Staffan Josefsson wrote:
Hi.
I'm trying to estimate the noise temperature in the DBSRX and USRP as a
part of a student project where we compare the USRP as an alternative
for the receiver in a radio telescope. I tried to convert the -165dBm/Hz
to Kelven but I'm not getting a sane value. I may be way out here but
would be very pleased to find out what the noise temperature might be,
in Kelvin.
Thanks
Staffan Josefsson
The MAX2118 chip that is used to do the downconversion has a very high
noise figure--10dB.
The DBS_RX board has a GaAsFET amplifier in front of it. which has a
roughly 0.9dB
noise figure.
Unless you have the DBS_RX right at the receive horn, you'll need your
own LNA chain
in front of the DBS_RX. Go with an LNA with the lowest noise figure
you can afford.
The LNAs produced by Radio Astronomy Supplies are pretty good, and
there's a fellow
in Switzerland who produces semi-custom LNAs for the 21cm band that
are really well
built. I use three LNAs in series in my system--all from Down East
Microwave. My
first LNA is attached directly to the feedhorn, followed by a ceramic
filter cut for 1420Mhz,
followed by another pair of LNAs--I did it this way because I had a
lot of feedline to drive,
and certainly you could get away with less front-end gain. But your
first low-noise amplifier
element had better be right at the feedhorn--no feedline (high-quality
RF connectors between
the feedhorn and the first LNA are OK, but keep them to an absolute
minimum).
My overall system temperature with this setup was about 95K.
--
Marcus Leech Mail: Dept 1A12, M/S: 04352P16
Security Standards Advisor Phone: (ESN) 393-9145 +1 613 763 9145
Strategic Standards
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