Tuesday, February 12, 2008

triple use circuit cards

One of the common requests is RF, Digital and and an antenna on the same printed circuit card. Requirement one is to keep the digital out of the RF, and requirement two is to get an antenna to work well on the same board.

Digital signals need to be contained. Any lost energy will seen as system noise and thats a bad thing. The technique I used on boards where noise and loss was critical was a 5 layer board. The top, middle and bottom were ground planes. Layers 2 and 4 were used for signals - one side for digital and one side for RF. The edges of my boards were plated from top to bottom creating a very low impedance bond between all ground layers.

You want a dielectric constant around 2 o 2.5 to help contain the energy of the signals on the traces.
Antennas want to launch waves and cannot be built on the same substrate. They want a substrate that allows the signals to escape easily and to this end, antennas rarely want ground plane under them.

Antennas can be mounted on printed circuit cards, especially on the edge and raised just a bit so there is air between the antenna and the circuit card.

Antennas on high dielectric substrate simply will not work, the energy will not launch waves effectively.

Antennas can be built on FR4 if the frequency is low, simply by using one layer only in the antenna area.

No comments: