SDR

Software-defined radio moves the majority of radio processing into software, facilitating relatively inexpensive wide-band hardware interfaces to the electromagnetic spectrum, especially those frequencies below 3GHz. Pairing advanced SDRs with software-defined antennas yields dynamically optimal cognitive radio. Perhaps most famously, DVB-T television tuners built around the RTL2832U chip, and available in USB form factor for less than $30 (particularly the Rafael Micro R820T2), can reliably provide 2MHz of RX bandwidth anywhere from ~30MHz to ~2GHz (still lower frequencies are supported via direct sampling). For $500, powerful units capable of tremendous bandwidth and range (as well as transmission capabilities) are available, and from there it's not that great a leap to building your own stingray.

Hardware

Device	Tuner	Bandwidth	Sample Rates	ADC	Freqs	Xmit?
NooElec NESDR SMArt	R820TR2	2.4MHz sustained 3.2MHz peak	225--300kHz .9--2.56MHz	8	25--1750MHz	No
NooElec NESDR SMArt XTR	E4000	2.4MHz		8	65--1100MHz 1200--2300MHz	No
HackRF One		20MHz		8		Yes
AirSpy		10MHz		12	24--1800MHz	No
RTL-SDRv3	R820TR2	2.4MHz sustained 3.2MHz peak		8	25--1750MHz	No