TECHNICAL BRIEF
Generating Precise FMCW and Chirp Radar Test Signals with Low-cost Devices
RF/microwave signal generators offering high speed frequency modulated continuous wave (FMCW) and frequency modulation on pulse (FMOP), or “chirp” capabilities, are depended on by radar system engineers in many fields. In particular, those designing microwave test sets for receivers in highly precise radar systems, such as anti-spoofing and anti-jamming applications. To achieve the reliable FMCW and chirp modulation necessary, one would typically rely on a full-featured bench-top unit. But many are discovering that today’s robust, portable, and programmable RF/microwave signal generators offer a real, low-cost, and flexible alternative. This Tech Brief provides insight on this self-managed approach and offers tips and tricks on how to get up and running fast and with confidence in every pulse. TECHNICAL BRIEF Generating Precise FMCW and Chirp Radar Test Signals with Low-cost Devices
Intoduction...... 2
FMCW and Chirp Modulation Radar. . . 3
The Signal Generator’s Key Role. . . . .5
Chirp Radar Field Test Challenges. . . . 6
Anti-jamming/Anti-spoofing Radar Test with Multiple Signal Generators. . . 6
Conclusion...... 7
Testing in Remote Locations or in Harsh Conditions is a Challenge for Traditional Bench-top Test Instruments.
Frequency modulated continuous wave However, there are still uses and the laboratory or the field. In the (FMCW) radar technology was devel- benefits to older radar techniques, case of field testing, chirp radar can oped back in the 1950s following such as linear FMCW for aircraft height be found in harsh environments from World-War II to provide better resolu- measurement during landing proce- remote radar locations, naval plat- tion and discrimination of radar dures, automotive radar, smart ammu- forms, aircraft maintenance facilities, targets. Once unclassified in the 1960s, nition sensors, liquid filling levels of or even servicing land-mobile systems there has been steady development of tanks and bulk solids in silos, and wherever they may be. radar technology beyond a simple naval tactical navigation radar. linear FMCW chirp to more advanced Given that a significant amount of radar technologies, such as coherent An extremely useful tool in develop- FMCW and chirp radars use frequen- pulsed radar using pulse compression ment and testing of FMCW radar and cies well into the microwave and techniques. These techniques have more advanced chirp radar is a signal millimeter-wave spectrum, such as resulted in frequency modulated chirp generator with frequency sweeping C-band, X-band, and Ku-band, the test (FM Chirp), barker codes, and other and with chirp modulation, which may equipment used to develop and test more complex modulated pulse also be used to develop and test pulse these radars are general large bench- compression radar techniques. compression techniques and devices in top devices with a wide range of
www.vaunix.com Page 2 TECHNICAL BRIEF Generating Precise FMCW and Chirp Radar Test Signals with Low-cost Devices
built-in features and a price-tag to re-calibrate, which most often must be modulation. Fortunately, Vaunix match their size. Moreover, large done at a certified facility taking weeks produces LabBrick signal generators bench-top signal generators are of lead time on repairs. that are portable, programmable, and typically designed for laboratory can produce self-triggered and exter- conditions, and usually don’t operate A modern and much more viable nally triggered chirp modulation, phase well in extreme environments. This is solution for many space constrained continuous linear frequency sweep, especially true if there is a substantial and rugged applications is the use of and pulse modulation. These devices amount of vibration or shock forces a portable signal generator. However, aren’t much larger than a standard occurring during transportation of most of the portable signal generators wallet, and are available at a fraction these units. Along with a high price- on the market do not reach common of the cost of repairing a typical tag, these bench-top units are also FMCW and chirp radar bands, and bench-top signal generator with generally expensive to repair and don’t offer any form of chirp or pulse the same capabilities.
