77 GHz Radar for Automotive PCBs: Routing and Signal Integrity
Technology moves quickly these days, and automotive radar has transferred from primarily operating near 24 GHz to 77 GHz shortly after its introduction into new vehicles for object detection. Recent regulation changes have allowed the transition to 77 GHz, which provides a number of benefits. Shorter wavelengths facilitate wider bandwidths, and provide better resolution, smaller device form factors, and longer range. This band just happens to fall between two absorption bands for diatomic oxygen, while the 24 GHz band overlaps with an absorption band in water.
The use of higher frequencies creates a range of design, simulation, and testing challenges for 77 GHz radar modules. In addition to the design of radar modules themselves, device layout, integration into smaller form factors, and integration into the larger ecosystem within a vehicle are all design challenges on the long road to fully-autonomous vehicles.
Long-Range vs. Short-Range 77 GHz Radar
As we described in a previous post, chirped GHz pulses are used to discriminate between multiple targets within a radar system’s field of view. The use of chirped pulses provides velocity and distance detection of multiple targets by measuring the Doppler shift and beat frequency with respect a signal from a reference oscillator. The use of a phased array antenna (3 Tx and 4 Rx SFPAs) provides directional emission, allowing angle-of-approach to be determined alongside the two aforementioned quantities.
Antenna array geometry used in 77 GHz radar for automotive applications
The chirp length (measured as a frequency range) is the primary determinant of the applicability of a given radar system. Long-range radar (LRR) uses 1 GHz linear chirped pulses (76 to 77 GHz), while high resolution short-range radar (SRR) has up to 4 GHz bandwidth with linear chirped pulses (77 to 81 GHz). The frequency spread in these FMCW pulses has the potential to create some signal integrity and power transfer problems that can be solved with the right routing and layout scheme.
The rate at which the pulse is chirped (i.e., the amount of time required to sweep over the entire chirp range) defines the length of the radar pulse. In forming a radar pulse, a technique very similar to mode locking in lasers is used to actively define the pulse length. Different frequency components are actively delayed by different amounts in the transmitter side.
The pulse length is one important factor that affects the sensitivity and useful range of a system. Using shorter pulses provides higher resolution as smaller beat frequencies and Doppler shifts can be reliably detected, but these shorter pulses are more difficult to amplify as the amplifier must have broader bandwidth. This is particularly important on the receiver side of a 77 GHz radar module as the limited capability of an amplifier to properly amplify a shorter pulse skews measurement results. If the measurement determined for a driverless is incorrect, this could result in a serious accident. This particular issue needs to be addressed by RF circuit designers; working with some basic analog simulation techniques can help significantly in this area.
Routing in 77 GHz Radar Systems
If you’re in the business of designing SRR or LRR modules, there are a number of important points to consider. These points include a routing and grounding strategy, as well as a basic layout strategy to ensure signal integrity as the module operates. The corresponding grounding strategy is also important in these systems, and the grounding strategy may need to be adjusted to accommodate integration of a 77 GHz radar module into a larger system.
The trace geometry you will have a major effect on signal integrity as you route the analog output from the transceiver module to your antenna module. If you look at data on insertion loss in different trace configurations, you’ll find that traditional microstrip traces start to have much higher losses than grounded coplanar waveguides at frequencies between ~30 and ~45 GHz.
Comparison between insertion loss in microstrips and grounded coplanar waveguide from Rogers Corp.
In order to keep form factors small, the Tx and Rx antennas are normally placed on the same board. This is where some isolation is required to ensure the Tx side does not self-jam the Rx side while emitting a radar pulse. Grounded coplanar waveguides provide excellent isolation without requiring extra shielding methods. Because current tends to be confined at the edge of the central conductor in a grounded coplanar waveguide, this helps suppress intermodulation products and harmonic that can arise in other structures with rough conductors.
These aspects make grounded coplanar waveguide ideal for routing traces in 77 GHz radar systems for vehicles, in addition to plenty of other applications. Note that you’ll need to optimize these waveguides to work at 77 GHz, which will be a function of your board thickness (see below).
Single or Multiple Boards?
In general, boards for 77 GHz radar are very small, and the use of grounded coplanar waveguides can prevent inclusion of a transceiver module on the board, depending on its size. If the transceiver appears on the same board as the antenna array, the RF ground plane should span below the transceiver and run just past the edge of your antennas. If the transceiver and other circuitry take up too much space, then they can be placed on their own board.
This is actually done in some commercially available 77 GHz radar systems. The board with the antennas is placed on a ceramic or high frequency laminate (e.g., Isola or Rogers substrates), while the transceiver and other signal conditioning and processing circuitry are placed on FR4 or similar substrate. As the operating wavelength for the 77 GHz radar signal will only be about 4 mm in free space (~1 mm in FR4), your layer thickness should be as thin as possible (ideally, between one-eighth and one-quarter wavelength) in order to suppress resonance between conductive elements in different layers.
At this point, you will need to figure out the best way to connect such a high frequency line to the antenna module. Your interconnect length needs to be as short as possible, although at these frequencies your interconnects will behave as transmission lines. This requires proper termination at each end of the interconnect, and at least one return path should be routed through the conductor to provide a return path for high frequency signals.
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