The disclosed invention is directed to noise generating circuitry, and is more particularly directed to a pseudorandom noise generator which utilizes digital modulation to produce a pseudorandom noise output which has a relatively wide spectral band, a relatively flat in-band spectrum, a substantially constant probability density function across its bandwidth, substantially constant dwell times for fixed frequency intervals within its bandwidth, and substantially constant revisits for such fixed frequency intervals.
Pseudorandom noise generators are utilized in various applications such as system testing, secure communications, and electronic jamming. Depending on the application, certain characteristics of the output noise signal may be desirable. For example, a relatively wideband pseudorandom noise output may be advantageous for jamming a class of victim receivers which are known to be operating within such relatively wide bandwidth. A pseudorandom noise generator that provides such a relatively wideband noise output also be useful for other uses such as a noise-like channel sounder or as a spread spectrum source for a spread spectrum communications system.
Considerations with known wideband pseudorandom noise generators include the lack of constant probability density across the bandwidth of the noise output. That is, different frequencies of the noise output have different probabilities of occurrences. Such lack of a constant probability density reduces the effectiveness of applications such as electronic jamming and may affect the apparent randomness of the output noise signal.
Further considerations with known pseudorandom noise generators involve characteristics known as "dwell time" and "revisits" which are based on dividing the noise output bandwidth into a plurality of fixed frequency intervals of the same width (e.g., 5 MHz) which are also known as "resolution cells." Dwell time refers to the long term average time interval that the noise output is within a specified fixed frequency interval or resolution cell. Revisits refers to the long term average number of times per unit time that the noise output is in a specified fixed frequency interval.
With known pseudorandom noise generators, dwell time for a specified fixed frequency interval depends on the location of the specified fixed frequency interval within the noise output bandwidth. Also, the number of revisits for a specified fixed frequency interval depends on the location of the specified fixed frequency interval within the noise output bandwidth.
The variation in dwell time and number of revisits are important considerations in applications such as receiver jamming. Such variation reduces the apparent randomness of the noise output, and may allow for increased detection of the jamming function since patterns of frequency changes may be detected. Further, such variations reduce the predictability of the effects of jamming on receivers whose locations within the noise output bandwidth are unknown.