Tomasz Watorowski has published a new build, a NE612 RTL SDR Upconverter:
This is a simple yet completely functional upconverter for the popular SDR donlge. The construction was build around cheap and easily obtainable mixer IC marked as NE 612, manufactured by Phillips Semi. Despite its limitations, it performs outstandingly well, at least when price-to-quality factor is discussed. This IC is used to upconvert 0-30MHz to a region acceptable by SDR dongle: 100-130MHz, all of that was achieved by employing a 100MHz 4-pin crystal generator.
Project details at MightyDevices site.