FMCW and Chirp Modulation Radar
FMCW modulation is similar to contin- FMCW waveforms are sawtooth, The basic concept of FMCW modula- uous wave (CW) radar modulation triangular, square-wave, and stepped, tion is that the received signal from with the exception that instead of or staircase, modulation, though the an object is mixed with the transmitted holding a single tone the frequency most common is saw-toothed or signal producing an intermediate of the CW radar increases or decreases “swept-frequency” FMCW radar. frequency (IF) tone that is proportional linearly, or non-linearly. Common to the delay, hence distance to the object. Multiple objects will produce multiple tones, which can be separated f f and identified using a Fourier Trans- t Δ T C form. The result is an FMCW radar that R C consists of a relatively simple circuit with a transmitter, receiver, FMCW Δ f synthesizer, mixer, digitizer, signal fD S processor, and display. In the case of t t t a radio altimeter, the IF signal is sent f directly to an analog gauge with a A TC frequency dependent coil meter that exhibits greater inductive impedance at higher frequencies (though not linear, IF this is still an effective method for t S determining relative altitude). f t
(Top) FMCW “chirp” signal as a function of time and FMCW radar response and processing. frequency. (Bottom) FMCW “chirp” signal as a function of amplitude and time
www.vaunix.com Page 3 TECHNICAL BRIEF Generating Precise FMCW and Chirp Radar Test Signals with Low-cost Devices
The benefits of FMCW over CW is that function of pulse width. However, high range resolution experienced by pulse the frequency modulation allows for pulse powers and short pulse widths radars with long pulse widths while much closer minimum range measur- require specialized high power modula- maintaining a high energy pulse ing capability, which is roughly limited tors and transmitters, which are without extremely high peak power to the transmitted signals wavelength. generally large and expensive. Using levels. Frequency modulation on pulse, Moreover, FMCW radar enables the solid-state technology, a pulsed radar or FMOP, is often referred to as “chirp” simultaneous measurement of a can be made much smaller at a radar, though somewhat confusingly, target’s range and its relative velocity possible cost savings, though sol- linear frequency modulation is also with much higher accuracy range id-state technology typically doesn’t often called a chirp signal. measurements than CW. FMCW outperform vacuum tubes in high modulation also can be used to frequency and high pulse power Additional benefits of chirp radar discriminate between multiple targets, applications. is that the broadband and random requires relatively simple circuitry, and distributed noise in the receiver is doesn’t require any pulsing with high Hence, the use of intrapulse modula- greatly reduced by the pulse compres- peak powers. tion and pulse compression allows for sion filtering process, which allows for longer pulse widths at relatively high the sensing of signal returns that are An advancement of this technique is power levels. By modulating a pulse, below the noise floor of the pulse. to combine pulse radar concepts with the benefits of range resolution and This is commonly known as the pulse intrapulse modulation/pulse compres- multi-object discrimination can be compression gain (PCG). Moreover, sion. Pulsed radar performance achieved. Pulse compression is a the pulse compression filter adjusts benefits from extremely high pulsed technique where a longer pulse is the relative phases of the receive signal power and short pulse width, as a processed in shorter sub-pulse seg- frequency components, which actually pulse radar’s range resolution is a ments, thus mitigating the loss of yields better maximum range than
B f ∑
f2 ∑
f3 VC ∑ f ∑
f5
f5 f f3 f t 2 f t D A
A D f f2 f3 f f5
FMCW radar block diagram A diagram of the pulse compression circuitry for a linear FM chirp radar. www.vaunix.com Page 4 TECHNICAL BRIEF Generating Precise FMCW and Chirp Radar Test Signals with Low-cost Devices
t t
The input and output signals of a pulse compression state with the receive signal within the noise and output echo clearly discernible. would be predicted by the standard advanced driver assistance systems circuitry. Long test ranges and precise radar range equation. This phenome- (ADAS), and aerospace altimeter and positioning of radar targets would non is known as the pulse compression other sensing applications. There are otherwise be required to test radar in ratio (PCR). also many legacy radar systems that the field, where a signal generator with are still in use today in aircraft and the appropriate modulation could be The performance of FMCW and chirp naval ships that require servicing used instead to simulate radar targets. radar depend on the performance of and retrofit testing. Moreover, high purity signal generators the radar components. Hence, added can also be used to test the radar noise/noise figure, linearity, phase Signal generators are a key test receivers signal-to-noise (SNR) perfor- noise, and spurious harmonics are all instrument for radar testing, and mance, which can be used to deter- key parameters for the active compo- are most often used to gauge the mine the maximum range capability nents within a radar system. While any performance of the radar receiver of the radar and if that falls within the of the active and passive components within a radar’s transmit/receive signal chain can substantially degrade the Transmitter performance of the radar system, it is onv ion extremely important that the frequency a an i na synthesizer excels. oc in mo u a ion The Signal
