Re: [Openbts-discuss] RF tweaking

[<ope...@ol...>]

Hi all,

On Sat, 2010-02-27 at 17:25 +0100, Nor Gsm wrote:
> 2010/2/26 Sylvain Munaut <24...@gm...>
>         Nor,
>         
>         Yes thanks a lot :) I'm working on a RF front end for openbts
>         so
>         that's precious info :)
>         
>         First, I'm a little surprised that moving the RX2 SMA before
>         the
>         antenna is such a gain. After all, once C202 is removed and
>         the pin
>         grounded, there is no more connection to the TX side through
>         there.
>         What am I missing ?
> My best guess is that the signal leaks through the common power
> supply. 
>  
>         Second, when looking at the figures choosing a LNA, we noticed
>         that we
>         couldn't put a very strong one if we didn't want to saturate
>         the input
>         stage for close phones (think more picocell where the bts can
>         be only
>         a couple meters from the antenna). My reasoning was such :
>         
>          - Phone TX power is regulated but the lowest then can
>         transmit at is
>         0 dBm on a 0 dB gain antenna
>          - Traveling trough 1 m of air: about 40 dB loss
>          - Simple RX antenna 3 dB gain
>          - Duplexer insertion loss 3 dB
>          - Our external LNA : x dB gain
>          - Our external RX filter insertion loss : 3.5 dB
>          - The RFX LNA MGA82563: 13.5 dB
>          - Maximum input level at the AD8347 (when set at minimum
>         gain) : - 2 dBm
>         
>         So :
>         
>         -2 dBm = 0 dBm + 0 dB - 40 dB + 3 dB - 3 dB + x dB - 3.5 dB +
>         13.5 dB
>         => x = 28 dB  absolute max for the LNA (after that ->
>         saturation in the mixer)
>         
>         And that's without much safety margin.
>         In the end, we chose a 15 dB LNA because there wasn't one ~ 25
>         (only
>         single stage 15-16 or double stage 31-33 ...).
>         
>         Do you see anything wrong with my math here ?
> No. 15 dB sounds like a good choise for close range operation. And,
> provided you have the required tx/rx isolation (two RFX cards), you
> can easily reconfigure for wide area coverage by turning up the AD8347
> gain.
>  
I am working with Sylvain on the RF front.

Apart from the duplexer and LNA there will be place for one or two
saw-filters on the USRP RX input side.

Our calculations show that one saw filter should be sufficient, but just
in case we put room for a second one.

Did any of you experiment or do measurements with saw filters on the
USRP RX input.

Best Regards,

Martin Dudok van Heel



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