PCB Feeds

This topic discusses RF feed basics applying to RF PCBs and the specific ways to set up feeds in a PCB component.

Basic Feed Concepts

Feeds provide signal input and output interfaces to printed circuit boards (PCB). Feed ports on the PCB may be placed along a board edge, or within the interior of the board (away from any edge). Feeds can be implemented on single layer and multi-layer PCBs. From the feed port the signal will be routed internally within a PCB layer. In a multi-layer board the signal may also be routed between various layers through conductive vias.

There are two fundamental types of signal feeds, whether serving as an input or output port:

• Single-ended (also called unbalanced) feed.

  A single-ended feed consists of one signal line and a ground return. In a single-ended configuration the signal is measured between the signal trace and the ground return. The most common type of single-ended PCB feed is a microstrip line, that consists of a signal trace, usually on the top board layer, and a ground plane on a layer directly below the top layer.

  This figure shows a microstrip transmission line created using the microstripLine object. Two red balls indicate signal ports locations. Note that the red balls do not represent any kind of physical connection or connector, but only port locations. For information on physical feeds see User-Defined Feeds.

• Differential (also called balanced) feed.

  A differential feed consists of two coplaner signal lines. An optional ground plane is included to control impedance and provide shielding. The signal is measured across the two signal lines, that are 180 degrees out of phase. A common differential configuration feed implementation is an edge-coupled differential stripline where two parallel lines share identical electrical and physical characteristics.

  Differential feeds provide excellent immunity to external ingress of electromagnetic signals, including noise, because coupled external signals result in a common mode signal across the pair of lines that cancel out when measured across the pair. Differential feeds reduce unintentional radiation of internally generated signals because the external electromagnetic fields from the pair of lines cancel each other, resulting in EMI reduction, and improved signal integrity within the PCB.

  The figure shows a balanced transmission line created using the coupledStripLine object. Four red hemispheres indicate signal port locations. Note that the red hemispheres do not represent any kind of physical connection or connector, but only port locations. For information on physical feeds see User-Defined Feeds.

Location-Defined Feeds

By default, feeds in a PCB Components Catalog object or in a pcbComponent object only show locations of signal feed ports. The type of feed at a port is automatically set up by the software based on the type of object and its geometry. The feed details are not revealed. The advantage of using location-based feeds is that you do not have to select the appropriate feed type, and manually create the feed geometry. When using location-defined feeds, any analysis of a PCB element, such as s parameters, employ the automatically created feed. To use a default feed set the FeedFormat property to "FeedLocations" in the pcbComponent object.

Note

When starting with a PCB Components Catalog object you must first convert the object to a PCB component object by entering pcb = pcbComponent(catalogcomponent) in the command line, where "catalogcomponent" in the name of the catalog object.

User-Defined Feeds

The other way to work with feeds is to manually create feeds using available feed definition objects within the pcbComponent object .The advantage of this method is that you can manually select the feed type and control its detailed geometry. Experience with RF PCB feed applications and geometry is recommended to efficiently use this method.

To create user-defined feeds:

1. The FeedFormat property must be set to "FeedDefinition" in the pcbComponent object.

2. Use the FeedDefinitions property to set the feed type in the pcbComponentobject. There is a choice of six feed types that can be configured. The choice of which feed type to use depends on the application of the PCB element.

3. Configure the chosen feed type from within the individual feed-type object.

For further details on creating user-defined RF PCB feeds refer to individual feed-type reference pages.

This table shows typical applications for feed definitions supported in pcbComponent.

Feed-type Applications

Type	Applications
ProbeFeed	Microstrip patch antennas; vertical launch feed
EdgeFeed	Microstrip elements whose signal originates on the PCB edge with one plane; stripline elements with two ground planes
DeltaGapFeed	Balanced structures, dipole antennas, lumped loads
CoaxialFeed	Higher-resolution version of a probe feed
FiniteGapFeed	Coplanar multi-ground edge feeds, finite-width delta-gap feed
ArbitraryFiniteGapFeed	Complex feed in which one or more signal locations are excited with respect to one or more ground locations

Probe Feed

The ProbeFeed is a type of contact feed used for patch antennas, or where a vertical PCB feed configuration is required. Feed point is placed anywhere in the antenna element, which facilitates impedance matching. Figures show probe placement, and feed properties.

Edge Feed

The EdgeFeed is a type of contact feed used when an edge connection to a PCB element is required. An edge feed is used with either microstrip elements, or stripline elements with two grounds.

The figure shows an exploded view of an edge feed to a stripline element with two grounds. Captions in the figure correspond to feed Properties you can specify.

Delta Gap Feed

The DeltaGapFeed is a type of contact feed used for balanced structures, dipole antennas and lumped loads.

Captions in the figure show Properties you can set.

Coaxial Feed

The CoaxialFeed is a type of direct contact feed. It is similar to the probe feed, but provides higher resolution. The figure shows an exploded view of a coaxial feed. Signal is fed to the element layer using a via, which runs from the center conductor of a coaxial cable to the via's pad. The via's antipad isolates the signal from ground. Coaxial ground runs to the ground layer. Captions in the figure correspond to feed Properties you can set.

The figure provides a detailed coaxial feed view.

Finite Gap Feed

The FiniteGapFeed is a type of contact feed used for coplanar feed applications. The figure shows a single-gap configuration. The captions correspond to Properties you can set.

Arbitrary Finite Gap Feed

The ArbitraryFiniteGapFeed is a type of contact feed used for complex configurations that include multiple signals and multiple grounds. The figure shows an exploded view of an arbitrary finite gap feed. The captions correspond to Properties you can set.