replacing the RX filter with a balun on the c139

246tnt at gmail.com 246tnt at gmail.com
Thu May 26 20:41:22 UTC 2011


Hi,


> 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.

Mmm, I doubt that. The filters probably had 50 ohm impedance as spec.
But the circuit before that / the antenna maybe was a bit off and they  
tuned it, or it might just be cost optimized, or they just tested and it  
worked better like that.

But anyway, it's probably tuned for 50 ohm unbalanced, so you can use it as  
is.


> My question is, does this matter?

Truth is ... not that much. If you look at the end of the page, you can see  
I tried the most botched up job _ever_ and it _still_ worked pretty good !


> 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

Just use the same, it'll work fine.


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

The hi band one if the one recommended for both PCS and DCS.
For the low band it's indeed E-GSM but the graphs are virtually flat,  
insertion loss varies by less than 0.5 dB. Return losses are a bit higher  
but I doubt that it matters much.

So yeah, not perfect but certainly not gonna be noticeable.


Cheers,

Sylvain
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