Re: [uracoli-devel] Question regarding LQI and ED values [Update]

[Eric Jennings]

On Aug 28, 2012, at 10:23 AM, Joerg Wunsch wrote:

>> I haven't done any UHF design either.
> 
> During a discussion on avrfreaks.net about a couple of years ago, I
> ran a short "design study" for how *I* would start with designing an
> ATmega128RFA1 prototype board:
> 
> http://www.avrfreaks.net/index.php?name=PNphpBB2&file=viewtopic&t=94891&start=20#724170

This is a great thread--I wished I had come across it before designing our 
first prototype.  Still--super good info to have.  I took a bunch of notes in 
my design workbook for the next revision.


> Also, in order to meet the timeline to order the batch of our PCBs (we
> needed 15 pairs of boards at an acceptable price, which requires quite
> a few weeks of lead-time), I have not been able to really fine-tune
> the antenna circuitry on the prototype PCBs before starting the full
> PCB batch.  Later on, it turned out shortening the antenna to an
> overall width of 40 mm (by simply scratching off a bit of copper on
> each side, or by drilling a couple of holes), radiated energy could be
> improved by another 10 dB, without any much impact on the radiated
> harmonics.

Really interesting antenna design.  Did you try to see what your maximum 
working distance was between two boards (despite the fact that the design goal 
only needs them to be a few meters away from each other?)  I've been curious 
how trace antennas do vs the chip/whip/etc antennas w/ these chips.


> The first and most important thing is: good grounding.  Of course, if
> you can afford a multilayer PCB, that's always quite easy, because
> you've got a full ground plane inside, and can tie any partial ground
> plane on the top layer to the full GND plane inside by a number of
> vias through a really low impedance path without any much problems.

Trying to avoid the 4 layer, as it's twice the price at the fab house we're 
using.


> If you only have two layers, the job is a bit more challenging.  Try
> maintaining large ground areas on at least one side.  The more they
> are distributed in both, X and Y dimensions, the smaller is their
> inductance.  Spend as many GND vias as possible in the vicinity of the
> RF circuitry, to make the ground connections a distributed net rather
> than small wires.  Just keep the 1 nH / mm in mind.  1 nH is 15 Ω
> impedance.  (Now you might understand why Johanson really wants *four*
> GND vias at their balun.)

Excellent.  Duly noted!


> The RFN/RFP pins are a *symmetric* input/output, so all circuitry
> attaching to them should be kept symmetric; either till a symmetric
> antenna, or to the balun that converts the signal to asymmetric.

Ah, my current design is not.  I'll note this as a change too.


> If you follow that, I'm sure the first prototype will already become a
> success, which only needs minimal rework for the second revision
> (after making some RF measurements).

I certainly hope so!  Your tips and hints have been super.  Thanks so much.  
I'll be getting these boards tested soon and we'll see just how bad the RF 
capabilities are.  I have some power section testing to do as well, and once 
all design errors are accounted for, will get the second version done with the 
changes you've recommended.  I'll also be referring to the avrfreaks forum 
thread you posted too, there's a wealth of info for RF design for newbies in 
there.

Thanks again!
Eric