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Implementation of Low Power All Digital Phase Locked Loop

Rajani Kanta Sutar1, M.Jasmin2 and S. Beulah Hemalatha3
  1. PG Scholar, Bharath University, Tamilnadu, India
  2. Assistant Professor, Department of ECE, Bharath University, Tamilnadu, India
  3. Assistant Professor, Department of ECE, Bharath University, Tamilnadu, India
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Phase locked loop is a familiar circuit for high frequency application and very short interlocking time. In this paper we have implemented and analysed All Digital Phase locked loop (ADPLL), as the present applications requires a low cost, low power and high speed Phase locked loops. The design is synthesized in Xilinx ISE software. The pure digital phase locked loop is attractive because it is less sensitive to noise and operating conditions than its analog counterpart. This project gives details of the basic blocks of an ADPLL. In this project it is been planned to implementation of ADPLL. Its simulation results are verified for all the corners of inputs. The ADPLL is planned for 200 MHz central frequency




A modern advance technology in integrated circuit technology makes fabrication processes very suitable for digital designs. Small-area and low-voltage designs are mandated by market requirements. Another advantage of digital PLL is easy to redesign with the process changes. Since analog blocks are present in a number of digital and mixed-signal ICs, their redesign is an important factor in the release of a new product. However, the performance requirements of analog blocks necessitates a complete redesign in a new process, thereby increasing the design cycle time. Reducing the amount of analog circuitry can improve the redesign of these mixed-signal ICs. A Phase Locked Loop is mainly used for the purpose of synchronization of the frequency and phase of a locally generated signal with that of an incoming signal. There are three components in a PLL. The Phase Frequency detector (PFD), the loop filter and the Voltage Controlled Oscillator (VCO). The VCO is the heart of any PLL. The mechanism by which this VCO operates decides the type of the PLL circuit being used. There are basically four types of constructing PLLs namely Linear PLL (LPLL), Digital PLL (DPLL) and All Digital PLL (ADPLL).
The analog PLLs (APLLs) are still widely used, but digital PLLs (DPLLs) are attracting more attention for the significant advantages of digital systems over their analog counterparts. These advantages include superiority in performance, speed, reliability, and reduction in size and cost. DPLLs alleviated many problems associated with APLLs. The following is a brief comparison:
1. APLLs suffer from the sensitivity of the voltage-controlled oscillator (which decides the center frequency) to temperature and power supply variations, hence the need for initial calibration and periodic adjustments. DPLLs do not suffer from such a problem.
2. the most familiar error detectors used in APLLs utilize analog multipliers (balanced modulators) which are sensitive to d.c. drifts, a problem that does not exist in DPLLs.
3. DPLLs can operate at very low frequencies that create problems in APLLs. These problems are related to the operation of the analog low-pass filter in extracting the lower frequency component, as it needs larger time for better frequency resolution, and this will reduce the locking speed.
4. Self-acquisition of APLLs is often slow and unreliable; while DPLLs, a basic block diagram is shown in Figure 1.1, have faster locking speeds. This is due to the basic operation of the analog low-pass filter and the analog multiplier in the phase detector (PD).


A survey of PLL is presented by Gupta in his paper. Both classical and modem approaches are discussed. He suggested that although the design of the A PLL is fairly well documented, more needs to be done to pinpoint the formal design of the DPLL, the primary use of the PLL has been in more sophisticated communication systems, however, with the rapid development of IC Technology, time is not far when PLL’s will be used widely in consumer electronics.[Gupta somvanshi 1975].
One of the important applications of Phase –lock loop is described by Yinqing. According to him Phase-lock loops are widely used in carrier synchronization and bit synchronization systems to improve their synchronization properties. Phase-lock loops are used in synchronization systems due to their tracking, narrow band filtering and memory properties. [Yinqing,, 1991].
Another important application of Phase –lock loop as frequency synthesizer and its model is described by Yang in 1997. Yang developed a model of a phase-locked loop (PLL) - based frequency synthesizer. The voltagecontrolled oscillator (VCO) utilizing a ring of single-ended current-steering amplifiers (CSA) provides low noise, wide operating frequencies, and operation over a wide range of power supply voltage. A programmable charge pump circuit automatically configures the loop gain and optimizes it over the whole frequency range. The measured PLL frequency ranges are 0.3–165 MHz and 0.3–100 MHz at 5 V and 3 V supplies, respectively. The peak-to-peak jitter is 81 ps (13 ps rms) at 100 MHz. The chip is fabricated with a standard 0.8-m n-well CMOS process. The chip is implemented in a standard 0.8- m CMOS process. [Yang, 1997]
From the continuous survey it is observed that foundry of technology and supply voltage range is continuously decreases with the advancement of technology. Phase-lock loop with 0.35- m CMOS technology at a supply voltage of 1.8 V has been designed in 2002.Chen and Sheen presented a phase-locked loop for clock generation that consists of a phase/frequency detector, charge pump, loop filter, range-programmable voltage-controlled ring oscillator, and programmable divider. Circuits as having low power dissipation, is widely adopted. The modified phase detector and charge pump have been extensively used to enhance the performance of the PLL. By applying the TSMC 0.35- m CMOS technology, the proposed phase-locked loop that uses the power-switch scheme can yield clock signals ranging from 103 MHz to 1.02 GHz at a supply voltage of 1.8 V. Moreover, power dissipation that is proportional to the number of paralleled inverter rings is measured with 1.32 to 4.59 mW. By using the TSMC 0.35- m CMOS technology, the designed PLL using the power-switch scheme can generate clock frequencies ranging from 103 MHz to 1.02 GHz with a power dissipation ranging from 1.32 to 4.59 mW, at a supply voltage of 1.8 V. [Chen and Sheen, 2002]


