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

[Marcus Leech]

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
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