power plane emi
Should we implement power plane in RF PCB design at 2.45GHz?
Or wide tracks sufficient for the current required will do the job.
I have read few contradictory notes regarding use of power plane in RF PCBs.
Cheers,
Its better
It depends on the power handling /drop by the track.
Share with us, we will be able to let you when to use what.
I seldom use a power plane anymore. They just serve to distribute stray RF signals and digital noise to where you do not want them. Also, they force you to use your first two layers as signal and ground plane, and then your dielectric thickness is very small (lossy lines).
Rich
www.MaguffinMcrowave.Com
I avoid power planes like the plague.
I see them like the ultimate low impedance transmission line -- once something gets on there, you really can't fight it. Of course, something *will* get on there. I don't mind bussing power around with wide strips, but always bring the power up/down to the layer where your device is, pass through your decoupling network and feed what you need to feed -- you'll get isolation in both directions this way and you've defined the flow (and when debugging, you can redefine the flow without drilling vias).
Lance
The contradictory notes which i am talking abouts are:
One document says:
Placing a distributed Power plane between 2 ground plane layers enables an evenly distributed RF decoupling capacitance between the supply and ground. In addition, the power plane provides a very low impedance at radio frequencies.
Another suggets:
Power planes can be disastrous if used on the RF sections of a PCB. The Power planes tend to act as patch antennas and spurious signals can find their way all over the PCB.
Kindly suugest.
Cheers,
These statements are not contradictory.
The reason is the first statement talks about multilayer signal, GND power GND combination and is correct
where as the later talks signal with power plane (split) on top layer itself. This is also correct.
Power planes were very popular back in the 1980's for digital designers. They thought that the power plane made one big capacitor, and that somehow helped with digital crosstalk. What really was going on is that they did not have a clue. As the digital electronics moved up in clock rate, the power plane became a good way to CAUSE digital crosstalk and supply glitches.
I suppose there might be low frequency applications where there is a good reason for one, but I can not think of one off the top of my head.
If you want high capacitance between your power supply line and the ground plane, try a chip capacitor.
In RF I seldom use them because most of my circuits require lowpass filtering in the various DC supply lines. This is to either keep spurious signals from re-entering a signal path, or to keep EMI radiation to a minimum for communications circuits. If you have a "power plane", it is had to drop a couple of ferrite beads along the way to de-Q the bias lines!
Jeez, power planes! I have enough trouble trying to keep the ground plane EMI free!
Rich
www.MaguffinMicrowave.com
right on!