RF Gain and BB gain of XCVR2450

Hello,

I will rephrase former question.

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.

On Mon, Nov 1, 2010 at 11:52 AM, Eric B. [email protected] wrote:

http://www.gnuradio.org/redmine/wiki/gnuradio/ReportingErrors

is shared.
Many thanks,

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

On Mon, Nov 01, 2010 at 05:36:35PM +0100, Jorge M. 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 :slight_smile:

“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

Jason,

That was very helpful. I use the USRP2 with both, Linux/GNURadio drivers
and
Windows/UDP drivers.
Now looking at the datasheet of the MAXIM2829 I can figure out how the
gain
is sorted.

Many thanks

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?

Probably, that is what you are looking for:
http://gnuradio.org/cgit/gnuradio.git/tree/usrp2/firmware/lib/db_xcvr2450.c?h=next#n445

Cheers,
-Gregor

On 11/01/2010 04:19 PM, Jason A. wrote:

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)

I think that if you really care about noise figure, though, you have
to use an
external LNA. I think at its very best, the XCVR2450 has a noise
figure
of about 8dB. A GaAs HEMT or HJ-FET LNA in front of that with a
sub-1dB noise figure and 15-20dB of gain would give you a much nicer
Tsys than with the XCVR alone.

It’s typically the case that RF VGAs use a fixed-gain amplifier
followed by a switchable attenuator matrix. (or worse, an
attenuator matrix followed by a fixed-gain amplifier–yuck!).

For any serious weak-signal work, you pretty much have to use an
external LNA right
out at the antenna. None of the downconverter chips in common use for
SDR
have spectacular noise figures, because they’re generally designed for
applications
where noise figure isn’t that important. The XCVR2450 downconverter
(MAX2829), for example,
was designed for WiFi applications, which don’t have particularly
stringent noise figure requirements.

Similarly the DBS_RX MAX2118 (and now MAX2112) are designed for
direct-broadcast-satellite applications
where there’s a low-noise block-downconverter at the dish, and that
down-converter typically has
a sub 0.8dB noise figure, and usually 40-50dB of gain. The noise
figure of the direct-conversion part
essentially vanishes when divided by the gain of the LNB, so there’s
no attempt made by the designers of
these chips to optimize noise figure to any great extent.

Any weak-signal work will necessarily require that the first low-noise
gain element be right up at the receive
antenna, usually hung directly off the feed, with as little
feed-line/connectors as possible between the feed and
the first gain stage. That pretty-much means an external LNA,
although you could somehow mount a USRP2
right up at the antenna, I’d be concerned about radiated “goop” from
the USRP2/USRP1 in that kind of scenario.
And you’d still need a decent LNA in front of the
USRP2+daughtercard–none of them are particularly low noise.

Marcus L.
Principal Investigator
Shirleys Bay Radio Astronomy Consortium
http://www.sbrac.org

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