Toward Solving Multichannel RF-SoC Integration Issues Through Digital Fractional Division

SAR Ahmadi Mehr, M Tohidian, RB Staszewski

Research output: Contribution to journalArticleScientificpeer-review

8 Citations (Scopus)
53 Downloads (Pure)


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.
Original languageEnglish
Pages (from-to)1071-1082
Number of pages12
JournalIEEE Transactions on Very Large Scale Integration (VLSI) Systems
Issue number3
Publication statusPublished - 12 Jun 2015


  • system on chip (SoC)
  • Digital fractional divider
  • digitally controlled oscillator (DCO)
  • frequency pulling
  • injection locking
  • multi-core radio
  • RF-SoC


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