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.
:-)
