Page 209 - Radar Technology Encyclopedia

199 frequency, critical (ionospheric) frequency, intermediate (IF)

12
where f is in hertz. For typical daytime conditions, N » 10 , Rayleigh distribution. Diversity is distinguished from fre-
c
e
f » 9 MHz. AIL quency agility in that the latter implies pulse-to-pulse changes
c
Ref.: Barton (1988), pp. 309–313. in frequency, while diversity implies change on a burst-to-
burst or scan-to-scan basis or use of parallel channels at dif-
The critical frequency shift is the frequency separation nec-
ferent frequencies. To be effective, the spacing between chan-
essary to obtain independent samples of target or clutter sig-
nels must exceed the jamming bandwidth or a critical
nals in a frequency agility or diversity radar system.
frequency shift (or correlation frequency), generally given by
The cutoff frequency is (1) the frequency defining the c
boundaries of frequency-selective filter passband (if the pass- f = --------
c
2L
band is restricted from both ends, the higher boundary fre- r
quency in termed the upper cutoff frequency and the lower is where L is the radial length of the target or clutter source, or
r
lower cutoff frequency); or (2) the frequency in waveguide the difference in pathlengths between direct and reflected
transmission lines below which no significant propagation components.
occurs. Energy propagates, without significant loss, in vari- One frequency diversity option uses k sequential bursts,
ous “modes” at frequencies above cutoff. These modes are each consisting of m constant-frequency pulses. Coherent
defined in terms of the type of the propagating electromag- processing over the m pulses is possible, followed by nonco-
netic wave, transverse electric, which has no electric field herent integration of the k outputs during the dwell of n = km
component in the z-direction, and TE m,n or transverse mag- pulses. Another option uses scan-to-scan frequency change,
netic, TM m,n , having no magnetic field component in the z- with all n pulses from a given scan at the same frequency.
direction (Fig. F36). The subscripts m and n indicate, for a Other options divide each transmitted pulse of width t into k
rectangular guide, the number of half-wave variations of the subpulses transmitted at different frequencies, giving kn sub-
transverse fields in the x- or y-directions, respectively. pulses per dwell with coherent processing available over n
subpulses per channel. In this last case, a broadband receiver
front-end passes all k frequency channels, which are then pro-
cessed in separate narrowband amplifiers and aligned in time
/
by incremental delays tk for postdetection integration. DKB
Ref.: Barton (1988), pp. 85–87.
doppler frequency shift (see DOPPLER EFFECT).
frequency drift (see frequency stability).
Figure F36 Rectangular waveguide dimensions and coordi- Frequency hopping refers to frequency agility or sequential
nate system (after Skolnik,1970, p. 8.7). frequency diversity. The term is normally applied to commu-
nications systems rather than to radar.
The cutoff frequency of a rectangular waveguide
depends on the mode of operation and is equal to c/2a for the image frequency (see RECEIVER).
dominant mode, TE . Tables of cutoff frequencies for stan-
10
dard waveguides are available in the radar literature. PCH, frequency instability (see frequency stability).
AIL Instantaneous frequency is 1/2p times the rate of change of
Ref.: Skolnik (1970), pp. 8.7–8.11; Popov (1980), p. 97. phase. In the expression for a sine wave, e(t) = cos q(t) = cos
Frequency deviation is a maximum deviation of the carrier (w t + q ), where w , the derivative of q(t), is the radian fre-
c
0
c
frequency within the modulation interval t for frequency quency and q is some initial phase angle. When q(t) varies
0
r
modulation (Fig. F37). The typical notation is Df. AIL with time, as in the case of an FM carrier, the value w in the
c
equation above no longer represents the frequency of the
waveform, and it is necessary to define an instantaneous fre-
quency w such that w = dq/dt. PCH
i
i
Ref.: Schwartz (1959), p.113.
Intermediate frequency (IF), in a superheterodyne receiver,
is the frequency that results when the received RF signal at f rf
is mixed with a signal of frequency f from the local oscilla-
lo
tor, and filtered to eliminate frequency components at or
Figure F37 Frequency deviation of sawtooth FM waveform. above f . Thus, f = f - f . It is formed by frequency conver-
lo
rf
rf
if
Ref.: Terman (1955), p. 586; Popov (1980), p. 103. sion in a mixer. AIL
Frequency diversity refers to the technique of transmitting Ref.: Terman (1955), p. 568; Popov (1980), p. 324.
and receiving on two or more carrier frequencies, to over- A frequency multiplier is a device for increasing the fre-
come target or propagation path fading, narrowband jam- quency of a basic oscillator by some integer factor. The oper-
ming, or clutter having pulse-to-pulse correlation or non-