Why is Chirp Used in Radar?
Pulse manipulation using modulation is one of the techniques that brought us the first radio broadcast and later advances involving wireless communications. Modulation involves superimposing information on the amplitude, frequency, or phase of a broadcasted signal, essentially broadening its bandwidth and causing one of these to vary over time. In an application like radar, modulation does more than superimpose some information on a signal, it allows certain information about a reflecting object to be determined from a collected signal.
Chirp is one of the modulation formats used in most radar systems that can allow fast collection of slow-moving and fast-moving objects. Chirp is better known in the lasers community as pulse compression, a form of time-dependent phase modulation that generates a change in the instantaneous frequency of an emitted pulse. In radar, its usage allows simultaneous resolution of an object’s heading and range, which can then be used to track targets within the radar’s field of view.
Modulation Formats in Radar
Radar systems operate on a principle similar to echolocation, but they use a modulation format to enable tracking of two important metrics (range and heading) for an identified object. Range is a simple matter of measuring the time-of-flight for the collected radar pulse, and the use of an antenna array will provide an estimate of the direction towards the object. The heading for a fast object can be more difficult to determine from a single radar pulse, and it requires the signal emitted from a radar to be modulated.
This leads to two modulation formats commonly used in radars: pulse-Doppler and chirp. The latter is sometimes known as frequency modulated continuous wave (FMCW) emission as chirp is a form of phase modulation that results in frequency modulation. Other quadrature modulation formats can be used, such as amplitude modulation, however these two modulation techniques have proven to be technically viable over time and have persisted as the primary modulation formats used in radar systems.
Advantages and Disadvantages of Chirped Radar
There are some important reasons for using these two modulation schemes; these are outlined in the table below.
|
Pulse-Doppler |
FMCW (chirp) |
|
Emits single pulses (square wave)
|
Emits linearly ramped pulses
|
|
Pulse train (Power vs. Time): |
Pulse train (Frequency vs. Time): |
|
Advantages:
|
Advantages:
|
|
Disadvantages:
|
Disadvantages:
|
FMCW chirp is the standard modulation format used in most solid-state radars, and the advantages listed above should illustrate why the technique is used in automotive. In an automotive setting, the objects to be tracked tend to be moving slower, they tend to be closer to the radar system, and tracking of nearby objects (other vehicles) would need to occur in very short time periods.
How Radar Chirps Are Generated
FMCW radar involves broadcasting a signal where the frequency is modulated linearly over time. In general, chirp generation can involve linear or nonlinear phase modulation that modifies the instantaneous frequency. Linear chirp is used because the signal processing required once the pulse is received is simpler to implement and can occur faster in the radar transceiver or DSP block.
Chirped pulses can be generated in modern FMCW radar with a DAC and VCO. The goal is to continuously broadcast a single frequency within the radar’s bandwidth for a specific duration, after which the frequency is iterated to the next value in a stair step fashion. Each frequency to be broadcast is directly synthesized with the DAC and VCO. The result is emission of a sinusoidal wave with the frequency increasing linearly over time. The image below shows a block diagram for this type of system and the resulting waveform.
The graph in the bottom half of the image shows the emitted waveform in the time domain. Each time window corresponds to a different frequency that has been generated digitally. Analog generation of a chirped waveform is also possible and it would produce the smooth transition between different frequencies depicted above, however it is more complex to implement in an integrated transceiver. It is much easier to design a processor that can integrate these functions with a digital approach.
Signal Processing
Once the waveform is received, the RX signal is passed into a mixer where it generates a beat frequency with high accuracy. The beat frequency can be read out with an ADC, extracted with a Fourier transform, and finally used in the standard radar equations for range and velocity determination. The timing can be used to first determine the range, and the Doppler shift can be extracted by comparing the expected shift in the received frequency with the actual frequency.
The challenge in vehicular chirped radar systems has been their low angular resolution, which can allow multiple objects to contribute to the reflection from a radar scene. These would produce multiple beats and timing measurements that have to be distinguished with a constant false alarm rate (CFAR) algorithm. To reduce this signal processing burden, newer radars are using larger arrays with the goal of finer angular resolution.
Design teams that want to build and demonstrate advanced radar systems can comprehensively evaluate their system functionality with the complete set of system analysis tools from Cadence. Only Cadence offers a comprehensive set of circuit, IC, and PCB design tools for any application and any level of complexity.
Subscribe to our newsletter for the latest updates. If you’re looking to learn more about how Cadence has the solution for you, talk to our team of experts.