Millimeter-wave radio signals are hitting the roads, in the form of radar sensors which have become part of Advanced Driver Assistance Systems (ADAS) for vehicles. Radar signals at 77 GHz, for example, have proven to be quite effective for ADAS and are a key component in the development of “self-driving” autonomous vehicles. But what kind of printed-circuit-board (PCB) materials are the best fit for millimeter-wave signals that will be used for ADAS and autonomous vehicles? Millimeter-wave frequencies have their own sets of circuit material requirements, often different than those for circuits at lower RF and microwave frequencies.
Keeping losses to a minimum is a worthwhile goal for any high-frequency circuit design, but especially important at millimeter-wave frequencies, where signal power is harder to come by. This ROG BLOG is Part One of a two part series introducing the key criteria to consider when selecting a PCB substrate which will minimize circuit losses for 77 GHZ radar PCB antenna applications. First we will discuss components of PCB circuit loss, and then introduce six key material properties critical to developing low loss millimeter wave circuits at 77 GHz.
The total loss of a millimeter-wave circuit, such as a 77 GHz automotive radar PCB antenna, is usually described as its insertion loss. A circuit material’s insertion loss has four components: conductor loss, dielectric loss, radiation loss, and leakage loss. The first three have significant impact, while the last one, leakage loss, is usually considered negligible, even at millimeter-wave frequencies.
Conductor loss is usually the most significant of the four and is typically tied to the choice of transmission-line technology, and to the parameters related to that technology for a circuit material, such as type of copper, skin depth, and conductor width.
The type of copper, for example, contributes a great deal to the conductor loss of a millimeter-wave circuit, with smoother copper having less loss than copper with a rougher surface. For example, circuit material with rolled annealed copper conductor has a smoother copper surface and will suffer less loss, especially at 77 GHz, than standard electrodeposited (ED) copper conductor with its rougher surface. The skin depth of the copper—how deep the RF current is within the copper in the copper-substrate interface—also affects conductor loss. Skin depth is frequency dependent, with the loss increasing with increasing frequency. When the surface roughness of the copper conductor has the same dimension or is near the same dimension as the skin depth, the surface roughness of the copper conductor will significantly increase the conductor loss of that circuit material.
The ROG Blog posed in the past (December 7, 2016), “why not just stick with a familiar transmission-line technology, such as microstrip, at millimeter-wave frequencies (see http://www.microwavejournal.com/blogs/1-rog-blog/post/27523-comparing-transmission-lines-for-millimeter-wave-circuits)?” Microstrip is certainly straightforward for the design and fabrication of high-frequency circuits, with its copper top signal plane and copper bottom ground plane separated by a dielectric layer. But other transmission-line formats, such as stripline and grounded coplanar waveguide (GCPW), each offer benefits at millimeter-wave frequencies.
Microstrip is a popular transmission line for circuits from RF through 30 GHz, with relatively low conductor losses. Above 30 GHz, it tends to have higher radiation loss than stripline and GCPW. Radiation loss is frequency dependent, increasing with increasing frequency. It is also dependent upon a circuit material’s dielectric constant (Dk), decreasing with increasing Dk.
The thickness of a circuit material will also impact its radiation loss, with thicker circuits having greater radiation loss. Spurious wave mode interference can increase radiation loss in millimeter-wave circuits. To minimize such spurious wave mode interference in millimeter-wave circuits, thinner dielectric materials are usually combined with narrower conductors. For this reason, millimeter-wave circuits with their short wavelengths tend to be fabricated on thinner circuit materials, regardless of transmission-line architecture.
A circuit material’s dielectric loss is mostly related to the dissipation factor (Df) of the circuit material. This type of loss can affect some circuit structures more than others, where more dielectric material is used. For example, dielectric loss can be worth tracking in stripline circuits in which several layers of dielectric material are used. It is also more noticeable in thicker rather than thinner microstrip and GCPW circuits. Circuits with a soldermask will also add to a circuit material’s dielectric loss, more for GCPW than for microstrip circuits.
Parameters at 77 GHz
Circuit design and fabrication is less forgiving at higher frequencies, such as 77 GHz, where signal wavelengths are short and signal power levels are limited. But several different transmission-line technologies can be effective at millimeter-wave frequencies, if high-resolution circuit features can be fabricated, and if circuit materials can provide consistent characteristics.
Low-loss stripline circuits at millimeter-wave frequencies, for example, depend on multiple consistent dielectric and copper conductor layers interconnected by means of plated through holes (PTHs). Each PTH adds some capacitance and inductance to the circuit layers, which can alter performance at millimeter-wave frequencies, especially if the conductor and dielectric layers differ in their characteristics.
Of course, at 77 GHz it is important to minimize loss as much as possible, since the signal levels tend to be small. No one transmission-line technology is an automatic choice for circuits at such a high frequency. However, the characteristics of circuit materials at millimeter-wave frequencies can serve as guidelines for choosing the best materials for circuits at millimeter-wave frequencies. Along with minimizing circuit losses, six key circuit material properties are critical to implementing effective, low-loss millimeter-wave circuits at 77 GHz:
- Dk tolerance
- Circuit material Df
- Copper conductor surface roughness
- Thermal coefficient of Dk and Df
- Moisture absorption
- Glass weave effect
When these six material properties are analyzed in terms of using RO3003 circuit material from Rogers Corp. at millimeter-wave frequencies, the material clearly has the properties that favor its use for such high frequencies. It may explain why RO3003 laminate has become a widely used circuit material for 77 GHz and for other millimeter-wave circuit applications. Understanding these six key material properties and how they relate to electrical performance at millimeter-wave frequencies can help circuit designers seeking optimum performance at 77 GHz. Just as 2.4 GHz became associated with widespread wireless communications applications like Wi-Fi, 77 GHz has become the “automotive radar” frequency. The next ROG Blog will take a closer look at the six material properties, especially how they relate to RO3003 laminate and its use at 77 GHz and other millimeter-wave application frequencies.
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