Variable delay lines offer continuously variable range selection for radar target simulators.
Radar test and calibration require target simulators possessing considerable testing flexibility within a single instrument. Features such as smooth scanning of a target through range, Doppler simulation and radar corss section (RCS) variability are desirable for a wide variety of radars and radar waveforms. This capability would benefit the radar community during simulation and hardware-in-the-loop (HWIL) testing. One of the major shortfalls of current target simulators is the lack of available technology and components to realize smooth target motion across a long range window. Until now, options for range simulators have been limited to fixed delays or discrete delay selection through the use of tapped delay lines. A new technique for realizing continuous range-motion is currently available for radar target simulators and other applications.
This paper describes a radar target simulator that utilizes continuously variable delay line (CVDL) technology.(1)(2) The availability of this technology allows the radar tester to simulate continuous target motion and to perform complex target maneuvering. Other advantages of this technology include low cost (relative to digital radio frequency memory (DRFM)) devices and low spurious signals. In this paper, the theory pertaining to CVDL technology is presented and simulator design issues are presented. The prospect for multiple threat simulators and clutter simulations will also be discussed.
Many delay line types are available for radar target simulators, including surface acoustic wave (SAW), bulk acoustic wave (BAW) and fiber optic. However, all produce only fixed delays when used alone or quantized delays when used in combinations. Recent advances in photonic technologies have led to the development of CVDLs that provide long, adjustable delays for RF signals. A serious limitation of SAW and BAW delay lines for radar target simulator applications is that the output is fixed in location on the crystal, thereby limiting the time delay to a constant value.
An acousto-optic (AO) CVDL is shown in Figure 1. The principal components are a laser diode, a focusing lens, an AO cell, a mechanical translation device and a photodetector. The signal to be delayed is input to the AO cell, where it drives a piezoelectric transducer that launches an acoustic wave into the AO crystal. The acoustic wave propagates through the AO crystal, at velocity [v.sub.a], much like SAW or BAW delay lines. At a distance d from the transducer, the acoustic wave is converted back into an electrical signal having a delay of
[MATHEMATICAL EXPRESSION OMITTED]
The basic concept defining the CVDL allows the laser beam to be injected into the crystal at a variable distance d, where it is modulated by the acoustic column and converted into a delayed replica of the input by a photodetector. To change the time delay, the position of the optical beam is moved, with respect to the transducer, thus changing the distance d and the associated time delay.
Table 1 lists specifications for CVDL-based radar target simulators. Other technologies, such as SAW, BAW or fiber-optics, might compete with some of these specifications, but cannot produce variable delays with repeatability on the order of 5 ns. The center frequency and bandwidth are determined by both the AO material choice and the configuration of the AO transducer. A wide range of center frequencies are achievable through pre- and post-delay mixing. In addition, system dynamic range is determined by several factors, including laser diode power, and the laser diode and output amplifier noise figure. Total insertion loss is affected by many factors, including laser diode power and AO efficiency. However, internal amplification can be used to minimize insertion loss while maintaining range performance in excess of 45 dB.
TABLE 1 SPECIFICATIONS FOR CVDL BASED RADAR TARGET SIMULATOR
Center Frequency(1) (MHz) 30 to 18,000 Bandwidth (MHz) 10 to 80 Delay [T.sub.3][O.sub.2] ([micro]s) > 50 (max) 0.4 (min) flint glass ([micro]s) 7 (max) 0.15 (min) Delay Repeatibility (Digital Control) (ns) < 5 Dynamic Range (10 MHz Bandwidth) (dB) > 45 Input Power (dBm) (max) (1 dB compression) 0 Spurious signal suppression (dB) > 50 Insertion Loss (dB) < 10 Maximum Range Rate (m/s) 24,000
(1.)User selectable RF center frequencies available through pre- and post-delay mixing.
Maximum delay is another important key parameter determining the choice of delay line configuration. CVDLs utilizing AO cells made from flint glass deliver a maximum delay of 7 [micro]s, but delays greater than 50 [micro]s are achieved with tellurium dioxide (Te[O.sub.2]) as the AO material. In radar target simulator applications, a 50 [micro]s delay would permit simulating target ranges over a 7.5 km range window.
Electronically controlled delay selection is implemented through the use of a translation stage driven by a high precision stepper motor. When a new delay value is desired, a command is sent to the stepper motor. This command repositions the stage corresponding to the desired delay. To affect target motion, the translation stage position is controlled, as a function of time, enabling both constant velocity and accelerating/decelerating targets to be tested.
CVDL-Based Radar Target Simulators
Incorporation of the CVDL into current radar target simulators solves the problems of the simulation of a large set of range delays and the simulation of a target smoothly moving through radar range gates. In the radar target simulator, the CVDL and other RF components are used to test a radar signal processor by simulating the time delays, amplitude changes and Doppler signatures that correspond to target range changes. The delay can be imposed at either the system intermediate frequency (IF) by directly inserting the delay line into the IF chain; or at the RF frequency (X-band and mm-wave) by providing frequency translation both before and after the variable delay line.
