Page 297 - Radar Technology Encyclopedia

287 noise, automatic video noise leveling noise, power (spectral) density

the next sweep. (See CONSTANT-FALSE-ALARM where k is Boltzmann’s constant, N is the output from the
o
RATE.) SAL network, T is the standard temperature (290K), T is the
e
0
Ref.: IEEE (1990), p. 6. effective noise temperature of the network, B is the noise
n
bandwidth, G is available gain, and S /N and S /N are sig-
i
a
i
o
o
Noise bandwidth is the bandwidth of an equivalent rectangu-
nal-to-noise ratios at the input and the output of the network.
lar filter whose noise-power output is the same as the filter
The noise factor may serve as a measure of signal-to-noise
with frequency-response characteristic H(f). The usual nota-
degradation when the signal passes the network. Typically, it
tion is B , given by the formula:
n
is measured in decibels (10 log F ). If the receiver incorpo-
n
¥
rates several cascaded stages the resultant noise figure may be
2
ò Hf ()df shown as
– ¥
B = ---------------------------- F – 1 F – 1
n 2 n2 n3
Hf () F = F n1 + ----------------- + ----------------- + ¼
n
0
G G
G
where f is the frequency of maximum response (usually at 1 1 2
0
ni
midband). The measurement of noise bandwidth involves a where i = 1, 2, ... is the number of the stage and F and G ni
are noise figure and gain of corresponding stage. So if the
complete knowledge of the response characteristic H(f), so in
gain is rather high, it is the first stage (usually RF amplifier)
practice the 3-dB bandwidth is used as an approximation. SAL
that determines the noise properties of the whole receiver.
Ref. Skolnik (1980), p. 18.
SAL
Colored noise is a noise component having nonuniform spec- Ref.: IEEE (1993), p. 849; Skolnik (1980), pp. 19, 344; Morchin (1993), p.
tral density, as opposed to white noise. SAL 411; Leonov (1988), p. 56.
cosmic noise (see solar noise). Gaussian noise is described by the random process with the
Gaussian distribution of probability density function:
Direct amplified noise is noise directly amplified without a
2
carrier frequency, used in jamming to saturate the radar 1 ( U – U )
n
n
receiver noise level. In this case the radar operator may not Wu () -----------------exp= – ---------------------------
2
2ps n 2s
realize the radar is jammed if AGC or automatic noise level- n
ing are employed. It is used as barrage noise jamming with a where u is noise voltage, U n is the constant component of
2
bandwidth of more than 50 MHz. This method is not widely noise voltage, and s is noise variance.
n
used, as linear wideband amplification is not a very efficient If U n = 0, Gaussian noise from k different independent
process. This technique is also termed direct noise amplifica- sources are summed as the powers:
tion (DINA). SAL 2 2 2 2
U nS = U n1 + U n2 + ¼ + U nk
Ref.: Johnston (1979), p. 58.
AIL
Noise duty cycle is the parameter sometimes used in descrip-
Ref.: Barton (1964), p. 11; Svistov (1977), p. 85
tion of receivers analogously to signal duty cycle and equal to
the product of average receiver range-gate-on-interval and industrial noise (see environmental noise).
pulse repetition frequency in the assumption that the receiver
Johnson noise (see thermal noise).
noise generated after the range-gate switch is negligible. SAL
Noise power is the power of the noise at the output of a net-
Ref.: Currie (1987), p. 794.
work. The usual notation is N. Noise power is given by the
Environmental noise is ambient noise arising from the natu-
formula:
ral environment. For much of the radar band this noise is rela-
N = kT B
tively low in comparison with internal receiver noise and has s n
to be taken into consideration usually only for radars employ- where k is Boltzmann’s constant, T is the system noise tem-
s
ing receivers with very-low-noise input stages. The main perature, and B is noise bandwidth. SAL
n
sources of environmental noise are atmospheric noise, urban Ref.: Morchin (1993), p. 407.
or industrial noise, and cosmic noise in which solar noise
Phase noise is random phase modulation of the carrier pro-
sometimes is separately distinguished. Sometimes this noise
duced by signal sources such as oscillators. This kind of noise
is called ambient or external noise. SAL
often referred to as single-sideband noise, it causes carrier
Ref.: Skolnik (1990), p. 463.
sidebands whose amplitude vary as a function of frequency
Noise factor [figure] is the ratio of the noise at the output of offset from the carrier frequency, and it is the predominant
a practical linear network (e.g., a receiver) to the noise at the factor of short-term stability. SAL
output of an ideal network at standard temperature. The typi- Ref.: Morchin (1993), p. 414.
cal notation is F and the evaluation equation is Noise power (spectral) density is the noise power per unit
n
¤
S N i T e bandwidth, n/B .
i
F = N ¤ ( kT B G ) ---------------- = 1 + ----- n
=
n o 0 n a S ¤ N T
o o 0