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.