Off The Shelf Digital Radio

摘要

Since I started this project, I've realized that the best approach to an electronically steerable antenna system involves the appropriate mixture of hardware and software. Even though a pure software approach works, a modest amount of hardware to mix RF down to base-band considerably reduces the computing burden. Gerald Youngblood, AC5OG, describes one such circuit, that allows one to mix an HF signal down to baseband while preserving dynamic range. Then, one can complex sample the baseband signal with an intermediate-speed, wide ADC. Gerald used a 24 bit sound card. So, whether the approach is all-software, or a mix of front end hardware and software, the next step would be to add a second evaluation board so that two receive channels can be combined in phase to achieve a steerable receiving beam. Naturally, both ADC boards would have to use a common clock. Since two ADCs doubles the throughput, some additional performance gains will be required. Using RTLinux with the National Instruments board demonstrated that real time programming does indeed permit one to extract every last bit of performance from a machine. Unfortunately, as of the time of writing, Techniprise had not yet finished their Linux driver, so I don't know what performance gains are yet possible using the PCI Protoboard.