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
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
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
have spectacular noise figures, because they’re generally designed for
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
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.
Shirleys Bay Radio Astronomy Consortium