On the Generation of Waveforms Having Comb-Shaped Spectra
Abstract
Two techniques, direct sequence and spread spectrum and frequency hopping are widely used to provide an antijam capability for communications systems. To test the effectiveness of such systems, suitable jammers must be devised. A frequency hopping system can be jammed by wideband noise, by a following frequency hopped carrier (if the hopper is slow enough), or by a multitone jamming signal. Multitone jamming signals are considered advantageous in that jamming power is not wasted on frequencies never visited by the frequency hopper, as is the case when wideband noise is used. This report considers several classes of waveforms having power spectra that are approximately comb-shaped. The report relates the properties of the comb spectrum to the time-domain properties of the waveforms. These are the repeated pseudorandom sequence, used to modulate a carrier, and the frequency-swept sinusoid. A good quality comb spectrum should have uniformly strong teeth over its passband, and the spectrum should fall away rapidly at the band edges. To minimize the distortion caused by saturating power amplifiers, it is helpful if the time-domain function has a constant envelope. It is shown that a pseudorandom sequence that is low-pass filtered prior to modulation can produce a very uniform comb with sharp band edges, provided that the required width of the comb is not excessive. A swept-frequency sinusoid can produce a comb of very wide bandwidth, also with sharp band edges. The swept-frequency sinusoid has the additional advantage of a constant envelope, buy it is found that the comb itself may not be very uniform. For some cases of practical interest the variation in power from tooth to tooth of the comb can be as great as 35 dB.
Document Details
- Document Type
- Technical Report
- Publication Date
- May 06, 1988
- Accession Number
- ADA196927
Entities
People
- Bruce A. Black
Organizations
- United States Naval Research Laboratory