrier being
affected.

T = Physical temperature of the conductor
R = Resistance
f = Frequency
k = Boltzmann constant = 1.38044 x 10^-23 (Joules/Kelvin)
h = Planck constant = 6.62 x 10^-34 (Joules-second)

Intermodulation Distortion (IM) occurs in many forms. The more common
forms are passive intermodulation (PIM) and transmitter intermodulation (TIM).
In most cases PIM is caused by error in systems design (antenna, cable,
component) or by physical damage/improper techniques of connection,
grounding, and shielding. Improper contact within a system typically leads
to non-linear interaction. These nonlinearities mix with the desired signals
and generate undesirable harmonics leading to instability and failure of the
desired mode of operation. TIM is the result of harmonics within the non-linear
portions of amplification stages interacting with each other to contribute to
system instability. These distortion harmonics have the ability to affect receivers
within close proximity as well as contributing to the overall system noise figure of
merit. Design techniques offer many stable methods electronically and
mechanically reducing these distortion within allowable levels. Further
enhancements can be applied in some situations (such as combined carriers)
by the application of devices such as linearizers that can apply filtering to cancel
intermodulation products by some degree.