RF Systems Phase Detector Filter Voltage Controlled Oscillator for PLL Phase-Lock Loop Phase Detector Filter Voltage Controlled Oscillator for PLL Complete PLL Targil3: PLL Design .

PLL applications The phase-lock-loop (PLL) is commonly used in microprocessors to generate a clock at high frequency (Fout=2GHz for example) from an external clock at low frequency (e.g.; Fref = 100MHz). The PLL is also used as a clock recovery circuit to generate a clock signal from a series of bits transmitted in serial without synchronization clock. The PLL may also be found in AISC as local synchronization clock tree with master clock, also utilize in frequency demodulation circuits, to transform a frequency varying waveform into a voltage.

PLL principle of operation The PLL uses a high frequency oscillator with varying speed, a counter, a phase detector and a filter .The PLL includes a feedback loop which aligns the output clock Clockout to the input clock ClkrefOscIn through a phase locking stabilization process. When locked, the high input frequency freqOut is exactly N*ƒin. A variation of the input frequency ƒin is transformed by the phase detector into a pulse signal, which is converted into variation of the analog signal Vc. This signal changes VCO frequency, which is divided by the counter and changes clkDiv according to ƒin.

Phase Detector The most simple phase detector is the XOR gate. The XOR gate output produces a regular square oscillation PD_Out when the input clkIn and the signal divIn have one quarter of period shift ( 90° or π/2). For other angles, the output is no more regular. Here two clocks with slightly different periods are used in Dsch2 to illustrate the phase detection.                                                                                                                                                                                                                                                                                          

Phase Detector At initialization, the average value of the XOR output VPD is close to 0. When the phase between clkDiv and clkIn is around Π/2, VPD is VDD/2. Then it increases up to VDD. Consequently, VPD and the phase difference are linked by expression: For example, when ΔΦ=Π/2, VPD is VDD/2. .                               

XOR Phase Detector Principles When the output of the XOR is a pulse with 50% duty cycle, the pll is in lock, or in other words the clock out of pll is sync with the incoming clock (or data..). Let’s start with data consist of string of zeros and clk divide by 2 (N=2): The XOR output is just a replica of the dclock, thus if dclcok is 50% duty cycle, the PLL is in lock. If logic “1” is suddenly applied there is no way to know if the clock out is sync with it (falling edges coincide with center of data bit), thus: Incoming data must have a minimum number of bit transitions over a given time interval. Consider again bit tream of zeros: with simple RC loop filter (RC>>Tclock), the output of the filter is simply VDD/2, thus: b) With No input data, the filtered output of the PD is VDD/2. This voltage is the input for the VCO.                               

c) The time it takes the loop to lock is dependent on the data pattern Typically we design the VCO with Fcenter = nominal bit rate., it is important that it oscillate even w/o data. Suppose the data is leading the dclcok, if the output of the PD is low pass filtered the result is as average voltage less than VDD/2, which cause VCO frequency to decrease until dclock edge is centered on data: c) The time it takes the loop to lock is dependent on the data pattern Input and the loop –filter characteristic Since the output of the PD is averaged, noise injected to data pattern (false bit) can be rejected, thus: d) The XOR PD has good noise rejection. Another important feature is whether the pll will lock on a harmonics of input data, the XOR PLL will lock on harmonics. To prove this replace any of clock signals at previous figures with twice/half the frequency, the average output will remain the same, thus: e) To avoid lock on harmonica, the VCO operating frequency range should be limited to frequency lower than 2Fclock and higher than 0.5Fclcok (fclcok = bit rate or target (reference) frequency).                               

Targil Kita We saw b4 that XOR PLL can lock on a harmonics of input data, the Prove this in timing diagram: 1) Replace any of clock signals at previous figures with twice and half the frequency, 2) Show that with simple ‘RC’ loop filter the average output will remain the same.                               

Df= fdata – fdclock = Dt *2pi/Tdclock = Dt *2pi/Tclock,out The loop filter we used here is simple low pass RC, since the output of the PD is oscillating, the output of the filter will show some ripple even when PLL is locked. This will modulate the clock frequency, thus: c) A ripple on the output of the loop filter with frequency equal to the clock frequency will modulate the voltage of the VCO. To characterize the PD we can define the time difference between the rising of dclock and the data bit as Dt, then the phase difference is : Df= fdata – fdclock = Dt *2pi/Tdclock = Dt *2pi/Tclock,out At lock the clock is centered at bit, then Dt = Tclock /4 -- Df= Pi/2 and the time difference between data and dclock is also Pi/2. Consequently, we can expressed the average voltage PD output as: Vpd,out = VDD*Df/Pi = Kpd * Df The gain of the PD is Kpd (VDD/Pi) . Thus: center freq of VCO should be equal to clock frequency when the VCO control voltage is VDD/2, Max/Min freq of VCO should be selected to avoid lock in harmonics, VCO duty cycle should be 50%, otherwise problems in locking or/and cause clock jitter.                               

