In this paper, we exploit an idea of coupling multiple oscillators to reduce phase noise (PN) to beyond the limit of what has been practically achievable so far in a bulk CMOS technology. We then apply it to demonstrate for the first time an RF oscillator that meets the most stringent PN requirements of cellular basestation receivers while abiding by the process technology reliability rules. The oscillator is realized in digital 65-nm CMOS as a dualcore LC-tank oscillator based on a high-swing class-C topology. It is tunable within 4.07-4.91 GHz, while drawing 39-59 mA from a 2.15 V power supply. The measured PN is -146.7 dBc/Hz and -163.1 dBc/Hz at 3 MHz and 20 MHz offset, respectively, from a 4.07 GHz carrier, which makes it the lowest reported normalized PN of an integrated CMOS oscillator. Straightforward expressions for PN and interconnect resistance between the cores are derived and verified against circuit simulations and measurements. Analysis and simulations show that the interconnect resistance is not critical even with a 1% mismatch between the cores. This approach can be extended to a higher number of cores and achieve an arbitrary reduction in PN at the cost of the power and area.@en

In modern RF system on chips (SoCs), the digital content consumes up to 85% of the IC chip area. The recent push to integrate multiple RF-SoC cores is met with heavy resistance by the remaining RF/analog circuitry, which creates numerous strong aggressors and weak victims leading to RF performance degradation. A key such mechanism is injection pulling through parasitic coupling between various LC-tank oscillators as well as between them and strong transmitter (TX) outputs. Any static or dynamic frequency proximity between aggressors (i.e., oscillators and TX outputs) and victims (i.e., oscillators) that share the same die causes injection pulling, which produces unwanted spurs and/or modulation distortion. In this paper, we propose and demonstrate a new frequency planning technique of a multicore TX where each LC -tank oscillator is separated from other aggressors beyond its pulling range. This is done by breaking the integer harmonic frequency relationship of victims/aggressors within and between the RF transmission channels using digital fractional divider based on a phase rotation. Each oscillator's center frequency can be fractionally separated by ~28% but, at the same time, both producing closely spaced frequencies at the phase rotator outputs. The injection-pulling spurs are so far away that they are insignificantly small (-80 dBc) and coincide with the second harmonic of the carrier. This method is experimentally verified in a two-channel system in 65-nm digital CMOS, each channel comprising a high-swing class-C oscillator, frequency divider, and phase rotator.@en