For the last few years, I've been kicking-around "paper-tiger" designs
for a new digital-sampler+RF "front end" for small-scale radio astronomy.
I've been developing software tools for small radio observatories,
based on Gnu Radio "core" software, and the USRP1 and USRP2
front-end hardware from Ettus Research.
For smaller observatories (hobbiests, amateurs, and smaller academic
institutions) a less-expensive, more-focusssed hardware option
needs to be available, but isn't.
I've been tossing ideas back and forth between myself and the board of
SARA (The Society for Amateur Radio Astronomers) for a
SARA-sponsored project to develop such a piece of hardware.
A mid-level design is shown here, which reflects current thinking:
http://www.sbrac.org/files/digital_receiver_cheap.pdf
The design shown above is relatively simple, compared to much of what
CASPER does, and I'm wondering if anyone in the CASPER
community has any interest in helping out.
The design as shown is tunable from roughly 30MHz to 2.2GHz, and
provides quadrature downconversion to baseband,
along with fixed-rate sampling at 40Msps, with decimation happening
inside an FPGA, after which data are presented to
a 1GiGe network interface. This is functionally identical to what
the USRP2 series products from Ettus do, but without a
Tx side, and using a cheaper, lower-sample-rate ADC, along with a
smaller FPGA. The FPGA Verilog and firmware
for the FPGA-resident aEMB microcontroller are all available under
the GPL. The ADF4351 synthesizer shown isn't actually
available yet from Analog Devices, but is an extended-capability
version of the existing ADF4350, including a lower minimum
frequency, it is both pin and software compatible with the ADF4350.
The quadrature ADL5387 mixer is a standard 2XLO design,
requiring an LO that is twice the desired direct-conversion
frequency (it uses an internal flip-flop type quadrature generator).
The design is significantly less-ambitious than the overwhelming
majority of projects undertaken by the CASPER group in the last
few years, and as such it may be attractive for volunteer labour in a
"what I do when I'm waiting for the coffee to be ready"
regime :-)
The host-software side of the "problem" has already been pretty-well
solved, with Gnu Radio offering an excellent
software-defined-radio framework, and higher-level
radio-astronomy-specific applications like IRA already able to
process 25MHz of bandwidth on a relatively-modest 6-core AMD 1090T
system for a single antenna.
--
Marcus Leech
Principal Investigator
Shirleys Bay Radio Astronomy Consortium
http://www.sbrac.org
