pcb-rnd knowledge pool
High frequency boards
Ihighfreq by Tibor 'Igor2' Palinkas on 2020-06-07
Tags: index, high, frequency, microwave, radio, signal, integrity
Abstract: This index node collects references to other pool nodes giving an overview on what tools and techniques pcb-rnd offers to help the design of high frequency boards.
The tree classes
pcb-rnd offers three classes of tools:
It is possible to design a high frequency board without using any of these, but using these features can speed up the design process and can help making sure the resulting board fulfills all high frequency related requirements.
1. Board editing aids
1.1. Set up the layer stack properly
Layer groups have attributes . Make sure you have set at least thickness on all copper and substrate layer groups and dielectric on all substrate groups. Easiest way to do this manually:
- 1. right click on any copper layer
- 2. select the Grouping menu from the context menu that pops up
- 3. in the cross section view that's coming up, right click on any copper or substrate layer group (on the hatched area)
- 4. in the context menu that pops up select the group properties menu
- 5. the property editor pops up; attributes are at the bottom, in the a/ subtree; new attributes can be added clicking the add button.
1.2. Set up the routing style
Most high speed or high frequency boards will have transmission lines which will require to follow an exact geometry (trace width and clearance). The easiest way to do this is using routing styles:
- 1. click the new button of the route styles subdialog of the top window (normally bottom left)
- 2. a new routing style is appended and a dialog is popped up to edit it; fill in all fields (e.g. for an 50 ohm microstrip line) and close the dialog
- 3. whenever you need to draw a transmission line of this specific type (e.g. 50 ohm microstrip line), just click on the new style in the route styles and make sure you are drawing the trace in the right layer.
1.3. Transmission line calculator + autosetup
It is possible to get pcb-rnd do both of the above semi-automatically, using a calculator dialog box, using the imp_setup user script from edakrill . The advantages are that it makes it much easier and faster to set up layer group and routing style properties, plus there's no room for copy&paste error while manually coping data. It can also be used to look at current settings and calculate approximate impedances.
1.4. via fencing
Some transmission lines require via fencing: post-wall waveguides, laminated waveguides. There's an extended object for making it very easy to draw and maintain line-of-via objects , removing the step of manually creating and moving each via.
2. Design rule checker
The following DRC scripts can help checking high speed designs:
- network length matching: netlen
- enforce number of vias per net: netvianum
- microstrip impedance check: ustrip
- detect floating copper objects: floating
It is possible to use OpenEMS to simulate parts of the board. TODO: figure how this can be used for transmission lines.