Issue link: https://iconnect007.uberflip.com/i/1464168
68 DESIGN007 MAGAZINE I APRIL 2022 0.023"-sized anomaly is part of the propagating wave path, it is now equivalent to 24% of the wavelength or about 87 degrees of the wave. Having a 24% portion of the wave affected by an anomaly may have an impact on the prop- agating wave and cause distortion and other unwanted wave properties. Additionally, at mmWave frequencies, the high frequency material used in the PCB con- struction is typically thin. A thin laminate will translate to a narrower conductor to maintain a certain impedance. A typical conductor etch- ing tolerance of ± 0.5 mil is usually acceptable for most lower frequency applications, but with mmWave applications this tolerance may not be tight enough to give good RF results. e reason is a narrower conductor with a total variation of 1 mil will have more impedance shi, as compared to a wider conductor. Some impedance matching networks in the antenna array areas of the auto radar sensor have con- ductors as small as 5 mils in width. A difference of 5 to 6 mils on a thin substrate can be a dif- ference of 6 ohms impedance. However, when using a thicker substrate for lower frequen- cies and the appropriate wider conductor to maintain a given impedance, a 1 mil difference in conductor width will make an impedance difference of less than 1 ohm. ere are many items in circuit fabrication which can alter the impedance of a circuit and having very good etching control is a major advantage for a PCB fabricator working with mmWave applications. ere are several different RF structures used in mmWave applications, and the struc- ture that is normally least affected in RF perfor- mance by the normal variations in circuit fabri- cation processes is microstrip. e microstrip construction is typically the top two conductor layers on a multilayer RF board and has a signal conductor on top and a ground plane below. Another structure oen used in mmWave circuitry is grounded coplanar waveguide (GCPW). is structure is also a two-copper layered circuit; however, the top signal plane has three conductors electrically oriented as ground/signal/ground. is configuration is very good to minimize certain unwanted wave characteristics at mmWave frequencies, how- ever this structure may be impacted more by normal variations of circuit fabrication. e space between the ground/signal/ ground in the GCPW is critical and if that space varies much, so will the RF performance of the circuit. However, it is more complicated than that. On the top signal plane, there are strong electric fields between the neighboring ground planes which are on both sides of the signal conductor. In a cross-sectional view, if the side- walls of these conductors are straight, there will be more fields in air. However, if the conduc- tors are trapezoidal shaped, there will be fewer fields in air and more fields in the substrate. e difference of these fields being in air impacts the effective Dk the propagating wave experiences. Air has a Dk of about 1 and when the fields use more air, the wave will experience a lower effec- tive Dk, and that can impact multiple RF circuit properties. at is especially true at mmWave frequencies. To have more consistent RF per- formance when using a GCPW at mmWave fre- quencies, controlling the conductor width and spacing is important, but having a consistent conductor shape is also very important. ere are many things to consider in the cir- cuit fabrication process for achieving a consis- tent conductor width, space, and shape. e image and etching process using an additive Having a 24% portion of the wave affected by an anomaly may have an impact on the propagating wave and cause distortion and other unwanted wave properties.