Page 79 - Radar Technology Encyclopedia

canceler, double-delay canceler, multiple-delay(-line) 69

f r 4 v b 4 Radar Auxiliary antennas
æ
æ
antenna
I = 2 ------------ ö = 2 ------------ ö 1 2 3
2 è 2ps ø è 2ps ø
c v
M A A A
where f is pulse repetition frequency, s is rms clutter spread 1 2 3
r
c
in frequency, v is the blind speed, and s is the rms clutter Adaptive Adaptive Adaptive
v
loop
b
loop
loop
spread in velocity. SAL
R R 4
Ref.: Barton (1964), p. 219; Skolnik (1980), p. 109. 1
Adaptive R 2 Adaptive
A double-delay feedback canceler is a double-delay can- loop loop
celer in which the feedback is added to adjust the shape of the R
3
R
filter passband, as shown in Fig. C2. The addition of feedback Adaptive 5
around the canceler makes it possible to reduce the width of loop
the notch, and, to a large extent, to preserve the cancellation R
6
characteristics of the double-delay system in the immediate
vicinity of the blind speed. SAL Figure C4 Gram-Schmidt canceler (after Lewis, 1986,
Fig. 3.7, p. 122).
Ref.: Barton (1964), p. 220.
Voltage The Howells-Applebaum canceler is a coherent sidelobe
feedback adjusted canceler implemented with a closed-loop technique. Origi-
for narrow notch
4 nally this canceler was invented by P. Howells, and S. Apple-
wide notch baum was the first to analyze such systems mathematically.
The conventional analog Howells-Applebaum canceler for a
2
single auxiliary antenna case is shown in Fig. C5. The cancel-
no feedback
lation ratio of Howells-Applebaum loop is give by formula
frequency
0
2 2 2 – 1
0 f /2 f r CR = ( 1 – 2 r + |r k )
r
Figure C2 Frequency response of a double-delay canceler where r is the normalized correlation coefficient between the
with and without feedback (after Barton, 1964, p. 220). main and auxiliary signals and k is the closed loop gain.
A four-pulse canceler is the configuration of a delay-line V m =A(t)cos(w 1 t)
canceler employing three delay lines connected as a transver- V a =aA(t)cos(w 1 t+f )
sal filter (see Fig. C3). The weights are 1, -3, 3, -1. SAL Multiply Local osc.
f
w = 2p
2
2
Ref.: Skolnik (1980), p. 110.
Input BPF
Delay Delay Delay
w -w
t r t r t r 1 2
+1 -3 +3 -1
+ - BPF
S Multiply
Output 1 w -w
2
Summer
r
W = GV V a cos(w 2 t+f )
Amplify
Figure C3 Block diagram of four-pulse canceler. G
V V cos(w 2 t+f )
r
a
A Gram-Schmidt canceler is a sidelobe canceler imple-
NBF
mented with an open-loop technique. Typically, it uses series- w 2
connected adaptive loops employing decorrelated auxiliary V V
ra
signals. One of the configurations is shown in Fig. C4, where V r
V Multiply
an adaptive loop decorrelates the signal picked up on auxil- r
iary antenna 1, another loop decorrelates the signal picked up
on auxiliary antenna 2, and the result is used by a second Figure C5 Howells-Applebaum canceler (after Lewis, 1986,
series loop in the radar channel, and so forth. Since the auxil- Fig. 3-2, p. 118.
iary signals have been decorrelated prior to use, none of the The original Howells-Applebaum canceler was imple-
series loops in the radar channels can reinsert interference mented at IF by using analog devices. More recently, the SLC
that has been removed by the previous loop. Such a configu- has been implemented with digital techniques. SAL
ration has better speed of convergence and stability than Ref.: Farina (1992), pp. 104–111; Lewis (1986), pp. 117–122.
closed-loop configurations, and the fact that the loops do not
A multiple-delay(-line) canceler is a delay-line canceler
interact simplifies the analysis. SAL
composed of sections of single-canceler circuits. One form of
Ref.: Lewis (1986), p. 122; Farina (1992), p. 122.
multiple delay canceler is the cascaded canceler (Fig. C6).