This paper presents a 500 kBit/s modem for space applications, designed for interfacing the satellite equipment with the On-Board Data Handling (OBDH) bus system. The 28 mm2chip, fabricated in a 0.8 µm SOI radiation-hard CMOS process, achieves a bit-error rate as low as 10-14with 120 mVRMSof noise consuming 64 mW from a 5 V power supply.