Constructing A $50 SDR With 20 MHz Bandwidth

Though the RTL-SDR is affordable, accessible, and succesful sufficient for a lot of tasks, it does have some essential limitations. Particularly, its bandwidth is restricted to about 3.2 MHz, and the value of SDRs tends to scale quickly with bandwidth. [Anders Nielsen], nonetheless, is constructing a modular SDR with a goal worth of $50 USD, and has already reached a bandwidth of virtually 20 MHz.

If this mission seems acquainted, it’s as a result of we’ve coated an earlier iteration. On the time, [Anders] had constructed the PhaseLoom, which filters an incoming sign, mixes it all the way down to baseband, and converts it to I/Q indicators. The subsequent stage is the PhaseLatch, a board housing a 20-MHz, 10-bit ADC, which samples the in-phase and quadrature indicators and passes them on to a Cypress FX2LP microcontroller growth board. [Anders] had beforehand related the ADC to a 6502 microprocessor as an alternative of the FX2LP, however this makes it a sensible SDR. The FX2LP was a very sensible choice for this mission due to its USB 2.0 interface, massive buffers for streaming information, and parallel interface. It merely reads the information from the SDR and dumps it to the pc.

The FX2LP didn’t help the ADC’s clock price, and overclocking the ADC led to points, so [Anders] related the ADC to an unbiased 20 MHz oscillator. The frequency spectrum of the SDR was oddly bell-shaped, which turned out to be as a result of restricted analogue bandwidth of the PhaseLoom (about 650 kHz) falling behind the digital bandwidth of 20 MHz. The PhaseLoom’s bandwidth gave the impression to be restricted largely by an amplifier, and reducing its acquire vastly improved issues. The SDR doesn’t but have a 20 MHz bandwidth in keeping with the traditional definition, nevertheless it’s shut sufficient to be sensible, and additional enhancements must wait on an up to date PhaseLoom board.

The Cypress growth board used right here is surprisingly succesful – we’ve beforehand seen it used to construct an SDR GPS decoder. Many of the custom-built SDRs we see don’t concentrate on technical efficiency, however do use such fascinating parts as a tube-based receiver or a {custom} silicon chip.