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High frequency PCB designers generally choose high-frequency PCB boards from several aspects: low dielectric constant, low loss factor, frequency and temperature stability, and cost (material cost, design testing manufacturing cost). The RO4350B produced by ROGERS is a low loss material for carbon hydrogen resin and ceramic filler laminates and semi cured sheets, with excellent high-frequency performance (generally applicable below 30GHz). Due to the use of standard epoxy resin/glass (FR-4) processing technology, RO4350B also has low circuit processing costs. It can be said that RO4350B has achieved the optimization of cost and high-frequency performance, and is the most cost-effective low loss high-frequency board. In order to better meet the design requirements, the insertion loss of microstrip transmission lines based on RO4350B PCB board at 24GHz was studied when designing microstrip array antennas.
The insertion loss of microstrip lines mainly includes conductor loss, dielectric loss, surface wave loss, and radiation loss, among which conductor loss and dielectric loss are the main ones. The skin effect causes high-frequency currents on microstrip lines to concentrate on thin layers in direct contact between the conduction band, ground plane, and dielectric substrate, resulting in significantly higher equivalent AC resistance than in low-frequency situations. When operating below 10GHz, the conductor loss of microstrip lines is much greater than the dielectric loss. When the operating frequency rises to 24GHz, the dielectric loss exceeds the conductor loss.
Measures to reduce microstrip line insertion loss
1. Reasonably choose board thickness and use green oil with caution
The conductor loss of microstrip lines with the same characteristic impedance decreases with the increase of dielectric thickness, while the dielectric loss remains basically unchanged. The reason is that the thicker the dielectric substrate, the narrower the microstrip line width, and the more concentrated the high-frequency current, resulting in greater conductor loss. It is worth noting that the loss tangent angle of green oil medium at 24GHz is relatively large, which will increase the insertion loss of microstrip lines. Therefore, when designing a 24GHz microstrip antenna, it is necessary to perform solder mask windowing in the antenna area.
2. Preferred LoPro copper foil
The surface roughness of the copper foil on the conduction band and ground plane is also an important factor affecting the insertion loss of microstrip lines. The smoother the surface of copper foil, the smaller the conductor loss. RO4350B offers two types of composite copper: electrolytic copper foil (ED) and low roughness reverse treated copper foil (LoPro). The surface roughness of ED copper foil is around 3um, while LoPro copper foil can reach 0.4um, effectively reducing conductor loss. Figure 2 shows the comparison of microstrip line insertion loss between these two copper foils, with a dielectric substrate thickness of 0.1mm for both. From the figure, it can be seen that the insertion loss of LoPro copper foil microstrip line at 24GHz is 40% smaller than that of ED copper foil.
3. Reasonable selection of surface treatment process
Surface treatment process is also one of the factors affecting conductor loss. There are four common surface treatment processes, including silver deposition, gold deposition (nickel free gold), nickel gold (nickel 3-5um, gold 2.54-7.62um), and tin deposition. Table 2 presents the electrical parameters of these metals, with nickel being a ferromagnetic material with a magnetic dielectric constant of 600. According to the skin depth calculation formula, the skin depth of nickel is one order of magnitude smaller than other metals, so the surface resistance of nickel is tens of times greater than other metals, resulting in much higher conductor losses in nickel gold processes than in other processes. Figure 3 compares the insertion loss of bare copper, deposited silver, and nickel gold surface treatment processes, with a substrate thickness of 20mil. From the figure, it can be seen that the insertion loss of the silver deposition process is similar to that of bare copper, but the insertion loss of the microstrip line after nickel gold surface treatment is 4dB/m (10GHz) larger. It can be foreseen that this difference will be even greater at 24GHz.
When designing a 24GHz microstrip antenna or microstrip circuit using the RO4350B dielectric substrate, we need to comprehensively consider the thickness of the dielectric plate, the type of composite copper, and the surface treatment process based on performance and cost requirements. The conclusion also applies to most of the boards in the Rogers RO4000 and RO3000 series.
