JED - March 2012 - (Page 37)
A SAMPLING OF DIGITAL RF MEMORIES
By Ollie Holt
ED last published a survey of digital RF memory (DRFM) devices in February 2000. Since then, many changes have taken place; size, weight and power requirements have decreased, faster analog-to-digital converters (ADCs) and digitalto-analog converters (DACs) have been developed, and higher density Field Programmable Gate Arrays (FPGAs), providing greater numbers of gates and more memory, have become available. These improvements have led to significant improvements in DRFM performance. Why is the DRFM so important to EW? Before the DRFM became available for use in EW systems, jamming threat radar systems was performed either by receiving the threat radar signal and storing the signal in a delay line for retransmission, or by measuring the frequency and then recreating the signal with an internal oscillator. Delay lines are bulky and have high signal loss, making them undesirable, while the jammer’s recreation of the radar signal via an internal signal generator was easily defeated in the threat radar’s signal processing. The DFRM provides a higher performance method of capturing and storing the radar signal for replay, when desired. Some of the first DFRMs were single-bit devices that “hard-limited” the radar signal, such that either a high or a low was recorded. At the time, the technology was available to build high-speed “Flip Flops” but not high-speed ADCs. These Flip Flops, when clocked at a high speed, provided a method of sampling and storing the radar’s RF signal. These devices were effective, but they had spurious signals, which could at times make them ineffective. With the development of ADCs with greater bit depth and faster sampling speeds, the spurious signal components could be reduced and thus provide much better deception performance. The figure below shows a simple block diagram of an EW jammer with a DRFM. The input signal is received and usually amplified and mixed down to some baseband through a mixer. This baseband is the center frequency (also called Intermediate Frequency or IF) as indicated in the survey responses. The bandwidth defines the lower and upper limit of the input signal to the ADC. Typically, after the mixer is a Bandpass filter with the center set at the IF, and the upper and lower limits set
by the bandwidth. The resulting signal is then sampled by the ADC at a sample rate of at least twice the input bandwidth (the Nyquist rate). The resolution, or bit-depth, defines the number of bits in each analog-to-digital sample. From the ADC, the samples are stored in a large memory device. The pulse width, or memory-depth (depicted in microseconds in our survey), defines the maximum memory size, which also defines how much of the radar signal the system can store. (Sample rate x memory size defines the maximum pulse length the DRFM can store.) After the signal has been copied and stored in the memory, countermeasures techniques can be applied to the signal. To apply the techniques, all that is required is some complex math, which can be implemented in hardware with the different gates within the FPGA. Time delays can be applied to create false radar targets in other positions, masking the true location of the target aircraft or ship. Correlated range (time) and Doppler (velocity) techniques can be generated to simulate multiple targets in different positions, denying the threat radar with useful targeting information. These modified radar samples are then loaded into the DAC and clocked out at the same rate that the radar created them. In some survey responses, the digital-to-analog output (resolution output), has more bits than the input sample. This just provides more fidelity in the output waveform. After the signal has been converted back into an analog signal, it is up-converted back to the same frequency at which it was originally received, retaining the entire coding original placed on the signal by the threat radar. (Note; it may not be exactly the same frequency depending on the Doppler applied for deception, but it will retain the same radar coding and will thus be accepted by the radar’s signal processing unit.) It is important that the same LO is used for both the down and up conversion to retain coherency.
The Journal of Electronic Defense | March 2012
Only information supplied by the survey respondents was used in this compilation. The number of responses was relatively small compared to most other JED surveys. This is primarily because the EW market for DRFM devices is limited to radar and communications jammers. In addition, commercial technology now enables many EW systems houses to manufacture their own DRFMs. This survey only covers DRFM manufacturers who sell their DRFMs as stand-alone products.
JED’s next product survey, which will run in the June issue, will cover spectrum analyzers.
Table of Contents for the Digital Edition of JED - March 2012
The View From Here
From the President
US Rotorcraft EW Programs
Technology Survey: DRFMs for EW Applications
2012 AOC Industry/Institute/ University Member Guide
Index of Advertisers
JED Quick Look
JED - March 2012