replacing the RX filter with a balun on the c139

Joshua Lackey jl at thre.at
Thu May 26 20:06:30 UTC 2011


I've been going over Sylvain's report on removing the RX filters,
http://www.246tnt.com/gsm/rx_filter.html, and I had some slight
differences on the c139 that I'm hoping someone can help me with.

Similar to Sylvain's experience, the schematic doesn't match reality.

Just as Sylvain describes, the signal from the balanced output appears
to be going through inductors rather than capacitors and there isn't a
bridging inductor.

However on my c139, the unbalanced input is missing the inductor to
ground on the EGSM path and is missing the capacitor to ground on the
DCS path.  (Sylvain had a capacitor to ground on DCS where the schematic
has an inductor to ground.)

You can find a picture of what I'm looking at here:
http://thre.at/c139/c139rxfilters.jpg .  I've placed the c139 schematics
in that directory as well, http://thre.at/c139 .

My guess is that the filters in the schematics had a different
unbalanced impedance than the ones they ended up placing on the c139.
But I'm not an EE, and that is just a guess.

I can't read the markings on the filters I have, so I can't check the
unbalanced input from a data sheet.  I also don't have a way to measure
the components that are there now so I can't try and compute the value.

My question is, does this matter?  That is, should I use the same balun
that Sylvain chose or should I find one with a different unbalanced
impedance?  Alternatively, should I use the same baluns and just install
an inductor (or capacitor on DCS) to ground so that it matches Sylvain's
c123?

Any ideas?

(As an aside, the baluns that Sylvain chose aren't perfect for the US
frequency range, but they should work okay.)





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