RO4350B PCB
Design Techniques for Microstrip Array Antennas Using Rogers RO4350B
1. Thickness selection: thick
The degree is mainly selected based on three factors: the operating bandwidth of the microstrip antenna, the design of the feeding network, and the antenna efficiency.
a. The thickness of PCB affects the impedance bandwidth of microstrip antennas. The smaller the PCB thickness, the larger the array size, and the smaller the operating bandwidth of the microstrip antenna.
b、 The thickness of the medium affects the conductor loss of microstrip lines, which in turn affects the efficiency of microwave and radio frequency antennas. Based on the above factors, the author's design experience is to choose a thickness of 10 or 20 mil for small arrays, 20 mil for large arrays, and 10 mil for microwave RF boards.
c. The thickness of the PCB determines the line width of the microstrip line in the impedance variation section of the feeding network. For the RO4350B PCB board, with a thickness of 20mil, the linewidths of the 50 Ω and 100 Ω microstrip lines are 1.13mm and 0.27mm, respectively. The corresponding resonant length of the microstrip antenna at 24GHz is about 3mm. If the impedance of the microstrip conversion section in the feeding network is too small or too large, it will cause the microstrip antenna line to be too wide or too narrow, and the microstrip antenna line to be too wide, which can easily cause structural interference. If the line of the microstrip antenna is too narrow, it will cause processing difficulties.
2. Antenna type
Microstrip array antennas are divided into parallel feeding arrays and series feeding arrays according to their feeding methods. The parallel feeding array has longer feeders, resulting in greater losses in the feeder network. For large-scale arrays, antenna efficiency is often limited, so a series fed array with simpler wiring is generally chosen. A series fed array is a resonant antenna with a smaller operating bandwidth than a parallel fed array, but the series fed structure is easier to achieve weighted excitation. The author designed series fed microstrip array antennas of different scales. They all use 20mil thick RO4350B. As the array size increases, the impedance bandwidth gradually decreases. The bandwidth is 1.2GHz with 16 elements, while the bandwidth is only 0.75GHz with 324 elements. The frequency modulation bandwidth of 24GHz radar using continuous wave systems is usually less than 250MHz, so the impedance bandwidth of the series fed array can meet most system design requirements.
3. Interconnection between antenna and RF chip
At present, chip manufacturers have mass-produced 24GHz RF chips in the market. In the zero intermediate frequency radar architecture, the pins of the RF chip are directly connected to the microstrip transceiver antenna port. When using an antenna circuit board (high-frequency board)+multi-layer FR4+microwave RF board (high-frequency board), the antenna and RF chip are interconnected through metalized vias. In the 24GHz frequency band, the discontinuity introduced by metalized vias with a length greater than 1mm will be very noticeable. The solution is to add several symmetrical metalized grounding vias around the metalized vias to form a coaxial transmission structure. When the antenna and the RF chip are located on the same side of the PCB circuit board, the RF chip and the transmitting and receiving antenna are directly connected through microstrip lines or coplanar waveguides. This design can minimize the insertion loss of transmission lines to the greatest extent possible.
4. Low sidelobe design
The sidelobe level of the directional pattern is an important design indicator for array antennas. Low sidelobe design can reduce environmental interference outside the radar main beam. Its function is equivalent to a spatial filter, which is very effective in improving the signal-to-noise ratio of radar. The sidelobe level of uniformly distributed array antennas is greater than -13dB. In order to obtain lower sidelobes, the power fed into each element forms a certain weighted distribution of low sidelobes through the feeding network. The commonly used equal phase and unequal amplitude low sidelobe weighted distribution methods include Chebyshev cloth and Taylor distribution. It is easy to synthesize an ideal weighted distribution based on the sidelobe level and the number of elements. The remaining work is to repeatedly optimize the feeding network, so that the power fed into each element approaches the ideal distribution.
The above is an explanation of the losses of microstrip array antennas using RO4350B, as well as design techniques for designing microstrip array antennas using Rogers RO4350B PCB.