This paper presents a versatile energy-efficient wireless transmitter (TX) for neural implants with high/ultrahigh channel counts. The TX supports a wide range of data transmission rates, from 50Mb/s to 1.2 Gb/s, through a novel dual-modulation approach. Our design combines 4-pulse position modulation (4PPM) and 2-phase shift keying (2PSK) for high data throughput, while adopting simple on-off keying (OOK) for energy efficiency at lower data rates. To support this modulation strategy, a twin oscillator-based wideband phase-locked loop (PLL) is proposed to generate the RF carrier at ultra-wideband (UWB). Thanks to the relaxed phase noise requirement for OOK modulation, a 5× frequency multiplier with edge combination is utilized, enabling a duty-cycled PLL-based RF carrier, thus enhancing energy efficiency. The proposed TX was fabricated in 65−nm CMOS and consumes 1.49−7.26 mW across all supported data rates (50−1200Mb/s). Our TX further incorporates frequency division multiple access (FDMA) for simultaneous, untethered multi-node operation in distributed brain implants. In addition, a custom-designed on-chip antenna facilitates a compact footprint of only 3 mm2, while achieving a record transmission range of 0.6 m compared to state-of-the-art implantable TXs. Index Terms-Impulse-radio ultra-wideband transmitter (IRUWB TX), on-chip antenna, multi-data-rate, frequency division multiple access (FDMA), duty-cycling, neural implants.