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

[Joerg Wunsch]

As Eric Jennings wrote:

> On page 496 of the rfa1 datasheet, the comment column of the balun
> record states that the filter is included within the balun.  Is that
> indeed the case for all baluns, or just the ones noted in the
> datasheet?

This is specific for the baluns mentioned there.

Normally, a balun is a wideband RF transformer using classic magnetic
coupling.  The wikipedia article about it isn't very exhaustive, it
simply mentions the transformer.  Usually, baluns are made using
specific techniques (bifilar, trifilar, N-filar windings) to ensure
the unavoidable capacitive crosstalk matches the inductive coupling.
See the bottom part of this (German) article to get the idea:

http://www.wolfgang-wippermann.de/balun.htm

Anyway, as I mentioned, a classic balun is a *wideband* transformer.

When it comes to upper UHF, structures for a magnetic balun get really
tiny.  It's possible to substitute it by a kind of stripline technique
then, for example see page 27 of this document:

http://csus-dspace.calstate.edu/bitstream/handle/10211.9/954/FINAL%20PDF.pdf?sequence=1

However, if there's not much emphasis on wideband behaviour but only
on impendance transformation and symmetrization on a main operating
frequency, you can replace the magnetic transformer by a combination
of L and C elements.  Atmel's appnote AVR2004 documents such a design.

The advantage of such a balun is that it can easily be integrated into
a single multilayer ceramic device, because this technique can create
small C and L devices easily.  That's these typical SMD 0805 or smaller
baluns you might be used to.

But then, if you're handling Ls and Cs already, integrating some
further components for filtering seems only logical.  This gets you to
the filter baluns mentioned in the datasheet.

One disadvantage of such a structure is that the transformation and
symmetrization is only well-defined for the design frequency.  Thus,
the harmonics might meet a mighty different impendance behaviour, and
thus "leak out" at differnt spots if you don't get the PCB around the
balun right.  For example, someone in a German Usenet group reported
that they couldn't make an AT86RF212-based board (868/910 MHz)
suppress the fourth harmonics well enough with the Johanson filter
balun.  It turned out this was simply due to poor grounding of the
balun: the resulting ground inductance was large enough to drastically
reduce the filtering behaviour for frequencies above 3 GHz.

-- 
cheers, J"org               .-.-.   --... ...--   -.. .  DL8DTL

http://www.sax.de/~joerg/                        NIC: JW11-RIPE
Never trust an operating system you don't have sources for. ;-)