Re: [avr-chat] Sample program for ATmeGA128RFA1

[David VanHorn]

That wasn't our experience.  The traces are symmetrical, and we used the filter balun from Johanson. 

We failed emissions by a large margin, and when we went looking for problems, found that the antenna impedance was way off.  It was copied from an app note, but that design was apparently for a PCB surrounded by lots of air where ours has plastic within a few mms.  So we re-designed the antenna for improved impedance match and the harmonic emissions are now 3dB or so better than spec, and we are running wide open on TX power.  No filtering other than the balun, which feeds the re tuned PIFA antenna.

We tried terminating with various impedances and verified that the harmonics increased strongly if the return loss was worse than 12dB but didn't have time to make measurements and plot a curve.

On Fri, May 17, 2013 at 2:40 PM, Joerg Wunsch <address@hidden> wrote:

David VanHorn <address@hidden> wrote:

> These chips also spew harmonics if the output impedance is too far
> off.

The key to a good supression of the second harmonics is to
keep everything symmetric.  That way, most of the 2nd harmonics
that still leave the internal PA will compensate outside.

It's really recommendable though to add external lowpass filters.
The baluns recommended in the datasheet do already have them.  But
if you use a symmetric antenna, you better add two PI filters.

(ETSI harmonics supression is just reachable even without them, but
not much margin is left, so Tx power should be reduced a bit to gain
margin.  FCC harmonics supression requirements are a bit harder, and
always require a lowpass filter.)

--
cheers, Joerg               .-.-.   --... ...--   -.. .  DL8DTL

http://www.sax.de/~joerg/
Never trust an operating system you don't have sources for. ;-)

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