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Re: [Discuss-gnuradio] Re: USRP / DBSRX calibration

From: Marcus Leech
Subject: Re: [Discuss-gnuradio] Re: USRP / DBSRX calibration
Date: Thu, 08 May 2008 12:03:23 -0400
User-agent: Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.7.12) Gecko/20060130 Red Hat/1.7.12-1.4.2

Staffan Josefsson wrote:


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.

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
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