[Top][All Lists]

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

[Discuss-gnuradio] 21cm spectra after adding the front end and another l

From: Marcus Leech
Subject: [Discuss-gnuradio] 21cm spectra after adding the front end and another line amplifier
Date: Thu, 12 Jan 2006 21:30:41 -0500
User-agent: Mozilla Thunderbird 1.0 (X11/20041206)

Last week, I observed that with the USRP/DBS_RX combination terminated at the input, there was still a couple of spectral spurs when I set the system up to observe at the neutral hydrogen line at 1420.405Mhz. This worried me a little, but I surmised that once I had the LNA and another
 line-amp on, their added noise would swamp any little spurs.

Here's a screenshot of my RA application with the feedhorn setup on a 1.25M dish, with a 15dBg/0.7dBNF LNA, a 15dBg/3.0dNF line amplifier, and roughly 6dB of cable loss to the DBS_RX front-end. The spectrum is nice and quiet, which is just what you want, with perhaps the occasional HI region passing through
 the beam to de-flatten the spectrum :-)

I measured the response of the setup to a blackbody source at 315K this morning, which produced a massive (at least in RA terms) 2.25dB increase in the total power reading. That's a ratio of roughly 56 (17.4dB) between the average magnitude of the random fluctuations seen in the total power display, and that produced by a blackbody source at 310K. It could stand a further increase
 in front-end gain, I suspect.

But the main thing is I was very pleased to see the spurs go away, and also to see how relatively stable the system is. I'll be making some attempts to capture transits of Cass. A--a standard calibration source that produces a flux density of about 2000Jy at the 21cm wavelength. If that is unambiguously successful (and in theory, it should be, but in amateur observatories, there's nearly-always a big gap between theory and practice), I'll let the list know.

A couple of good messages to tak e away from this are that GnuRadio forms a good basis for amateur radio astronomy observing, and that USRP/DBS_RX combination make this possible with a bare minimum of hardware futzing required, and at a fraction of the cost of
 the only comparable amateur RA system on the market today.

reply via email to

[Prev in Thread] Current Thread [Next in Thread]