Selectivity improvement solutions proposal for UmTRX

Andrey Sviyazov andreysviyaz at gmail.com
Wed Mar 14 15:52:23 UTC 2012


Hi all!

I am again about far-near problem.

If we have heterodyne noise -135dBc/Hz at 600kHz offset (ADRF6601), then
for blocking signal at the same offset and at 200kHz RBW we get additional
noise level 135-53=72dBc relative to blocking signal level.
To keep "normal GSM900 BS" sensitivity -104dBm we must keep additional
noise as low as -107dBm, therefore blocking signal maximum level must less
then -107+73=-39dBm.
But in GSM-05.05 (sec 5.1) I saw blocking characteristics requirements for
normal BTS must be -26 dBm at 0.6-0.8 MHz offset and -16 dBm at 0.8-1.6 MHz
offset.
So, I do not know how and who can meet those requirements and I am really
hope that there are fundamental mistakes in my calculations.
Correct me please.

Best regards,
Andrey Sviyazov.



16 января 2012 г. 19:22 пользователь Jean-Samuel Najnudel - BJT PARTNERS
SARL <jsn at bjtpartners.com> написал:

> Hi Alexander,
>
> These last days, I tried to find a solution for the selectivity
> improvement.
>
> I have 5 solutions to propose. 1st and 2nd are inboard solutions. 3rd, 4th
> and 5th uses an external board. Some seems to be much better than others.
>
> 1/ We could use an IF frequency above 375 MHz to be able to connect the IF
> signal dirtectly to the LMS, without any upconvertion back to RF frequency.
> This would save some components.
> We could use the ADRF6601 (PLL/VCO + mixer) and the TB0448A IF SAW filter.
> The ADRF6601 is single chip PLL/VCO and mixer. This would be quite
> convenient.
> The TB0448A is cheap (< 3 USD), narrow band (good selectivity) and 400 MHz
> center frequency (> 375 MHz LMS lower limit).
>
> Cost of this solution would be about 60 USD and selectivity would be
> really good.
> The main disadvantage of this solution is the filter would restrict the
> signal to a single GSM carrier. This would avoid us to get both GSM
> carriers on each LMS. We would not be able to get true diversity. We would
> only be able to get switched diversity.
>
> After the LNA, RF SAW filter and the RF switches, we can split the signal
> between the current RX path to LMS RX LNA 3 and a new alternate RX path
> (ADRF6601 => TB0448A => LMS RX LNA 1).
> Depending of our need for selectivity, we would be able to select 1 of
> these 2 RX path (direct RX path to LMS RX LNA 3 or IF filter RX path to LMS
> RX LNA1).
>
> This would allow to use the board either as a normal wideband SDR board or
> with a very selective filter.
>
>
> 2/ A very nice option would be to use a variant of the 1st solution with a
> wider bandwidth SAW IF filter. For example, if we use a 400 to 600 KHz
> bandwidth IF filter, we would also get a very good selectivity and we would
> also be able to sample both GSM carriers on each LMS. This would allow a
> good selectivity and full diversity.
>
> The problem is we would need a 400 to 600 KHz SAW IF filter, with good
> selectivity, reasonable price and an IF center frequency above 375 MHz. I
> was not able to find such a filter.
>
>
> 3/ As suggested a few days ago, we may use the external selectivity
> improvement board design I sent you. Instead of the Triquint 856378 IF SAW
> filter, we could use the TAISAW TB0448A narrow band filter. This TAISAW
> filter is really much cheaper than the Triquint. This would save a lot of
> budget. However, we would still need 4 mixer and 2 PLL/VCO for each LMS RX
> path. This external board would cost approximately 100 USD (excluding PCB
> and assembly). We would need 2 of these boards for each UmTRX board. This
> would make 200 USD per UmTRX. Including PCB and assembly, toatl cost would
> be around 300 USD. This is not compeltely unrealistic but it seems still
> quite expensive.
>
>
> 4/ Another solution would be to build a single carrier version of the 3rd
> solution design. We would need only 1 RF path (PLL/VCO + mixer) with only 1
> narrow band filter per LMS RX path. This would not need any splitter or
> combiner. Design would be quite simple and cost would be about 2 times
> lower. However, as we will have only 1 carrier on each antenna, we would
> not be able to get diversity at all.
>
> This solution would finally not have many advantages compared to 1st
> solution. It would cost more and would not allow any kind of diversity.
>
>
> 5/ Last solution would be to build an external diversity improvement
> board, as 4th solution, but with a wider band IF SAW filter.
>
> We could use the following RF path:
> LNA => RF SAW filter => mixer => IF SAW filter => mixer => RF SAW filter.
> Dual mixer could be the ADL5802 connected to the ADF4350 PLL/VCO.
>
> We could use the TB0218A IF SAW filter. This filter is quite affordable (<
> 10 USD). Selectivity is good and bandwidth is wide enough to select 2 GSM
> carriers (separated by 400 KHz).
>
> Cost of such external diversity improvement board would be quite
> reasonable.
> This would be a very nice solution to select 2 GSM carriers. Connected to
> the UmTRX, this selectivity improvement board would allow to get both
> switched or true diversity.
>
> As TB0218A center frequency is 140 MHz, we would not be able to connect
> directly the IF signal to the LMS. We would need to up convert the signal
> back to the RF frequency.
> As IF down converted signal is upconverted back to the original RF
> frequency, it would be possible to use this selectivity improvement board
> with any kind of existing OpenBTS (UmTRX, USRP, SSRP...) or OpenBSC
> (Sysmocom BTS, IP.access nanoBTS...) hardware to improve the Rx
> selectivity. This would offer a wider potential market than an inboard
> solution.
>
>
> Considering all these solution, I believe 1st and 5th solutions seems to
> be the best choices. 2nd solution would also be really nice but I was not
> able to find the appropriate IF SAW filter.
> Please let me know your opinion regarding each of these two solutions.
>
> By the way, the TB0448A and TB0218A SAW filters looks really good but I am
> not 100% sure the GSM carrier spectrum distortion due to the pass band
> ripple of the SAW filter is acceptable.
> Center part of the GSM carrier (f +/- 100 KHz) is fine but side parts of
> the GSM carrier (bellow f - 100 KHz and above f + 100 KHz) may be cut a bit
> by the SAW filter.
>
> Could you also please check the TB0448A and TB0218A datasheets to double
> check if the usable bandwidth is wide enough ? Especially, do you think
> cutting a bit the side parts of the GSM carrier may cause problem ?
>
> Anyway, please let me know your point of view regarding these selectivity
> improvement solutions.
>
> Best regards.
>
> Jean-Samuel.
> :-)
>
>
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