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Considerations for high-frequency designs beyond 20GHz
January 21, 2020
Below points need to be considered during the design of millimeter-wave products:
1. Material Selection:
- Much care has to be taken while choosing suitable substrate material for high frequency and high speed PCBs. Dielectric constant (Dk) and Dielectric loss (Df) should be as small and stable (the smaller, the better). Low dielectric constant allows rapid signal propagation and low dielectric loss maximizes the power delivered.
- Conductors on PCB do not have perfectly smooth surfaces. Rough copper improves peel strength of laminate. Finished Copper surface should have low roughness to avoid impedance control mismatching and signal loss caused by skin effect. Surface roughness values vary from 0.025µm to 50µm.
- Ensure that the impedance and trace width calculation and stack-up for the PCB design are accurate. Impedance matching is must to prevent the signals from being reflected as they move along the various parts of a circuit or system. This in turn ensures maximum power transfer from signal source to the load.
- PCB manufacturing tolerance should be considered for impedance calculations. Tolerance should be ≤1% to avoid deviations in the performance.
- Cavity analysis should be carried out for modules to avoid unwanted resonance. Gap between the PCB to top cover should be greater than or equal to ten times the PCB thickness.
- There should not be any gap between the connectors mounted on PCB through mechanical housing. Connector should be tightly fitted in to the mechanical enclosure with a tolerance less than 0.01%. Mechanical wall to PCB tolerance also should be less than 0.1% for better grounding which improves the insertion loss and return loss performance.
- Mechanical enclosure surface roughness for PCB landing area should be of fine roughness (0.025 to 1.6microns).
- Proper system grounding scheme need to be ensured. Proper grounding scheme results in better return loss plots. Thus, increase in the RF performance.
- The trace length must be avoided to become integral multiples of quarter wavelength (such as 1/4, 1/2, 3/4, 1, 5/4, etc.,) to overcome the EMI problems and Transmission line effects. These effects can be neglected if the trace length is less than 1/10 or 1/20 of wavelength.
- The height of the microstrip board should never exceed 1/10 of a wavelength at the maximum frequency of usage.
- The trace width of a package should generally not exceed one-half of a wavelength in free space at the maximum operating frequency in order to cut off spurious modes.
- For microstrip and stripline curved lines, a minimum radius of three line widths shall be used at X-band and below and at higher frequencies, five line widths shall be used. An optimum miter is preferable instead of a curve.
- Provide maximum exposed ground between PCB and enclosure to the extent possible for better ground connectivity. Complete exposed copper is recommended on bottom side of PCB for better contact with enclosure base.
- Board edge plating shall provide more ground connectivity. In this case, mechanical enclosure walls should be in contact with the edge plating for better results.
- Routing width should never be changed suddenly. Corner should be arc or with an angle degree of 135° to avoid radio interference.
- Circuit simulations, post-layout (co-simulations) need to be carried out to validate the RF performance before proceeding for fabrication. Any deviations in the performance can be altered and simulated back to achieve the desired performance in order to avoid the hardware repeatability.