JED - October 2013 - 62

EW 101

Digital Radio Frequency Memories – Part 1

Digital RF Memories
By Dave Adamy

T

he Digital Radio Frequency Memory (DRFM) is
an important development supporting electronic
countermeasures. It allows the rapid analysis of
complex received waveforms and generation of
countermeasure waveforms. It can increase the
effectiveness of a jamming system against complex waveforms by many dB.

DRFM BLOCK DIAGRAM

The Journal of Electronic Defense | October 2013

62

computer can typically analyze the first pulse received by the
system and generate subsequent pulses with the same, or systematically varied, modulation parameters.
The digital-to-analog converter (DAC) that generates the RF
output signal will have more bits than the ADC to assure that
the signal quality is not degraded in the reconstruction of the
RF signal.

WIDEBAND DRFM

As shown in Figure 1, the DRFM down-converts received
A wideband DRFM digitizes a wide IF bandwidth that may
signals to the appropriate intermediate frequency (IF) for digiinclude several signals. The jammer system tunes across the
tization. Then, it digitizes the bandwidth of the IF signal. The
frequency range of threat signals it must jam, and outputs an
digitized signal is placed into a memory for transmission to
IF signal with a bandwidth the DRFM can handle. As shown in
a computer. The computer makes any necessary analysis and
Figure 3, the frequency-conversion and the later reconversion
modifications to the signal to support the jamming technique
to the received frequency are done using a single system local
being employed. Then, the modified digital signal is converted
oscillator to preserve phase coherence. The DRFM bandwidth is
back to analog RF. This signal is frequency-converted back to
limited by the digitization rate of its ADC. Because there can
the received frequency using the same local oscillator used in
be expected to be multiple signals present in the bandwidth, a
the original frequency conversion. The use of a single oscillasignificant spurious free dynamic range is required, so the ADC
tor maintains the phase coherence of the signal through the
requires the maximum practical number of digitization bits.
down-conversion and up-conversion process.
Dynamic range is discussed in detail in the March 2007
The key element of the DRFM is the analog-to-digital conEW101 column. The dynamic range of a digital circuit is: 20
verter (ADC). It must support a digitization rate of about 2.5
log10(2n), where n is the number of digitizing bits. It is imsamples per hertz of the frequency band it digitizes, and it
portant to remember that the analog circuitry before the ADC
must output an I & Q (in-phase and quadrature) digital signal.
must have as much dynamic range as the digital circuitry. AnAs shown in Figure 2, the I & Q digitization has two samples
alog dynamic range is discussed in the same column.
per hertz of the digitized RF signal
that are 90 degrees apart in phase.
This captures the phase of the
digitized signal. Note that the 2.5
SIGNAL
FILTER
ADC
samples per hertz is greater than
IN
the Nyquist rate of two samples per
hertz that is required in a digital
receiver. This “over sampling” is required because the signal is being
COMPUTER
LOCAL
MEMORY
reconstructed. The digital signal
OSC
must typically have several bits per
sample, although there are cases in
which one bit digitization or phaseSIGNAL
FILTER
DAC
only digitization is used.
OUT
The computer performs analysis
of the captured signal, including
determination of its modulation
Figure 1: The DRFM digitizes a received signal, passes it to a computer for modification, and coherently
characteristics and parameters. The
regenerates the modified signal for rebroadcast.



