Re: [Discuss-gnuradio] RF Gain and BB gain of XCVR2450

[Jason Abele]

On Mon, Nov 1, 2010 at 11:52 AM, Eric Blossom <address@hidden> wrote:
> On Mon, Nov 01, 2010 at 05:36:35PM +0100, Jorge Miguel wrote:
>> Hello,
>>
>> I will rephrase former question.
>
> Jorge, I don't know the answer to your question, but I do that the
> answer is in the source code that is associated with the XCVR2450.
>
> You haven't specified whether you're using the UHD or not and whether
> or not you're on a USRP1 or USRP2, all of which may matter.
>
> In any event, I'd look for files that contain "2450" in their name :-)
>
> "find" and "grep" are powerful command line tools for this kind of
> stuff, and would allow you to answer your own question quickly, or to
> at least generate a more refined question, that indicated that you'd
> "tried the obvious".
>
> To increase your chances of getting assistance on this list (or any
> list for that matter), please read and follow the suggestions here:
>
>  http://www.gnuradio.org/redmine/wiki/gnuradio/ReportingErrors
>
> Eric
>
>
>
>> When using the XCVR2450, up to 92dB gain in rx mode is allowed.
>> However this gain is shared between to places. Gain at RF and then gain at
>> baseband (after the mixing)
>>
>> It is very important (from the point of view of the Noise Figure) how gain
>> is shared.
>> There is a register B7 to B1 which control the gain to the MAX2829 chip
>> placed inside the XCVR2450.
>> The first 2 bits are critical since they define 3 states: High gain (11),
>> Medium gain(10) and Low gain (0X)
>>
>> I would like to know how the overal gain configunation setting (0-92 dB) is
>> shared between theses two places and what the limits to be in high, medium
>> and low gain state are.
>> Where I can find it?
>>
>> Many thanks,
>> Jorge
>>
>>
>> I would like to know how the gain setting (up to 92 dB in Rx mode) of the
>> MAXIM2829 is sorted between the two amplifiers placed around the mixer.
>>
>> I is clear in the data sheet what the total gain is depending on the gain
>> settings but it is not clear where the gain is. Most of them can be before
>> the mixer (RF gain), or after the mixing (BB gain)......Is it possible to
>> find it out? I would like to model this receiver.
>>
>> Many thanks,
>> Jorge.
>

For the GNURadio drivers for either USRP2 or USRP1, the gain on
receive is add in a loosely noise-figure optimized fashion by adding
gain to the element closest to the antenna first and then filling in
from there.  (The mathworks UDP driver should work like the GNURadio
libusrp2 version)

In UHD, when using the single combined gain control, the intention is
to create the same effect as with the GNURadio drivers, although it is
possible that the recent addition of gain groups has caused subtly
different behavior, we are looking into that.  However, if you want
explicit control, the UHD C++ api exposes each controllable gain stage
through a gain group that you can use to change gains in a fashion
which suits your application.

http://www.ettus.com/uhd_docs/doxygen/html/classuhd_1_1gain__group.html

Jason