Mathematically, the narrowband representation of the radar received waveform can be written as
[MATHEMATICAL EXPRESSION OMITTED] where a(t) = amplitude modulation [phi](t) = phase modulation [f.sub.IF] = system IF
This representation is typical of actual applications where the radar waveform is reflected from the target, and experiences round trip delay, attenuation, Doppler shifting and degradation by receiver noise. Ideally, the signal exiting the target simulator would be described by
[MATHEMATICAL EXPRESSION OMITTED] where w(t) = amplitude weighting, such as RCS control [f.sub.D] = induced Doppler frequency of the target [[tau].sub.d] = time delay corresponding to target range Target range R and time delay are related by
[MATHEMATICAL EXPRESSION OMITTED] where c = 3 x [10.sup.8] m/s
The primary innovation of the CVDL-based simulator is its ability to implement the continuous delay function described in Equation 3. RCS and Doppler control are readily implemented using standard RF techniques. For example, a 50 [micro]s CVDL provides up to 7.5 km of range with 0.75 m repeatability (5 ns). To augment the total range of the delay coverage, fixed delay lines can be used in conjunction with the CVDL. For example, by using the CVDL and three additional fixed delays, range coverage of 50 km can be achieved. More importantly, it means that a target can be smoothly scanned through multiple range cells.
Using this simulator, a wide variety of radar tests can be performed. For example, target detection tests can be performed versus a wide variety of conditions, including stationary and moving targets, different target range, different target Doppler and different target RCS. This type of closed loop probability-of-detection test greatly reduces the cost of radar system acceptance, calibration and performance evaluation.
In addition, range tracking can be verified by changing the delay line in a manner consistent with actual target movement. Suppose it is desired to simulate a radial velocity of dR/dt. Since the time delay and target range are coupled, the required rate of change of the time delay, d[[tau].sub.d]/dt, that corresponds to the desired radial velocity can be calculated. Given the relationship between the [[tau].sub.d] time delay (distance to a target), and the acoustic velocity, the required translation parameters for the CVDL can be determined. As an example, suppose that a target is moving radially with respect to the radar at 150 m/s. The rate of the time delay is
[MATHEMATICAL EXPRESSION OMITTED] or 1 [micro]s/s. Since the AO tap distance and time delay are proportional, the physical speed of the laser beam tap (dl/dt), relative to the AO crystal, is
[MATHEMATICAL EXPRESSION OMITTED] where [v.sub.a] = acoustic velocity
For this example, the required translation speed of the mechanical stage is only 0.6 mm/s. Radial target velocities up to 24,000 m/s can be simulated using commercially available stages.
Figure 2 shows a block diagram of a CVDL-based radar target simulator. The unit consists of an RF front end, the delay unit (CVDL), Doppler signature and RCS enhancement unit. In addition, an interface/control unit is included to provide user control of the delay, RCS, and Doppler signature. The RF front end is used for frequency conversion from the signal RF to the CVDL IF (typically in the 30 to 150 MHz range). The interface control unit provides user control of the simulator through either an RS-232 or an IEEE-488 bus. Figure 3 shows a photo of a CVDL-based range delay unit (RDU) built for the US Army MICOM. This computer controlled RDU has an X-band center frequency, a 3 dB bandwidth of 10 MHz and a maximum delay of 50 [micro]s. Computer control of the delay and target motion is accomplished via an RS-232 interface.
Future Growth Options
In addition to single target simulation, producing multiple target returns from a simulator would provide a significant testing enhancement as well. With such a capability, complex clutter simulators could be developed for use in an HWIL simulation and multiple target generators could be used to test track and ECM functions.
Investigations are underway to extend the CVDL concept into a multiple false target generator. To produce multiple delays, the AO cell must be tapped with several (> 50) optical probes, as shown in Figure 4. Techniques for implementation include using a laser diode array or binary optical elements to generate the necessary optical taps. By varying the intensity of a laser in a diode array, for example, the signal output for that specific time delay can be varied in amplitude. In effect, this is equivalent to having a bank of attenuators followed by a bank of delay lines. The attractive feature about this approach is that these functions can be performed in a small integrated package.
A CVDL based multiple false target generator could be made to mimic a wide variety of scenarios by putting the laser diode intensity under computer control. The actual complexity of the false target generator depends on several factors, including the type(s) of false targets to be replicated, that is, fixed-wing aircraft, landing craft or helicopters, and the type of radar/processing to be defeated. However, with this unique delay line concept, it should be possible to develop a multifunctional false target generator meeting advanced deception requirements.
CVDL-based radar target simulators offer new opportunities that have never been available to test and characterize radars and other RF equipment. The availability of this technology offers continuous target motion, complex target maneuvering, low spur levels and low cost. Using AO techniques, CVDL based simulators with bandwidths up to 80 MHz are available at user specified frequencies.
(1.)C. Anderson, R. Berinato, M. Zari and T. Barrett, "Acousto-Optics Pulls Delays to Longer Lengths," Microwaves and RF, January 1993.
(2.)C. Anderson, M. Zari and R. Berinato, "Continuously Variable Delay Lines," US Patent No. 05247388, 21 September 1993.
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|Author:||Anderson, Chris S.; Zari, Michael C.|
|Date:||Apr 1, 1994|
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