Microstrip

Cross-section of microstrip geometry. Conductor (A) is separated from ground plane (D) by dielectric substrate (C). Upper dielectric (B) is typically air.

Microstrip is a type of electrical transmission line which can be fabricated with any technology where a conductor is separated from a ground plane by a dielectric layer known as "substrate". Microstrip lines are used to convey microwave-frequency signals.

Typical realisation technologies are printed circuit board (PCB), alumina coated with a dielectric layer or sometimes silicon or some other similar technologies. Microwave components such as antennas, couplers, filters, power dividers etc. can be formed from microstrip, with the entire device existing as the pattern of metallization on the substrate. Microstrip is thus much less expensive than traditional waveguide technology, as well as being far lighter and more compact. Microstrip was developed by ITT laboratories as a competitor to stripline (first published by Grieg and Engelmann in the December 1952 IRE proceedings).[1]

The disadvantages of microstrip compared with waveguide are the generally lower power handling capacity, and higher losses. Also, unlike waveguide, microstrip is typically not enclosed, and is therefore susceptible to cross-talk and unintentional radiation.

For lowest cost, microstrip devices may be built on an ordinary FR-4 (standard PCB) substrate. However it is often found that the dielectric losses in FR4 are too high at microwave frequencies, and that the dielectric constant is not sufficiently tightly controlled. For these reasons, an alumina substrate is commonly used. From monolithic integration perspective microstrips with integrated circuit/monolithic microwave integrated circuit technologies might be feasible however their performance might be limited by the dielectric layer(s) and conductor thickness available.

Microstrip lines are also used in high-speed digital PCB designs, where signals need to be routed from one part of the assembly to another with minimal distortion, and avoiding high cross-talk and radiation.

Microstrip is one of many forms of planar transmission line, others include stripline and coplanar waveguide, and it is possible to integrate all of these on the same substrate.

A differential microstrip—a balanced signal pair of microstrip lines—is often used for high-speed signals such as DDR2 SDRAM clocks, USB Hi-Speed data lines, PCI Express data lines, LVDS data lines, etc., often all on the same PCB.[2][3][4] Most PCB design tools support such differential pairs.[5][6]

  1. ^ Grieg, D. D.; Engelmann, H. F. (Dec 1952). "Microstrip-A New Transmission Technique for the Klilomegacycle Range". Proceedings of the IRE. 40 (12): 1644–1650. doi:10.1109/JRPROC.1952.274144. ISSN 0096-8390.
  2. ^ Olney, Barry. "Differential Pair Routing" (PDF). p. 51.
  3. ^ Texas Instruments (2015). "High-Speed Interface Layout Guidelines" (PDF). p. 10. SPRAAR7E. When possible, route high-speed differential pair signals on the top or bottom layer of the PCB with an adjacent GND layer. TI does not recommend stripline routing of the high-speed differential signals.
  4. ^ Intel (2000). "High Speed USB Platform Design Guidelines" (PDF). p. 7. Archived from the original (PDF) on 2018-08-26. Retrieved 2015-11-27.
  5. ^ Silicon Labs. "USB Hardware Design Guide" (PDF). p. 9. AN0046.
  6. ^ Kröger, Jens (2014). "Data Transmission at High Rates via Kapton Flexprints for the Mu3e Experiment" (PDF). pp. 19–21.