Digital phase-locked loops can be classified into two major categories depending on the type of sampling process. 1. Uniform sampling DPLLs 2. Non-uniform sampling DPLLs. The DPLLs can be also classified according to the mechanization of the phase detector into five types as follows namely The flip-flop DPLL, The Nyquist-rate DPLL, The lead-lag DPLL (LLDPLL), a.k.a binary-quantized DPLL and 4. Exclusive-OR DPLL Types 2 above belongs to uniform sampling, while the others belong to non uniform sampling. A brief discussion of each type is given below.

Flip-Flop DPLL

In this type the phase detector is realized by a set-clear flip-flop and a counter. The sinusoidal input signal is converted into a square wave through an operational amplifier acting as a comparator. The output “Q” of the flip-flop is set to logic “1” on the positive-going edge of the comparator, and to logic “0” on the positive-going edge of the digital controlled oscillator (DCO). Hence the duration when Q is at level “1” will be proportional to phase error between the input signal and the DCO. This error is used to gate the counter clock which has a frequency of 2Mfo where fo is the center frequency of the DPLL and 2M is the number of quantization levels of the phase error over period of 2π. The counter is zeroed and starts counting on the positive-going edge of the flip-flop waveform. The content of the counter, No, which is proportional to the phase error, is applied to the N-bit first-order digital filter which consists of proportional and accumulation paths. The output of the digital filter K controls the period of the DCO which consists basically of a programmable divide-by-K counter. It is the phase of the input signal that undergoes non-uniform sampling here rather than the amplitude.

Lead-Lag DPLL

This type of DPLLs has been developed by the work in and extended in to include a second-order sequential filter with memory. The LL-DPLL is characterized by the binary output of the phase detector that indicates whether the DCO waveform leads or lags the input signal. Due to this quantization it is often named “binary quantized DPLL”. The input sinusoidal signal should be converted to a square wave by a comparator. On the occurrence of a DCO pulse, either “lead” or “lag” terminal of the phase detector will give a pulse depending on the state of the input signal being “high” or “low”, respectively. These pulses are applied to a special type of digital filters known as “sequential filter.” The sequential filter deals with the input “lead” and “lag” pulses statistically; it observes them for a variable duration of time and gives a decision when a reliable limit is reached.
Figure 2 shows that the sequential filter is composed of an up-down counter whose length is 2N + 1. A pulse at the “lead” terminal causes the content of the counter to increase by 1, while the “lag” pulse behaves conversely. When the content of the counter reaches 2N or zero, the corresponding “Retard” output gives a pulse that resets the counter to “N” and triggers the phase controller. A “Retard” pulse causes the phase controller to delete one pulse from the clock pulse train that is applied to the divide-by-L counter, forcing the DCO phase to retard by 2π/L, where L is the number of quantization levels of the period 2π. An “Advance pulse does the contrary. When “lead” and “lag” pulses are equally probable, a case that indicates locking, the counter cycle has maximum duration. Other types of sequential filters exist like the N-before-M filter and the variable reset random walk filter.


Phase Frequency Detector (PFD)

Phase detector generates error signals depending upon the phase error in locked state and frequency error in unlocked state. We have used a synchronous version of PFD presented in [1]. This synchronous PFD can be easily implemented on an FPGA as all signals are synchronized to a clock.

