SDR Radio Telescope Receiver

Hello All,

I’m currently an undergraduate engineering student at Grand Valley State
University. For my senior design project, I’m working on designing a
SDR-based radio telescope to compliment the NASA RadioJove project (a
direct
conversion receiver for use in radio astronomy).

In a nutshell, the receiver must be able to receive an AM signal with a
center frequency of 20.1MHz and a bandwidth of 1MHz. On the software
side
of things, we need to be able to plot signal strength as a function of
time
and frequency in real time. We are planning to use analog mixing to get
an
IF centered around 500kHz and 1MHz bandwidth. The IF would then be
sampled
that with a 14-bit ADC clocked around 4MHz. The output of the ADC would
then be sent to some kind of controller (ie. FPGA, microcontroller, etc)
which would do a little bit of digital down converting to get the signal
into its I and Q centered around DC and send it over USB to the host PC.

While I am not completely against using an FPGA in the design of our
receiver, I would really like to use a fast microcontroller, PSoC, or
similar device as I have much more experience programming in C than I do
in
HDL. Has anyone hear of a SDR receiver with 1MHz bandwidth using a
microcontroller? I was thinking along the lines of using a ARM Cortex
M3
chip with high clock frequency (~80MHz), DMA, and external ADC clocked
around 4MHz, but I’m not sure if the ARM chip would be fast enough to
down
convert all that data and send it over USB quickly enough. If anyone
has
links to resources where this kind of thing has been done that would be
awesome.

I’m very new to SDR development so any input would be greatly
appreciated.

Thanks!
Phil

Hi Marcus:
FX2 dev kit at $475 ???
hardly inexpensive to get started, unless there’s a cheaper one out there.
Limited free dev software too, cost more for more than 4k code size???
Don

When I investigated it a couple of years ago, cheaper dev kits were
available.

Also, you can use the SDCC open-source compiler to generate code for it.
No
need to buy an expensive commercial devkit for it. The USRP1
firmware was
done with SDCC, and it works fine.


Marcus L.
Principal Investigator
Shirleys Bay Radio Astronomy Consortium
http://www.sbrac.org

a function of time and frequency in real time. We are planning to use
do in HDL. Has anyone hear of a SDR receiver with 1MHz bandwidth
Phil
A few comments:

check out the gr-radio-astronomy sub-tree. It’s old, but should still
function.

What you might consider doing, if you’re committed to building your own
hardware, is a direct-conversion receiver, perhaps with a fixed
LO at 20.1MHz, and then sample with a 1MSps ADC, feeding an FX2 for
USB-2.0. No need for an FPGA, provided you never want to
change sample rates before the signal gets to the host PC.

The FX2 uController/USB-2.0 interface is very common, very inexpensive
hardware, and has a nice FIFO interface that’s easy to interface
to ADC hardware.

Another thing to keep in mind, is that your usable bandwidth around
20.1MHz is likely to only be a few 10s of KHz, due to lots of
interference down there. So your 1MHz bandwidth may be utterly
wasted.

For narrowband stuff, you might consider something like the SoftRock
series of direct-conversion receivers, or the UHFSDR.
The UHFSDR tunes from DC to 700MHz, and gives a 96KHz
bandpass–perfect for audio. Combine the UHFSDR with the SDR-Widget,
and you have a pretty-complete narrowband platform.


Marcus L.
Principal Investigator
Shirleys Bay Radio Astronomy Consortium
http://www.sbrac.org

On 01/29/2011 12:37 PM, Phil Behnke wrote:

Thanks for the tips! I’m definitely going to investigate hooking the
ADC directly to the FX2. Looks like there are some inexpensive dev
boards for the FX2 on ebay, although I have no idea how good they are.
I figure I will try to develop the receiver to always grab 1MHz worth
of bandwidth, and then give the user finer filtering abilities by
using digital filters on the PC side.

That’s probably a reasonable approach. The FX2 is actually a
fairly-capable chip. It’s used in the
USRP1 from Ettus R., which can “pump” upto 16Msps(complex,
8-bit) over USB-2.0,
so 1Msps should be more than doable.

