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Multiband FDD operation requires numerous off-chip duplexers, which limit form factor. Widely-tunable low-noise RF active self-interference cancellation (SIC) (e.g. [1]) is a step towards enabling duplexers with reduced TX/RX isolation as well as adjacent-channel full duplex. However, SIC bandwidth (BW) is limited to a few MHz due to the selectivity of the high-Q duplexer isolation. Recent works suggest that same-channel full duplex (SC-FD) can greatly improve network performance [2][3][4]. SIC must be pursued in RF, analog/mixedsignal and digital for SC-FD to achieve the >100dB of SIC required, as filtering the SI is not an option. However, SC-FD self-interference channels can be frequency-selective due to ambient reflections [3], requiring silicon-averse bulky delay lines to replicate the delays in the RF SIC path [2].

In this paper, we propose an instantaneoushop frequency synthesizer based on a zero-initial-phase-error multi-modulus divider that breaks the fundamental tradeoff between hopping time, spectral purity and frequency resolution. In the proposed synthesizer, initial frequency error and phase error at the instant of hop are virtually eliminated through frequency presetting in the high-resolution voltagecontrolled oscillator (VCO) and the proposed zero-initialphase-error divider. This eliminates the acquisition process and enables "instantaneous hops" to within a frequency error that is only limited by the resolution of digital control. An IC prototype is implemented and fabricated in a standard 65nm CMOS technology. The implemented frequency synthesizer operates over 4-5.84 GHz with three discrete divider ratios (80,88,96) and achieves instantaneous hops to within an average of 3.64 MHz of the desired output frequency. The prototype dissipates 16.8 mW from 1.2V power supply.

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