Dual Mode Hybrid Pll Based Frequency Synthesizer for Cognitive Multi-radio Applications

This paper investigates a novel approach to implementation of multiband frequency synthesizer for cognitive multi-radio applications. The architecture here considered employs a dual-mode PLL (Phase Locked Loop) based frequency synthesizer for data applications, which is capable to switch between a fractional wideband high speed mode and an integer, narrowband and low spur mode after the settling. A secondary integer-N PLL synthesizer is proposed for voice applications. Both synthesizers are driven by the same reference oscillator. The overall architecture is described here and simulated along with frequency plan considering the most diffused standards in the band from 500 MHz to 6 GHz. 1.Introduction The increasing number of emerging wireless applications and standards calls for a multi-band and multi-standard radio operation. An approach, which is frequently used in multistandard wireless devices, employs multiple chips, each one dedicated to a particular communication standard. This approach provides the best RF performance for individual standards, but it significantly penalizes the performance in terms of IC integration and power consumption. Therefore, a single multi-radio transceiver able to fulfil requirements of multiple standards becomes a challenge. Suchlike multi-radio devices should be able to cope with various waveforms and types of modulation (high vs. low Peak to Average Power Ratio PAPR), different average power and power control dynamic range, different frequency of operation and different radio bandwidths (narrow vs. wideband operation). In addition to this, the power consumption becomes a very important issue. Different approaches for efficient multi-radio transmitters have been recently proposed and a lot of research efforts have been put particularly in the reconfigurability of RF components and high power efficiency [1], [2]. Moreover, as the frequency spectrum utilization reveals to be very poor [3], a new paradigm in wireless communications based on the spectrum aware cognitive radio principle becomes widely discussed. In order to adapt to the actual transmission request and the radio spectrum accessibility, the cognitive radio concept will require very high flexibility of RF elements. One of the most challenging components to design in the flexible multi-band RF front-end is undoubtedly the wideband reconfigurable LO (local oscillator). The LO frequency is used to convert the signal from the baseband to the RF and vice versa and it frequently determines overall performance of the system. In the ideal case, only one LO should be able to deliver the appropriate frequency (in quadrature) with respect to requirements of relevant communication standard and common performance metrics such as phase noise, frequency settling time, spurious output, frequency raster etc. Beside the FDD (Frequency Division Duplex) based communication standards, which require simultaneous LO operation in uplink and downlink directions, an additional LO is needed in cases, where the simultaneous voice and data communication is required (e.g. voice GSM/3G voice communications along with Bluetooth/WiFi/WiMAX data communications). Therefore, a frequency plan that suits to the simultaneous radio operation has to be considered. There have been proposed various techniques for multi-band LO operation, including a reconfigurable ADPLL [4], PLL with wideband or distribute VCO [5], [6] or employing multiple PLL’s. This paper deals with the analysis of a CP (Charge Pump) PLL based frequency synthesizer that combines two different modes of operation (fractional-N and integer-N) accompanied by the loop filter switching technique. Advantages and drawbacks of this approach will be pointed out along with particularities and system performance characteristics of both modes of operation. The first section briefly summarizes RF requirements related to the LO design given by communication standards in the band from 500 MHz to 6 GHz. Next, the overall principle and functionality of proposed multi-band synthesizer are given along with simulations and behaviour analysis of both modes of operation. Subsequently, different modes of operation are proposed and attributed to different communication standards, according to their requirements. 2.Multi-Radio Requirements Table 1 summarizes LO requirements in terms of the phase noise, settling time and channel raster, given by different communication standards. Stars correspond to more critical requirements. It can be seen, that the most critical requirement in terms of the phase noise and RF channel raster is claimed by the GSM standard. In terms of settling time, Mobile WiMAX (802.16e) calls for the fastest frequency switching performance (more detailed study of LO RF requirements can be found in [7]). These requirements will be considered in the initial design of the synthesizer architecture. Table 1: LO requirements given by different standards. Standard Phase Noise Settling Time Ch. Raster