Re: [Discuss-gnuradio] RF Gain and BB gain of XCVR2450
On 11/01/2010 04:19 PM, Jason Abele 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 Leech Principal Investigator Shirleys Bay Radio Astronomy Consortium http://www.sbrac.org ___ Discuss-gnuradio mailing list Discuss-gnuradio@gnu.org http://lists.gnu.org/mailman/listinfo/discuss-gnuradio
Re: [Discuss-gnuradio] RF Gain and BB gain of XCVR2450
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 ___ Discuss-gnuradio mailing list Discuss-gnuradio@gnu.org http://lists.gnu.org/mailman/listinfo/discuss-gnuradio
[Discuss-gnuradio] 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. ___ Discuss-gnuradio mailing list Discuss-gnuradio@gnu.org http://lists.gnu.org/mailman/listinfo/discuss-gnuradio
Re: [Discuss-gnuradio] RF Gain and BB gain of XCVR2450
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. ___ Discuss-gnuradio mailing list Discuss-gnuradio@gnu.org http://lists.gnu.org/mailman/listinfo/discuss-gnuradio
Re: [Discuss-gnuradio] RF Gain and BB gain of XCVR2450
On Mon, Nov 1, 2010 at 11:52 AM, Eric Blossom e...@comsec.com 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 ___ Discuss-gnuradio mailing list Discuss-gnuradio@gnu.org http://lists.gnu.org/mailman/listinfo/discuss-gnuradio
Re: [Discuss-gnuradio] RF Gain and BB gain of XCVR2450
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 On 1 November 2010 21:19, Jason Abele ja...@ettus.com wrote: On Mon, Nov 1, 2010 at 11:52 AM, Eric Blossom e...@comsec.com 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 ___ Discuss-gnuradio mailing list Discuss-gnuradio@gnu.org http://lists.gnu.org/mailman/listinfo/discuss-gnuradio