Loop Filter

Loop filter takes error signals from PFD as input, these signals are processed to generate control signals for dco. Loop filter effectively performs the following calculations once on each cycle of dco clock period for duration of mismatch. The implemented loop filter is presented in figure 3.This implementation is designed for low hardware requirements and high frequency of operation. Fixed point arithmetic is used to accommodate fractional values. The register error denoted in figure as ‘Error reg’ is included to enhance maximum frequency of operation. Including this register requires change in generated control signals denoted in figure as ‘ clear’ and ‘up date’ so that equation 1 is calculated for equal number of clk dco cycles for which phase error exist. The required change is that update and clear signals must be delayed by one clk dco cycle which has been represented in figure 4.


We have implemented dco proposed in which comprises of two parts. A ring oscillator with variable delay, the second part consists of control unit that constantly switches number of delay elements in the ring between two adjacent integral values to obtain fractional number of effective delay elements. This implementation has high frequency resolution. Switching between two adjacent values causes maximum peak to peak jitter of. Complete architecture of DCO is shown in figure 5

Fractional Frequency Divider (FFD)

For high frequency resolution and flexibility the feedback of DCO has been implemented using a reprogrammable fractional frequency divider presented in [2]. Scaling factor of this accumulator switches periodically between two adjacent integers realizing a fractional scaling factor on average. The Most Significant Bit (MSB) of register is the output of FFD. The MSB also controls selection of N or N-M values. If MSB is zero, negative value N-M is selected. If MSB is one, the positive value N is selected. On average MSB bit is high for N out of M cycles of clkdco. The register N and N-M are made reprogrammable using a separate clock signal ‘reprogram’. The values of these registers can be varied so as to attain different clkdco frequencies for different ref frequencies as per requirement. A complete architecture is shown in below Fig.6


ADPLL components are Phase Detectors (PDs), Loop Filters (LFs) and Digital Controlled Oscillators (DCOs). Based on all these components we can design various types of ADPLLs. For designing ADPLL, Verilog hardware description language is used. It is simulated in ISE Xilinx 12.1. All basic building blocks of ADPLL i.e. Phase Detector (PD), Loop Filter (LF) and Digital Controlled Oscillator (DCO) are designed using Verilog (HDL). EXOR gate and JK Flip flop are used as PD. Output of EXOR or JK Flip-flop PD is fed into K counter. This signal is represented by Dn/up. K counter clock is Kclk signal. Kclk is M time multiple of center frequency. Up counter and down counter are two independent counters of K counter. The Dn/up signal controls the K counter. If this signal is low then up counter is active and down counter becomes inactive. In other case when this signal is high then down counter is active and up counter becomes inactive. Outputs of K counter are carry and borrow pulses. Carry and borrow are MSB of the up and down counter respectively. Carry pulse is fed into INC input of ID counter .Whereas borrow pulse is fed into DEC input of ID counter .Output of ID counter is IDout .In general one carry pulse adds half cycle to IDout and one borrow pulse delete half cycle to IDout signal.ID clock is 2N time multiples of center frequency. Output of ID counter (IDout) is fed into divide by N counter, which is the last stage of DCO.ID out is used as clock pulses for divide by N counter. Design is verified and simulated in Xilinx ISE 12.1 tool. A diagram of designed and implemented ADPLL [8] is shown in the Fig.7. Also M=2N, so both clock frequency are taken from the same source.


Phase frequency detector and DCO simulations are done using Xilinx and simulation waveform of same is shown below. The design has been done keeping in mind the portability, flexibility and optimality criterion. It can be used in any design suiting the given frequency specifications. A system clock of 5 MHz is used. The design is implemented for a center frequency of 300 kHz. It’s mainly meant for low frequency applications. The current design offers a operating frequency range of 290 kHz to 320 kHz approximately. The top module of the designed ADPLL has been presented in Fig 8. The design can be extended beyond that also. But the logic criterion needs to be changed to overcome the propagation delay that is introduced due to greater number of bits involved in the computation.


According to review of ADPLL, digital circuits are more versatile, efficient, flexible, and less noisy as compared to analog circuit. This paper discusses the ADPLL design using Verilog HDL It also presents the FPGA implementation in detail. The ADPLL blocks used for the design are also given here. This PLL is designed for the centre frequency of 200 kHz and its operating frequency range of ADPLL is 189 kHz to 215 kHz, which is the lock range of the design. When the input was at fault, it automatically resumed output formation by employing the last known correct parameters.


  1. Aniruddha Chandra, Lecture on “Phase Locked Loop”, in winter school on VLSI Systems Design , ECE Department, NIT Durgapur, Jan. 2009

  2. KusumLata Department of Electronics and Communication Engineering, Indian Institute of Information Technology,Allahabad, INDIA

  3. Manoj Kumar Department of Electronics and Communication Engineering, Indian Institute of Information Technology, Allahabad, INDIA

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