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