Beginner question about rtlsdr

historical

Hello Michel

OK, I see you are way ahead of me. However, I have some experience using
these TV Dongles, they have pitfalls. Number one is its frequency Accuracy,
out-of-the-box; needs calibration. Another is that its frequency accuracy
drifts over time and from the Ambient temperature necessitating periodic
offset calibration. While it is possible to create a compensation cure for
frequency drift vs. temperature, it may be far more prudent to place the
Dongle in a temperature controlled enclosure (50C Heater) keeping it above
the ambient to help prevent drift, in your case, the all too important
Temperature related Phase Shift. I have thought of disconnecting the
Dongle's Crystal and placing it in a Crystal Oven, but I have not tried
that. The Entire Dongle is small enough to place it in a temperature
controlled enclosure which also keeps it dry.

-----Original Message-----
From: osmocom-sdr-bounces at lists.osmocom.org
[mailto:osmocom-sdr-bounces at lists.osmocom.org] On Behalf Of Michel Pelletier
Sent: Sunday, October 21, 2012 8:22 PM
To: Jay Salsburg
Cc: osmocom-sdr at lists.osmocom.org
Subject: Re: Beginner question about rtlsdr

On Sun, Oct 21, 2012 at 5:50 PM, Jay Salsburg <jsalsburg at bellsouth.net>
wrote:
> Hello
>
> May I suggested a few things not related to the "Programming" issues 
> but related to the Radio Astronomy subject? Just like retail, about 
> "Location, Location, Location," Radio Astronomy's is about the 
> "Antenna, Antenna, Antenna." One thing the TV Dongle offers for Radio 
> Astronomers is the ease of placing the receiver (Dongle) on the 
> Antenna. The USB must be extended from the Computer to the Antenna, 
> instead of bringing the Signal from the Antenna to the Dongle. The 
> advantage is lower noise and higher gain (without noise). There are 
> many references on the Internet for extending USB, some for purchase, some
DIY.

Thanks Jay, I'm personally fascinated by the scientific aspects of the
subject so I do appreciate your comment.  I'm not sure if you're referring
to the link I posted in this thread to a picture of my telescope, but I did
as you suggested an embedded the dongle directly in the cantenna.  My
extension is a simple usb cable as my computer is just across the wall,
although for my next antenna on the other side of my property, I intend to
use a small dlink access point device flashed with openwrt and using usb
over IP and power over ethernet to extend the other data source to my
computer.  This will give me a single interferometric baseline to begin
using packages like aipy.  I have the parts, I'm still piecing together the
receiver.  I live in Western Oregon so weatherproofing is key. :)

FWIW the EOR array uses simple, omnidirectional crossed dipoles and Python:

http://eor.berkeley.edu/

That's a pretty gorgeous picture of Centaurus A for some simple dipoles!
Here is an excellent presentation on the python technology behind it:

http://setiathome.berkeley.edu/~aparsons/papers/2008-08-10_LFSW_AIPY_Present
ation.pdf

there are some pretty pictures at the end.  All of the software parts of the
interferometry are basically done, what is needed is to vet the possibility
that something like an rtlsdr dongle and a simple antenna, arrayed across an
area, can produce data of sufficient quality to feed into a package like
aipy.  As roger-'s ecellent pyrtlsdr can already capture dongle complex data
directly into numpy arrays, it seems like all the pieces are laying around,
simply needing to be put together, and I intend to try it.

As an update, thanks to Scott's great help I am currently dumping data at
1420 Mhz center frequency into numpy arrays and averaging their squares.
Here's the simple "core" of my loop:

while True:
    yield numpy.average(numpy.absolute(radio.read_samples(2**20)))

I'm accumulating one average per secondish and dumping them into a data file
that I am plotting on a simple graph.  Cassiopeia reaches maximum at my
local position around midnight, and the graph is currently trending upward.
I'm really hoping that by morning I'll have a nice curve that peaks at
midnight and hopefully not a bunch of random squiggles. :)

-Michel

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