JED - October 2013

Table of Contents for the Digital Edition of JED - October 2013

The View From Here
Conferences Calendar
Courses Calendar
From the President
Letters
The Monitor
Washington Report
Growing AOC’s EW Advocacy Program
World Report
Beyond CREW: Army EW Fights for a Bigger Spectrum
US Army Electronic Warfare in the Post- Global War on Terrorism World
RF Decoys for Fighter Aircraft
New Products
EW 101
AOC News
AOC 50th Annual International Symposium and Convention Guide
AOC 2013 Award Winners
Index of Advertisers
JED Quick Look
JED - October 2013 - cover1
JED - October 2013 - cover2
JED - October 2013 - 3
JED - October 2013 - 4
JED - October 2013 - 5
JED - October 2013 - The View From Here
JED - October 2013 - 7
JED - October 2013 - Conferences Calendar
JED - October 2013 - 9
JED - October 2013 - Courses Calendar
JED - October 2013 - 11
JED - October 2013 - From the President
JED - October 2013 - 13
JED - October 2013 - Letters
JED - October 2013 - The Monitor
JED - October 2013 - 16
JED - October 2013 - 17
JED - October 2013 - 18
JED - October 2013 - 19
JED - October 2013 - 20
JED - October 2013 - 21
JED - October 2013 - 22
JED - October 2013 - 23
JED - October 2013 - 24
JED - October 2013 - 25
JED - October 2013 - 26
JED - October 2013 - 27
JED - October 2013 - Washington Report
JED - October 2013 - 29
JED - October 2013 - Growing AOC’s EW Advocacy Program
JED - October 2013 - 31
JED - October 2013 - 32
JED - October 2013 - 33
JED - October 2013 - World Report
JED - October 2013 - 35
JED - October 2013 - Beyond CREW: Army EW Fights for a Bigger Spectrum
JED - October 2013 - 37
JED - October 2013 - 38
JED - October 2013 - 39
JED - October 2013 - 40
JED - October 2013 - 41
JED - October 2013 - 42
JED - October 2013 - 43
JED - October 2013 - 44
JED - October 2013 - 45
JED - October 2013 - 46
JED - October 2013 - 47
JED - October 2013 - US Army Electronic Warfare in the Post- Global War on Terrorism World
JED - October 2013 - 49
JED - October 2013 - 50
JED - October 2013 - 51
JED - October 2013 - 52
JED - October 2013 - 53
JED - October 2013 - RF Decoys for Fighter Aircraft
JED - October 2013 - 55
JED - October 2013 - 56
JED - October 2013 - 57
JED - October 2013 - 58
JED - October 2013 - 59
JED - October 2013 - New Products
JED - October 2013 - 61
JED - October 2013 - EW 101
JED - October 2013 - 63
JED - October 2013 - 64
JED - October 2013 - 65
JED - October 2013 - 66
JED - October 2013 - 67
JED - October 2013 - AOC News
JED - October 2013 - 69
JED - October 2013 - 70
JED - October 2013 - 71
JED - October 2013 - 72
JED - October 2013 - 73
JED - October 2013 - 74
JED - October 2013 - AOC 50th Annual International Symposium and Convention Guide
JED - October 2013 - 76
JED - October 2013 - 77
JED - October 2013 - 78
JED - October 2013 - 79
JED - October 2013 - 80
JED - October 2013 - 81
JED - October 2013 - 82
JED - October 2013 - 83
JED - October 2013 - 84
JED - October 2013 - 85
JED - October 2013 - 86
JED - October 2013 - 87
JED - October 2013 - 88
JED - October 2013 - 89
JED - October 2013 - 90
JED - October 2013 - 91
JED - October 2013 - 92
JED - October 2013 - 93
JED - October 2013 - 94
JED - October 2013 - 95
JED - October 2013 - 96
JED - October 2013 - 97
JED - October 2013 - 98
JED - October 2013 - 99
JED - October 2013 - 100
JED - October 2013 - 101
JED - October 2013 - 102
JED - October 2013 - 103
JED - October 2013 - 104
JED - October 2013 - AOC 2013 Award Winners
JED - October 2013 - 106
JED - October 2013 - 107
JED - October 2013 - 108
JED - October 2013 - 109
JED - October 2013 - 110
JED - October 2013 - 111
JED - October 2013 - Index of Advertisers
JED - October 2013 - 113
JED - October 2013 - JED Quick Look
JED - October 2013 - cover3
JED - October 2013 - cover4
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