You have to make certain that your I and Q lines are low-pass filtered,
fairly stiffly, prior to sampling.
If you’re sampling at 1Msps, the I and Q lines need to be low-pass
filtered to 500KHz. There’s a nice
passive-filter designer on-line at:

http://www.wa4dsy.net/filter/filterdesign.html

I’ve used it for other radio astronomy projects in the past

Other things to keep in mind:

o you'll need enough low-noise gain ahead of the down-converter to

make up for the
terrible noise figure that’s typical of mixers and ADCs. At
HF, in radio astronomy, you’ll
probably need about 50dB.

o You'll need a good bandpass filter at RF.  Again, the

above-mentioned site should help here. A
good approach is to use a amp-filter/amp-filter/amp-filter
topology, which gives you distributed gain
and filtering, and makes the individual filter stages
manageable. If you designed your bandpass
RF filter for an Fc of 20Mhz, and bandwidth of 5Mhz or so, it’ll
improve your usable dynamic
range, and prevent driving your gain stages into compression,
due to “other muck” that your
antenna will inevitably “see”.


Principal Investigator
Shirleys Bay Radio Astronomy Consortium
http://www.sbrac.org

I have been working with Joe K5SO on the OpenHPSDR project
using two Mercury receivers for diversity reception. I mention
Joe because he is using his system for Radio Astronomy:
Perhaps there would something on his site.

http://www.k5so.com/

Jeff
K2SDR

On 01/29/2011 12:37 PM, Phil Behnke wrote:

Thanks for the tips! I’m definitely going to investigate hooking the
ADC directly to the FX2. Looks like there are some inexpensive dev
boards for the FX2 on ebay, although I have no idea how good they are.
I figure I will try to develop the receiver to always grab 1MHz worth
of bandwidth, and then give the user finer filtering abilities by
using digital filters on the PC side.

Don’t know whether you’ve seen these:

http://www.knjn.com/ShopBoards_USB2.html

$79.95 for the low-end board, which includes and FX2, and a small
Cyclone FPGA. Pretty good
value.


Principal Investigator
Shirleys Bay Radio Astronomy Consortium
http://www.sbrac.org

On 01/29/2011 01:24 PM, Phil Behnke wrote:

Thanks again, Marcus! Do you suggest breaking the signal up into I
and Q via analog mixing and having two ADCs (one for I and one for Q)?
I was thinking about mixing down to DC and sampling with a single
ADC, and then breaking the signal up into I and Q digitally on the PC
side. I’ve been looking at the Mercury SDR (
http://openhpsdr.org/wiki/index.php?title=MERCURY) and that seems like
what they do, only they have a digital down converter in the FPGA
which beaks the signal into I and Q.

-Phil
If you want to convert to I and Q in an FPGA, you need to use a
conventional IF architecture that brings
it into the range of the ADC sampling rate. If you convert to DC in
“real” mode, you lose
phase information and have the problem of “folding” about DC.

Use an analog approach to generating I & Q. The simplest is to use a
so-called 2XLO approach, in
which you generate a 50%-duty-cycle square wave at exactly twice the
desired Fc, feed that to
a pair of D flip-flops to produce two signals at exactly half the
frequency, and exactly 90 degrees
out-of-phase with respect to each other. Then use a pair of mixers to
produce the converted
I & Q signals. Use a dual-channel ADC, like the AD9288 – only $8.00
apiece for 40Msps, and
you can likely find cheaper dual-channel ADCs for only 1 or 2Msps.
The FX2 FIFO is 16-bits wide,
so if you use a dual-channel 8-bit ADC, you don’t have to play any
interleaving games into
the FX2.

Here’s an article on a 2XLO approach here, see figure 4:

rfdesign.com/mag/606RFDF3.pdf

Since your input frequency would be around 40MHz, you could use
pretty-ordinary CMOS gates
and flip-flops to do this–nothing exotic like ECL logic required.

Hi Jeff,

I’m interested to learn about Joe’s hydraulic antenna pointing system.
Do you know how he handles the feedback, ie what kind of sensors
is he using to “know” the antenna position?

At the moment we’re trying to use Pololu Jrk as a low cost motor control
with feedback
http://www.pololu.com/catalog/product/1393
and
http://www.pololu.com/catalog/product/1392

Patrik

----- Original Message -----
From: “Jeff Kelly” [email protected]
To: [email protected]
Sent: Saturday, January 29, 2011 20:39
Subject: Re: [Discuss-gnuradio] SDR Radio Telescope Receiver

This forum is not affiliated to the Ruby language, Ruby on Rails framework, nor any Ruby applications discussed here.

| Privacy Policy | Terms of Service | Remote Ruby Jobs