JED - February 2014 - 44

EW 101

Digital RF Memories

DRFMs - Part 5
By Dave Adamy

BARKER CODE MODULATION

The Journal of Electronic Defense | February 2014

44

OSCILLATOR

BINARY
PHASE
MODULATOR

AMPLITUDE
MODULATOR

POWER
AMPLIFIER

The other technique for pulse compression is the addition of a binary phase
PSEUDOshift keyed (BPSK) digital modulation
RANDOM
ISOLATOR
CODE
to each pulse as previously discussed in
the August 2010 "EW 101" column.
There are a fixed number of bits in
TAPPED
RECEIVER
DETECTION
DELAY LINE
FRONT END
this code during each pulse, and when
& ANALYSIS
ASSEMBLY
the pulse is received by the radar as a
skin return, it is passed to a tapped deFigure 1: A radar with Barker code places a BPSK modulation on each transmitted pulse and
lay line assembly as shown in Figure 1.
compresses the received skin return pulses by passing them through a tapped delay line.
The code, which can be a Barker code
by the speed of light. Thus, the range resolution is improved
or one of several other codes, is a "maximal length code." This
by the factor equal to the number of bits transmitted during
means that it is pseudo-random, and if the number of zeros
each pulse.
is subtracted from the number of ones, the sum will be zero
or minus one. The code on the pulse at the top of Figure 2
is a seven-bit barker code where "+" indicates a one and "-"
JAMMING BARKER CODED RADARS
indicates a zero. Note that this is a short code, but the typical
Now consider a non-coherent jammer operating against a
codes used in pulse compression radars are much longer (up to
system that has Barker coded pulses (as shown in Figure 4).
1000 bits). There are 180-degree phase shifts on some of the
The skin return pulse has coding that is matched to the tapped
taps. They are designed so that when the pulse exactly fills
delay line configuration. This means that the post processing
with the delay line, each of the zero bits is at a tap with a
pulse width will be effectively reduced to the bit duration. For
phase shifter. Thus, when the pulse fills the delay line and the
example, if there are 13 bits in the barker code, the pulse will
taps are summed, the pulse has full amplitude. At any other
be reduced by a factor of 13. On the other hand, a jamming sigtime, the summed output is significantly less. With a seven-bit code as shown,
INPUT TO TAPPED DELAY LINE
the summed output will be either zero or
180° Phase Shift
minus one when the pulse is not aligned
with the delay line. For longer codes,
+ + + - - + there will be some time periods during
BIT
which the sum is larger, but still signifiDURATION
cantly below the full pulse amplitude.
When the pulse leaves the delay line and
TIME
PULSE DURATION
summing process, the pulse duration is
effectively one bit wide, (i.e., the "efis180°
INPUT
fective post processing pulse duration
Phase Shift
is the time the pulse exactly fills the
SUMMED
tapped delay line).
OUTPUT
TAPPED DELAY LINE
Figure 3 shows the effect of this
pulse width reduction on the radar's
resolution cell. The cross-range dimenFigure 2: The coded pulse produces a large
OUTPUT FROM
sion of the resolution cell is still the 3output from the delay line only when all
TAPPED
of its bits align to the taps. This reduces
DELAY LINE
dB beam width of the radar's antenna,
BIT
the post processing pulse width to the
DURATION
but the depth of the cell is now one
duration of one bit of the code.
half of a code bit duration multiplied



JED - February 2014

Table of Contents for the Digital Edition of JED - February 2014

The View From Here
Conferences Calendar
Courses Calendar
From the President
The Monitor
World Report
Antennas: Shaping the Future of RF Applications
DARPA – No Surprise is the Prize
EW 101
AOC News
Views from the AOC 50th Annual International Symposium and Convention
Index of Advertisers
JED Quick Look
JED - February 2014 - Cover1
JED - February 2014 - Cover2
JED - February 2014 - 3
JED - February 2014 - 4
JED - February 2014 - 5
JED - February 2014 - The View From Here
JED - February 2014 - 7
JED - February 2014 - Conferences Calendar
JED - February 2014 - 9
JED - February 2014 - Courses Calendar
JED - February 2014 - 11
JED - February 2014 - From the President
JED - February 2014 - 13
JED - February 2014 - 14
JED - February 2014 - The Monitor
JED - February 2014 - 16
JED - February 2014 - 17
JED - February 2014 - 18
JED - February 2014 - 19
JED - February 2014 - World Report
JED - February 2014 - 21
JED - February 2014 - Antennas: Shaping the Future of RF Applications
JED - February 2014 - 23
JED - February 2014 - 24
JED - February 2014 - 25
JED - February 2014 - 26
JED - February 2014 - 27
JED - February 2014 - DARPA – No Surprise is the Prize
JED - February 2014 - 29
JED - February 2014 - 30
JED - February 2014 - 31
JED - February 2014 - 32
JED - February 2014 - 33
JED - February 2014 - EW 101
JED - February 2014 - 35
JED - February 2014 - 36
JED - February 2014 - AOC News
JED - February 2014 - Views from the AOC 50th Annual International Symposium and Convention
JED - February 2014 - 39
JED - February 2014 - 40
JED - February 2014 - Index of Advertisers
JED - February 2014 - JED Quick Look
JED - February 2014 - 43
JED - February 2014 - 44
JED - February 2014 - 45
JED - February 2014 - 46
JED - February 2014 - 47
JED - February 2014 - 48
JED - February 2014 - 49
JED - February 2014 - 50
JED - February 2014 - Cover3
JED - February 2014 - Cover4
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