SEMICONDUCTOR INTEGRATED CIRCUITS

A class-C VCO based Σ-Δ fraction-N frequency synthesizer with AFC for 802.11ah applications

Xiaobao Yu, Siyang Han, Zongming Jin, Zhihua Wang and Baoyong Chi

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Abstract: A 1.4-2 GHz phase-locked loop (PLL) Σ-Δ fraction-N frequency synthesizer with automatic frequency control (AFC) for 802.11ah applications is presented. A class-C voltage control oscillator (VCO) ranging from 1.4 to 2 GHz is integrated on-chip to save power for the sub-GHz band. A novel AFC algorithm is introduced to maintain the VCO oscillation at the start-up and automatically search for the appropriate control word of the switched-capacitor array to extend the PLL tuning range. A 20-bit third-order Σ-Δ modulator is utilized to reduce the fraction spurs while achieving a frequency resolution that is lower than 30 Hz. The measurement results show that the frequency synthesizer has achieved a phase noise of < -120 dBc/Hz at 1 MHz offset and consumes 11.1 mW from a 1.7 V supply. Moreover,compared with the traditional class-A counterparts,the phase noise in class-C mode has been improved by 5 dB under the same power consumption.

Key words: phase-locked loop (PLL)class-C VCOfrequency synthesizerlow power802.11ahtransceiver



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.  A block diagram of a sub-GHz transceiver.

figure 1.


.  Sub-GHz band distribution over different countries.

Figure 2.


.  The architecture of the proposed frequency synthesizer.

Figure 3.


.  A schematic of the proposed class-C VCO.

Figure 4.


.  A schematic of a quadrature divided-by-two divider.

Figure 5.


.  The timing diagram of the AFC.

Figure 6.


.  A flow chart of the AFC algorithm.

Figure 7.


.  Micro-photograph of the proposed frequency synthesizer.

Figure 8.


.  Measured VCO tuning range.

Figure 9.


.  The control words of switched-capacitor array during the AFC process.

Figure 10.


.  Measured VCO control voltage during the locking procedure.

Figure 11.


.  Measured frequency synthesizer output spectrum.

Figure 12.


.  Measured PLL phase noise at 1700 MHz.

Figure 13.


.   Component values of loop filter.

Table1.

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.   Performance comparison with the state-of-the-arts.

Table2.

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    Received: 12 February 2015 Revised: Online: Published: 01 September 2015

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      Xiaobao Yu, Siyang Han, Zongming Jin, Zhihua Wang, Baoyong Chi. A class-C VCO based Σ-Δ fraction-N frequency synthesizer with AFC for 802.11ah applications[J]. Journal of Semiconductors, 2015, 36(9): 095003. doi: 10.1088/1674-4926/36/9/095003 X B Yu, S Y Han, Z M Jin, Z H Wang, B Y Chi. A class-C VCO based Σ-Δ fraction-N frequency synthesizer with AFC for 802.11ah applications[J]. J. Semicond., 2015, 36(9): 095003. doi: 10.1088/1674-4926/36/9/095003.Export: BibTex EndNote
      Citation:
      Xiaobao Yu, Siyang Han, Zongming Jin, Zhihua Wang, Baoyong Chi. A class-C VCO based Σ-Δ fraction-N frequency synthesizer with AFC for 802.11ah applications[J]. Journal of Semiconductors, 2015, 36(9): 095003. doi: 10.1088/1674-4926/36/9/095003

      X B Yu, S Y Han, Z M Jin, Z H Wang, B Y Chi. A class-C VCO based Σ-Δ fraction-N frequency synthesizer with AFC for 802.11ah applications[J]. J. Semicond., 2015, 36(9): 095003. doi: 10.1088/1674-4926/36/9/095003.
      Export: BibTex EndNote

      A class-C VCO based Σ-Δ fraction-N frequency synthesizer with AFC for 802.11ah applications

      doi: 10.1088/1674-4926/36/9/095003
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      • Received Date: 2015-02-12
      • Accepted Date: 2015-04-14
      • Published Date: 2015-01-25

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