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Designing voltage-controlled oscillators (VCO) for a Global System for Mobile Communications (GSM) receiver is a challenge due to the phase noise requirement. The requirement comes from the stringent in-band blocking characteristics in the GSM standard. Based on the reference sensitivity, the blocking requirement, and the assumption of C/I=9 dB, the calculated phase noise specifications are summarized in this paper. Because typical VCO phase noise roll-off around 1MHz offset has a slope of 20 dB or 30 dB per decade, the key specifications are at offsets of 600 MHz and 3 MHz for Mobile Station (MS) and 800 kHz for Base Transceiver Station (BTS). The table is sorted by the difficulty of the requirement, with the easiest (DCS 1800 MS) on the left and the most stringent (GSM 900 BTS) on the right.