Michael Ossmann of Great Scott Gadgets is working on a new software radio called HackRF. His vision is to repurpose a highly-integrated transceiver IC to serve a wider tuning range, ideally from below 100MHz to over 4GHz.

I'm helping out with various tasks along the way. Here are the biggest challenges we've faced so far:

• Finding a front-end IC or combination of ICs that will convert 2.4GHz signals to/from baseband, digitize received baseband signals and "analogize" transmitted baseband signals. So far, the best solution seems to be a two-chip combination of the MAX2837 WiMAX transceiver and MAX5864 analog front end (quadrature ADC and DAC). This combination will get signals to/from 2.4GHz to digitized quadrature baseband data streams.
• Moving digitized baseband data between the HackRF device and the host PC – again, inexpensively and using little power. The most conventional, obvious interface is USB 2.0 high-speed. We've entertained several options: ARM microcontrollers, FPGAs, XMOS event-driven processors, and dedicated USB interface ICs like the Cypress CY7C68013 and FTDI FT2232H. In reviewing these options, we came across a new breed of ARM microcontrollers that have generalized high-speed interfaces on them. The most compelling so far is the LPC43xx SGPIO interface, which can be configured into arbitrary-width serial or parallel interfaces running at over 100MHz transfer rate.
• Understanding and configuring the Si5351C clock generation IC. Silicon Labs documentation for this part is rather scattered and inconsistent. Mike did a lot of testing with an oscilloscope to identify the true behavior of the IC.
• Interfacing the LPC43xx SGPIO interface to the MAX5864, which has two dual-data-rate (DDR) interfaces that the SGPIO port can't directly interface to. Our current solution is to use a low-cost CPLD (programmable logic device) to convert bus widths, voltages, and clocking schemes.
• Keeping total power consumption to below USB 2.0 limitations. Mike opted to put a switching regulator on the board, which seems to work well, though we haven't reached the point in development where we're drawing a lot of current and trying to receive weak radio signals. It's possible the switcher may produce enough noise to interfere with the radio circuitry.

Keep up with HackRF developments on the GitHub repository and Wiki, or join us in IRC (freenode.net #hackrf), or on the HackRF-dev mailing list.