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Andrey Sviyazov andreysviyaz at gmail.comHi 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. > :-) > > -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.osmocom.org/pipermail/umtrx/attachments/20120314/1ad5a018/attachment.htm>