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

[Eric Jennings]

On Aug 28, 2012, at 7:32 AM, Joerg Wunsch wrote:

> As Eric Jennings wrote:
> 
>> The datasheet says that the max ED value is 0x53, so with the boards
>> right next to each other, I'm only getting around -48dBm, or half of
>> the ED value that's theoretically possible?  Meh, maybe these
>> Sparkfun boards aren't as great as I first thought.
> 
> Well, how to put it politely? ;-)

You were very polite.  The problem it seems with RF design is that some people 
know how to do it well, others don't, and many ascribe it to some sort of black 
magic.  The best thing that can happen is to have an honest critique of what 
works and what doesn't--especially when discussing a specific circuit design 
like the Sparkfun board.  And the depth of description of what was done well 
and what wasn't done well is immensely helpful to someone like myself--tasked 
with getting a well-performing board designed using the rfa1.

The good news as I see it is; despite the poor RF design on the Sparkfun 
circuit, the board is still able to work for testing purposes.  That's a 
testament to the chip I'd say.  (Also reminds me of Eric W.'s story he told me 
about one of the Atmel engineers needing an antenna to test a dev board, and 
unbent a paperclip and stuck it in the through-hole--voila, instant antenna!)

> Whoever designed that PCB appears to have never done any UHF design
> before.

I haven't done any UHF design either.  I've studied the Eagle files for the 
Zigduino, the Picopter, and the Sparkfun board, and I now see how both the 
Zigduino and Picopter have several vias surrounding the balun to give a good 
connection to ground vs. the Sparkfun board.

I also reviewed Dresden Elektronik's manual for their rfa1 board, and see how 
they tuned the RF circuit to ensure 50Ω with inductors.  It would be ideal to 
see the board layout file for the DE board, but looks like they're not 
open-sourced. 

Regardless, every board design will probably require its own specific tuning.  
I'll keep both the ground vias and the 50Ω tuning in mind when laying out the 
next revision of our board.


> Oh, and don't forget to tie the TST pin to GND, they accidentally left
> it floating ...  (The pin is only required for parallel programming.
> In all other cases, it ought to be connected to GND.)

Thanks for the note.  I luckily found out about this before receiving the 
boards, and it was the first thing I fixed upon delivery.  (Though I tied both 
CLKI and TST to ground, per the datasheet recommendation.)

Thanks for the great info!
Eric