JED - May 2014 - 60

EW 101

Digital RF Memories

DRFMs - Part 8
By Dave Adamy

DRFM LATENCY ISSUES
In the January and February EW 101 columns, we discussed
the reproduction of chirped and Barker-coded pulses. In both
cases, the DRFM and its associated digital signal processor
captured and analyzed the first pulse received and copied the
characteristics of that pulse when rebroadcasting subsequent
pulses. This, of course, assumes that all of the pulses in a received radar transmission will be identical. The rebroadcast
pulses are coherent with the received pulses, and there are
other modulation elements applied to support the jamming
technique being employed. For example, each of the subsequent pulses may be delayed and/or frequency shifted.

Pulse Width

Analysis Increments
Received
Frequency
Modulation
Transmitted
Frequency
Modulation

IDENTICAL PULSES

The Journal of Electronic Defense | May 2014

60

When all of the pulses in a received radar transmission are
identical, the DRFM and its associated processor analyze the
first pulse received and generate jamming pulses with the
proper modulation to jam each subsequent pulse in the transmission. The process latency required is short enough to complete the necessary processing during the interpulse period.
This is a few tens of microseconds to a few milliseconds.

FOR IDENTICAL CHIRPED PULSES
As shown in Figure 1, the analysis of the first pulse must
be accomplished during the interpulse period. Consider a
tracking radar with a pulse width of 10 µsec and a 10 percent duty cycle. Remember that the pulse interval is the time
between the leading edges of subsequent pulses. This means
that the interpulse time available for the DRFM to make cal-

Received
Waveform

Pulse
Width
Maximum Process
Latency

Transmited
Waveform

Figure 2. During the interpulse time, the received chirped signal is
analyzed to determine its frequency at each analysis increment. Then, a
digital return signal is formulated with a stair-step approximation of the
modulating frequency. The return signal is offset in frequency and time
to support the chosen jamming technique.

culations is 90 µsec. The process throughput latency time,
during which the DRFM processor accepts the digitized pulse
data from the DRFM, determines the pulse modulation parameters, generates the desired jamming pulse and returns the
modified signal (in digital form) to the DRFM, must thus be
less than 90 µsec.
For a chirped pulse, the first received pulse must be digitized in the DRFM and passed to the processor. As shown
in Figure 2, the slope of the frequency modulation must be
measured. Note that the frequency modulation on the pulse
can be either linear or non-linear. Blocks of code are generated for each of many time increments over the pulse
width. A digital representation of the whole pulse is then
generated with the frequency during each increment determined from the measured frequency in the received
pulse and the desired Doppler shift offset. The digital
signal returned to the DRFM for rebroadcast will have
a stair-stepped representation of the frequency modulation of the received pulse and will be offset in frequency
and time by the amount dictated by the jamming technique employed.

FOR IDENTICAL BARKER CODED PULSES
Figure 1. In order to copy the first received pulse in subsequent pulses, the
DRFM and processor must complete the full process within the inter-pulse
time.

When there is a binary phase shift keyed (BPSK) signal
on a received radar signal, the DRFM digitizes the first
pulse received. Then, the processor determines:



JED - May 2014

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

The View From Here
Conferences Calendar
Courses Calendar
From the President
The Monitor
Washington Report
World Report
Interview
European Helicopter EW
Advanced ASE And ESM Already On Some US Helicopters
Ground-Based COMINT Solutions
Fighting the Kill Chain: Concepts and Methods of Electronic Attack
EW 101
AOC News
Index of Advertisers
JED Quick Look
JED - May 2014 - cover1
JED - May 2014 - cover2
JED - May 2014 - 3
JED - May 2014 - 4
JED - May 2014 - 5
JED - May 2014 - The View From Here
JED - May 2014 - 7
JED - May 2014 - Conferences Calendar
JED - May 2014 - 9
JED - May 2014 - Courses Calendar
JED - May 2014 - 11
JED - May 2014 - From the President
JED - May 2014 - 13
JED - May 2014 - 14
JED - May 2014 - The Monitor
JED - May 2014 - 16
JED - May 2014 - 17
JED - May 2014 - 18
JED - May 2014 - 19
JED - May 2014 - 20
JED - May 2014 - 21
JED - May 2014 - Washington Report
JED - May 2014 - 23
JED - May 2014 - World Report
JED - May 2014 - Interview
JED - May 2014 - 26
JED - May 2014 - 27
JED - May 2014 - European Helicopter EW
JED - May 2014 - 29
JED - May 2014 - 30
JED - May 2014 - 31
JED - May 2014 - 32
JED - May 2014 - 33
JED - May 2014 - 34
JED - May 2014 - 35
JED - May 2014 - 36
JED - May 2014 - 37
JED - May 2014 - 38
JED - May 2014 - Advanced ASE And ESM Already On Some US Helicopters
JED - May 2014 - 40
JED - May 2014 - 41
JED - May 2014 - 42
JED - May 2014 - 43
JED - May 2014 - Ground-Based COMINT Solutions
JED - May 2014 - 45
JED - May 2014 - 46
JED - May 2014 - 47
JED - May 2014 - 48
JED - May 2014 - 49
JED - May 2014 - Fighting the Kill Chain: Concepts and Methods of Electronic Attack
JED - May 2014 - 51
JED - May 2014 - 52
JED - May 2014 - 53
JED - May 2014 - 54
JED - May 2014 - 55
JED - May 2014 - 56
JED - May 2014 - 57
JED - May 2014 - 58
JED - May 2014 - 59
JED - May 2014 - EW 101
JED - May 2014 - 61
JED - May 2014 - AOC News
JED - May 2014 - 63
JED - May 2014 - 64
JED - May 2014 - Index of Advertisers
JED - May 2014 - JED Quick Look
JED - May 2014 - cover3
JED - May 2014 - cover4
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