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RFM12B and vRestrict

Added by MobileWill about 6 years ago

Hi, I am designing a small micro inspired by the JeeNode with the RFM12B radio. The eagle footprints I find all have a vRestrict area underneath a good portion of the radio. Can I remove that or is that really needed? I really need the space for routing since I am trying to fit everything in slightly over an inch squared.

I am using the JeeNode library footprint.

Thanks.
-William


Replies (8)

RE: RFM12B and vRestrict - Added by dzach about 6 years ago

Hi

This may be an answer to your question:

There are two RFM12B nodes in the picture: an Arduino Nano v.3 with an RFM12B module wired without any rf shielding and a JeeNode v6 with the RFM12B module on the pcb with the vRestricted area underneath. The copper on the pcb provides rf shielding to the RFM12B module reducing the noise in the receiving frequency, as it is shown in this snapshot of the spectrum of the two boards:

RE: RFM12B and vRestrict - Added by MobileWill about 6 years ago

Wow, thanks for the details. What a difference. I think I was trying to do too much in such small space. Now I am going to try a larger size, definitely worth keeping the vRestrict area. I am using the Dangerous Prototypes sick of beige PCB sizes.

RE: RFM12B and vRestrict - Added by dzach about 6 years ago

Well, as you see my temporary wiring leaves a lot to be desired so you might not get this much of a problem or any problem at all if you pcb is carefully laid out. Best thing, give it a try if you can, measure it and then decide.

The nRfMon program is available here https://github.com/dzach/nrfmon, if you want to give it a try. The github code is a little older than the one I’m using right now, but it’ll give you an idea. It’s open source.

RE: RFM12B and vRestrict - Added by MobileWill about 6 years ago

Thanks, I will try that. I think even without the radio I would have a hard time routing. Now I am closer with the 31x50mm size. Might even been room to add a LiPo charging IC.

RE: RFM12B and vRestrict - Added by tve about 6 years ago

Does the vrestrict area also act as a ground plane for the antenna?

RE: RFM12B and vRestrict - Added by martynj about 6 years ago

The vrestrict area is only a routing guide. It is preferable to keep any traces with fast edges and/or imposed HF noise out of the area. This is to reduce noise coupling directly into the RFM12B module.  Since the gap between trace and module is a few mm, the coupling capacitance is very small, but at MHz frequencies is enough to let through some energy right into the ISM band of interest, raising the noise floor of the receiver section.  Traces on the module side  have more effect than underside traces since the larger separation reduces the coupling capacitance value (but is increased again by a factor of k = relative permittivity of the PCB material). 

If vrestrict is observed, then this area is available for copper flood.  This can can do no harm in improving shielding. It is some  help to the ground plane effect for a directly-connected, vertical wire from the antenna pad, but this effect is weak anyway since an effective ground plane needs to extend ~lambda/4 from the driving point in all directions.  Clearly with the connector at the PCB corner, this is not true for most mounting arrangements.

The real issue here is the overall system target.  If the communication path is short and/or at low baud rate, then the received Signal/Noise ratio is high enough to swamp these effects.  If the system is to push the envelope on range/transfer speed, then these and other issues need attention.

RE: RFM12B and vRestrict - Added by MobileWill about 6 years ago

For the most part I use the auto router. The vRestrict only keeps out via’s but not traces. I was trying to fill the gaps with a ground plane. Sounds I shouldn’t eve have traces there. Makes sense then in the JeeNode design why it is at the end of the PCB. I am finding it very difficult to make a micro with the radio on the back side or on top next to the uC.

RE: RFM12B and vRestrict - Added by martynj about 6 years ago

IMHO, to squeeze MPU and an RFM12B with optimum RF performance onto a PCB similar in size to the RFM12B, you need to use a 4 layer board.
Since most free layout tools are limited to two layers, an alternative is ‘poor mans 4layer’ - meaning stack two thin 2layer by aligning the pads/vias and soldering thru to make the ‘upper’ to ‘lower’ connections.

A considerable challenge to Eagle et.al. layout skills !

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