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Although the RTL-SDR is cheap, accessible, and capable enough for many projects, it does have some important limitations. In particular, its bandwidth is limited to about 3.2 MHz, and the price of SDRs tends to scale rapidly with bandwidth. [Anders Nielsen], however, is building a modular SDR with a target price of $50 USD, and has already reached a bandwidth of almost 20 MHz.
If this project looks familiar, it’s because we’ve covered an earlier iteration. At the time, [Anders] had built the PhaseLoom, which filters an incoming signal, mixes it down to baseband, and converts it to I/Q signals. The next stage is the PhaseLatch, a board housing a 20-MHz, 10-bit ADC, which samples the in-phase and quadrature signals and passes them on to a Cypress FX2LP microcontroller development board. [Anders] had previously connected the ADC to a 6502 microprocessor instead of the FX2LP, but this makes it a practical SDR. The FX2LP was a particularly good choice for this project because of its USB 2.0 interface, large buffers for streaming data, and parallel interface. It simply reads the data from the SDR and dumps it to the computer.
The FX2LP didn’t support the ADC’s clock rate, and overclocking the ADC led to issues, so [Anders] connected the ADC to an independent 20 MHz oscillator. The frequency spectrum of the SDR was oddly bell-shaped, which turned out to be due to the limited analogue bandwidth of the PhaseLoom (about 650 kHz) falling behind the digital bandwidth of 20 MHz. The PhaseLoom’s bandwidth seemed to be limited mostly by an amplifier, and decreasing its gain greatly improved matters. The SDR doesn’t yet have a 20 MHz bandwidth according to the normal definition, but it’s close enough to be practical, and further improvements will have to wait on an updated PhaseLoom board.
The Cypress development board used here is surprisingly capable – we’ve previously seen it used to build an SDR GPS decoder. Most of the custom-built SDRs we see don’t focus on technical performance, but do use such interesting components as a tube-based receiver or a custom silicon chip.
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