Phase Detector Gain The gain of the phase detector is the ratio between VPD and ΔΦ. The gain is often written as KPD, with an expression derived from last equation which is valid for ΔΦ between 0 and π, as drawn here. When the phase difference is larger than π, the slope sign is negative until 2π. When locked, the phase difference should be close to π/2. .                                             

Filter  The filter is used to transform the instantaneous phase difference VPD into an analog voltage Vc which is equivalent to the average voltage VPD. The rapid variations of the phase detector output are converted into a slow varying signal Vc which will later control the voltage controlled oscillator. Without filtering, the VCO control would have too rapid changes which would lead to instability. The filter may simply be a large capacitor C, charged and discharged through the Ron resistance of the switch. The Ron.C delay creates a low-pass filter The XOR output charged the a large capacitor and a serial resistance to create the desired analog voltage control Vc.

Filter Layout The filtered version of the XOR gate output VPD is shown here. The XOR output charged the a large poly/poly2 capacitor and a serial resistance to create the desired analog voltage control Vc.

Filter simulation It can be seen that VPD is around VDD/2 when the phase difference is Π/2 or -Π/2. The duty cycle of the phase detector output should be as close as possible to 50%, so that Vc is very close to VDD/2 when the inputs are in phase. If this is not the case, the PLL would have problems locking or would not produce a stable output clock. The XOR gate layout has been modified so that the output voltage Vc is very close to VDD/2 when one input in fixed to ground and the other input is a regular clock.

Voltage Controlled Oscillator for PLL Important characteristics of the PLL can be listed: The oscillating frequency should be restricted to the required bandwidth. For example, in European mobile phone applications, the VCO frequency should be varying between flow=1700 to fh=1800MHz. Due to process variations, the VCO frequency range should be extended to fmin/fmax, ~ 10% higher and lower than the request range. When the control voltage Vc is equal to VDD/2, the VCO clock should be centered in the middle of the desired frequency range. The duty cycle of the VCO clock output should be as close as possible to 50% [Baker]. If this is not the case, the PLL would have problems locking or would not produce a stable output clock.

Voltage Controlled Oscillator for PLL, requirements and design                                                                              

The current starved oscillator can be used as a VCO for the PLL, with a modification of its voltage control circuit so that the center frequency is 2450MHz at Vc=VDD/2, and the frequency range do not exceed 2800MHz and do not drop lower than 1800MHz. The modification consists in providing a permanent current path through Rvdd2 to VDD/2 which helps keeping Vc around VDD/2. When VPD is VDD, Vc is increased and the VCO frequency is close to fmax. When VPD is 0, Vc is lowered and the VCO frequency is close to fmin. A second sub-circuit added in the PLL is the precharge to VDD/2. The nMOS device controlled by Vc_Prech helps the big capacitor Cfilter to reach VDD/2 during few nanosecond thus speeds up the PLL locking.

Complete Phase Lock Loop The implementation of the PLL shown here is a direct copy of the schematic diagram of PD, Filter and VCO. Notice that the resistor Rfilter (1000Ohm) and Rvdd2 (5000 Ohm) and the capacitor Cfilter (0.3pF) have been implemented as virtual elements. However, these resistance and capacitance are easy to integrate on-chip.

Complete Phase Lock Loop: Simulation The input frequency is fixed to 2.44GHz. During the initialization phase. The precharge is active, which pushed rapidly the voltage of Vc around VDD/2. The VCO oscillation is started and the phase detector starts operating erratically. The output Xnor is an interesting indication of what happens inside the phase detector. We see that the phase difference is very important during the first 10 nanoseconds. Then, the VCO output starts to converge to the reference clock. In terms of voltage control, Vc tends to oscillate and then converge to a stable state where the PLL is locked and stable. The output is equal to the input, and the phase difference is equal to one fourth of the period (Π/2) according to the phase detector principles.

Complete Phase Lock Loop: Simulation

Targil 4 Design frequency synthesizer using PLL principles. Target frequency is 80MHz while reference frequency is low cost 20MHz xtal oscillator oscillation