Document text
July 1972
NASA TECH BRIEF
Marshall Space Flight Center
B72-10361
NASA Tech Briefs announce new technology derived from the U.S. space program. They are issued to encourage
commercial application. Tech Briefs are available on a subscription basis from the National Technical Information
Service, Springfield, Virginia 22151. Requests for individual copies or questions relating to the Tech Brief program may
be directed to the Technology Utilization Office, NASA, Code KT, Washington, D.C. 20546.
A Study of the Power Spectral Density of an FM Signal
The problem:
Several communications problems can be better dealt
with when the power spectral density of an FM signal
is known. This knowledge would help in the design of
the corresponding band limiting network; it would help
in assuring that the FM signal does not interfere with
adjacent channels and that the bandwidth used is no
larger than necessary. In addition, this knowledge would
be useful in understanding how to stabilize video FM
signals.
The solution:
In a published study, several useful mathematical
descriptions of FM spectra have been derived.
How it’s done:
In the study, the derived mathematical expressions
describe an FM signal modulated by a composite video
wave. Using three types of video information, three
general spectrum equations for FM signals are obtained.
Of the three types of video information used, two
are periodic patterns and one a random pattern. The two
periodic patterns are the simple sinusoid and the bar
pattern. These signals are commonly used as test patterns
for testing video-frequency networks. Since their FM
spectra are known, the band-limiting network necessary
to make these signals into suitable test signals can be
readily designed. The bar pattern is of particular interest
in this application since, by varying the widths of the
bars, the maximum changes in video brightness levels
can be simulated. The information portion of the con-
tinually changing, or random pattern, is described using
a Gaussian noise distribution. This method simulates
the average properties obtained from a random picture.
A time-domain expression is derived. The expression
describes the FM signal formed by a composite video
signal and is very general. It applies to all video infor-
mation. Using this equation and the method used to
obtain the spectra from the three types of information
considered, the equation for the spectra of any video
information can be obtained.
Finally, the study presents specific calculated spectra
and compares them with those obtained by experiment.
Note:
Requests for further information may be directed to:
Technology Utilization Officer
Marshall Space Flight Center
Code A&TS-TU
Huntsville, Alabama 35812
Reference: TSP72-10361
Patent status:
No patent action is contemplated by NASA.
Source: H. L. Deffebach of
Auburn University
under contract to
Marshall Space Flight Center
(MFS-21070)
Category 02
This document was prepared under the sponsorship of the National
Aeronautics and Space Administration. Neither the United States
Government nor any person acting on behalf of the United States
Government assumes any liability resulting from the use of the
information contained in this document, or warrants that such use
will be free from privately owned rights.