In this paper, the authors present an X-band core chip in a production-ready 0.25 mu m SiGe BiCMOS technology, suited for implementation in a transmit/receive module for future phased-array antennas. The very compact chip (3x3mm(2)) incorporates separate circuits for the receive and transmit paths with a very accurate 6 bit phase shifting and amplitude variation capability. Once mounted in a QFN package, the core chip achieves an excellent gain of 18 dB between 8 GHz and 12 GHz, good linearity with a P1dB, in exceeding -20 dBm and noise figure below 10 dB for the receiving path. In transmit mode, the P1dB, out is in excess of 13 dBm between 8GHz and 12 GHz and even 17 dBm between 8GHz and 10 GHz, which is sufficient to drive an external high power amplifier. Additionally, an on-chip digital control is implemented as well as analog and mixed-signal functionality. The chip supports pulsed and continuous wave operation. The latter consumes 790mW in Tx or 330mW in Rx mode, with a supply voltage of 3.3V for the RF components and 2.5V for the digital as well as analog and mixed-signal components. The overall achieved performance makes this Xband core chip very well suited for implementation in next generation X-band transmit/receive modules for phased-array antenna systems.