Generator’s i na i na n c ion n c ion Key Role Though there are more complex radar modulation schemes, FMCW and chirp a an i na radar are still very common in a variety oc in mo u a ion of applications, including air defense, own onv ion marine navigation, automotive eceiver
FMCW radar block diagram www.vaunix.com Page 5 TECHNICAL BRIEF Generating Precise FMCW and Chirp Radar Test Signals with Low-cost Devices
specification for a given radar. Pulsed designed to be carted into in the field or externally triggered (pulse trigger chirp modulation signal generators can and set up in potentially harsh condi- port). Advantageously, the LMS-183CX also be used to test the pulse compres- tions. These units often cost upwards can be purchased for roughly a third of sion circuitry of a radar receiver by of $100,000 and are large/heavy units the price of repairing a typical bench- sending calibrated compressed pulses that contain their own displays and top signal generator with comparable to the pulse compression delay lines computing processors. A downside performance. Also, the PC-driven and measuring how well these circuits of this complexity and one-size-fits-all nature of the Vaunix LabBricks allows meet specifications. approach is that these units tend to for the testing personnel to use their become inoperable if any of the own secure PCs or laptops, and no A radar receiver may also be tested components fail, which is likely under testing information is stored on the under varying signal conditions using the harsh travel and field conditions LabBrick, unlike with bench-top test degraded radar signal characteristics these units are exposed to. Costs for units with their own processors with and noise. This can be done with repairing these units can be in the area networking capability. multiple signal generators, one produc- of a quarter of the cost of a new unit, ing the simulated radar response and and can take several weeks to months. another producing the calibrated Anti-jamming/Anti- impairments, such as in-channel and These units also often have a variety of out-of-channel interference signals and features that are unnecessary for this spoofing Radar noise. This type of testing can be used type of testing, which only complicates to determine the anti-blocking and the test procedures. Some full-feature Test with Multiple sensitivity of a radar receiver. signal generators may be externally Signal Generators programmable and triggerable, which Radar transmission circuitry may also can be advantageous in reducing the In the case of newer radar and during be tested using a signal generator in complexity and time of the testing retrofitting of radar systems on older place of the radar systems frequency setup, but often require the purchase platforms, testing and maintaining the synthesizer. In this way, prototype radar and use of proprietary software specific radar once installed is essential. As transmission circuitry may be tested to the test instrument brand. radar technology has progressed, so and undergo troubleshooting separate- has radar jamming and spoofing ly from the frequency synthesizer and Another more affordable and practical technology. This is why it is important modulation circuitry. approach to chirp radar field testing is to not only test the operation of radar, to use PC-driven and portable signal but also their anti-jamming/anti-spoof- generators that are already designed ing capabilities. With modern software Chirp Radar Field to be compact, rugged, and easy to defined radar, new anti-jamming/ use. An example of this is the Vaunix Test Challenges anti-spoofing techniques can be LabBrick Signal Generator LMS-183CX. implemented in existing radar by Though virtually any signal generator This signal generator provides a changing the programming. capable of linear FM chirp pulses can calibrated signal output from 6 GHz be used to test such radar in the field, to 18 GHz with frequency sweep, Testing the effectiveness of this most signal generators with frequency linear FM chirp modulation, and pulse programming, however, is much more capability of C-band and beyond are modulation that can be internally complicated. In some cases, it may be relatively expensive units and are not triggered (programming or PC-control) useful to test anti-jamming/anti-spoof-
www.vaunix.com Page 6 TECHNICAL BRIEF Generating Precise FMCW and Chirp Radar Test Signals with Low-cost Devices
ing techniques using signal generators simulating various radar targets and Conclusion jamming/spoofing systems. In these Signal generators are a key tool for situations multiple signal generators the development, testing, and mainte- are extremely useful to simulate the nance of critical FMCW and chirp radar targets and jamming/spoofing systems. technology. Though traditional bench- PC-driven and low-cost portable signal top signal generators may have their generators with a compact footprint place in the development and testing can be extremely beneficial in these of radar, there are many circumstances testing situations, as multiple-synchro- where the cost, size, complexity, and nized signal generators can be used fragility of these units mitigates their simultaneously to simulate a more usefulness. In these cases, rugged and realistic scenario than a single bench- portable signal generators, such as top signal generator, still at a fraction Vaunix Lab Bricks, can be a low-cost of the cost. Signal generators, such and extremely adaptable solution to as Vaunix Lab Bricks, are even rugged solving a variety of challenging radar enough to be deployed in field testing testing scenarios. scenarios of anti-jamming/anti-spoof- ing technology with minimal environ- mental protection.
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