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This paper reports a flexible direct digital modulation based low power transmitter in 90 nm CMOS that supports constant-envelope modulation using phase or frequency modulation for carrier frequencies from 100 MHz to 2.5 GHz. For rectangular filtered 8-PSK modulation from 1 K symbol/s to 20 M symbol/s the RMS phase error is less than 4 deg. Frequency modulation is measured via filtered frequency shift keying (FSK) schemes used in two common standard protocols, GSM and APCO-25. GSM phase error at 800-900 MHz as well as 1900 MHz is less than 1 deg RMS. APCO-25 FSK error is less than 2% across bands from VHF (~130 MHz) to UHF (~400 MHz) to 800 MHz. Measured adjacent channel power ratio (ACPR) are within specification.The synthesizer power drain is 130 mW with an active area of 0.7 mm 2 .