High-speed PCBs are the pinnacle of PCB technology. These PCBs can further be categorised as digital high speed, mixed signal and RF microwave PCBs. Out of these, RF and Microwave PCBs handle the highest frequencies among the high-speed PCBs, and one of the most prominent applications of RF and microwave PCBs is in RADAR or Radio Detection and Ranging. Many Radar PCBs also host digital signals as well which requires design considerations of mixed signal PCBs.
RADAR PCBs are considered one of the most advanced PCBs since they can be very difficult to design and manufacture. This article will give you a brief idea about various aspects of Radar PCBs.
What are Radar PCBs?
Radar is technology that detects objects while determining the speed, distance, etc., using Radio waves and microwaves. A basic radar system transmits radio waves using an antenna, and these waves reflect off objects. Based on the returned radio waves’ time delay, speed is determined using various techniques depending on the radar being used. For processing of these radio waves, they must travel from the antenna to the RF components with minimum attenuation, where the radar PCBs play a vital role.
Radar PCBs generally handle signals from 1 GHz to 77 GHz, which demands the highest standards of signal integrity and EMC since at these frequencies signals are more prone to electromagnetic interferences, crosstalk, signal reflection and skin effect. In addition to the electrical challenges, these PCBs are used in critical applications like aerospace and defence, which also adds the factor of high reliability of the PCB. All the challenges combined make Radar PCBs quite difficult to manufacture and design.
Design Considerations for Radar PCBs
Radar PCBs are notoriously difficult to design. However, by following some industry-set practices and procedures, Radar PCB Designs can be achieved. The following are some design guidelines for designing Radar PCBs –
- Choosing the Right Layer Stack – RF PCBs handle both digital and RF signals that may require different grounds, and therefore, you might require more than 2 dedicated ground planes that may increase the number of layers. In some cases where the digital signals do have very high frequencies, the PCBs can have a hybrid material layer stack. The layer stack will determine the characteristics of the traces and also the manufacturability of the PCB.
- Choosing the Right Material – General materials like FR-4 can not handle signals higher than 1-2 Ghz, such materials are also known as standard loss materials (technically, standard loss materials can handle high frequency signals if the trace length is short enough). For High-frequency signals, ideally, one must check the datasheet of the material. Additionally, an estimate can be made by checking the loss tangent or the dissipation factor. A lower dissipation factor translates to lower attenuation at higher frequencies.
- Impedance and Length Matching – High frequency signals traces, be it analog or digital signals, are required to be impedance matched and length matched. Generally, RF signals use 50 Ohm impedance matched traces, which are required to be maintained along the length of the trace. Radar PCBs also transmit and receive digital signals, which use differential impedance in accordance with the protocol that is used for communication. However, many digital signals used in such protocols have very low rise and fall times and use more than one channel for data transmission and reception. Such signals are required to be length-matched to ensure data integrity.
- Maintain Proper Ground Shielding – There are various ground shielding methods that are used to protect against crosstalk and EM, thereby improving signal integrity and EMC. Use of Ground traces and Vias along the path of high-speed signal traces, and uninterrupted ground planes beneath high-speed signals to provide an uninterrupted return path.
Please note that you do not use the ground plane beneath the trace antenna or antenna modules mounted on the PCB. This will severely restrict the reception of the antenna, thereby deteriorating performance.
Materials Used in Radar PCBs
Like all the RF and microwave PCBs, the material of the PCBs plays a significant role in the performance of the PCB with respect to electrical, thermal and mechanical performance as well as the cost and manufacturability of the PCB. A few commonly used laminates in Radar PCBs are –
- RO4350B – RO4350B is one of the most popular high-speed materials considered as one of the best low-loss materials available in the market, considering its availability, its performance and its cost. This laminate from Rogers Corporation has a dielectric constant of 3.48 and a dissipation factor of 0.0037. The fabrication process of this laminate is similar to that of FR-4, hence, it can be easily used in hybrid layer stacks at a cheaper cost.
- RO3003 – RO3003 is high high-performance laminate from Rogers Corporation that is significantly expensive than RO4350B but is several times better in performance. It is an ultra-low-loss material that was engineered for RF applications. It has a Dielectric Constant of 3.00 and a dissipation factor of 0.0013. This laminate is extensively used for Radar PCBs since it is been in the market for quite some time now.
- Astra MT77 – Astra MT77A is a high-performance laminate specifically designed for RF and millimetre applications. This laminate from Isola Group is an ultra-low-loss material with exceptional electrical and mechanical stability across a wide temperature range. It has a dielectric constant of 3.00 and a dissipation factor of 0.0017. Its wide operating temperature makes it an ideal choice for automotive Radars.
- Taconic RF-35 – Taconic R-35 is a popular laminate widely used in high-frequency applications. It is an ultra-low loss material with a Dielectric constant of 3.5 and a dissipation factor of 0.0018. It is the preferred material for Frequency Modulated Continuous Wave (FMCW) Radar and Doppler Radar.
Application of Radar PCBs
Different Radar PCBs are required for different types of Radar. The following are a few types of Radar that are used in various applications according to the Radar’s working principle –
- FMCW or Frequency Modulated Continuous Wave – Used for measuring range and velocity, widely used in altimeters
- Doppler Radar – Measures velocity via frequency shift of reflected signals, widely used in motion detectors.
- Pulsed Radar – Measures range using the time of flight of short pulses, widely used in long-range surveillance Radars.
- Phased array Radars- Electronically steer beams without moving parts, unlike traditional Radars, widely used in 5G base stations.
- SAR (Synthetic Aperture Radar) – Creates high-resolution 2D/3D images usually of Earth’s surface, mostly used in satellites.
- MIMO (Multiple Input Multiple Output Radar) – Enhances the resolution of a Radar input and detects objects using multiple antennas. This radar is gaining popularity and is widely used in Automotive radars.
Conclusion
Radar PCB designs usually require a team with expertise in RF, Simulation and most importantly, PCB design. With Radar PCB designer requires extensive knowledge of materials, the working of radar, design techniques and the manufacturer who has the capability to fabricate the PCB with the precision required. For new designers, staying informed about the topic is highly recommended.
