TELECOMMUNICATIONS Dr. Hugh Blanton ENTC 4307/ENTC 5307

Dr. Blanton - ENTC More PLL 2 The phase lock loop (PLL) is a frequency selective feedback system which can synchronize with a selected input signal and track the frequency changes associated with it.

Dr. Blanton - ENTC More PLL 3 The basic PLL system is comprised of three essential blocks: A phase detector, A loop filter, and A voltage-controlled oscillator (VCO).

Dr. Blanton - ENTC More PLL 4 Phase Detector The phase detector compares the phase of the periodic input signal V s (t) with the output frequency of VCO and generate an error voltage V d (t). The error voltage is then filtered by the loop filter and is applied to the VCO in the form of the voltage V e (t) to control the frequency of oscillation.

Dr. Blanton - ENTC More PLL 5 When the PLL is locked on the input signal (normal PLL operation), the VCO frequency is identical to the input frequency f s, except for finite phase difference,  o. This net phase difference of phase error,  o, is necessary to generate the corrective error voltage, V e (t), to shift the VCO frequency to higher frequencies if f s starts to increase, or to lower frequencies if f s starts to decrease in order to maintain lock.

Dr. Blanton - ENTC More PLL 6 The figure shows the case when the input frequency is equal to  o (the so called free- running frequency) and increases to 2  o.

Dr. Blanton - ENTC More PLL 7 The self-correcting ability of the system allows the PLL to track the frequency changes of an input signal, once it is locked. The range of frequencies over which the PLL can maintain lock with the input is defined as the lock range. The lock range cannot exceed the range of control of the VCO.

Dr. Blanton - ENTC More PLL 8 Lock does not exist all the time. With no input signal applied to the PLL, the filtered error voltage V e (t) in the feedback loop is equal to zero. Thus, the PLL operates at the free-running frequency:

Dr. Blanton - ENTC More PLL 9 If a periodic input signal V s (t) is applied to the PLL such that  s is sufficiently close to  o, the feedback nature of the PLL causes an error voltage V e (t) to be generated. The forces the VCO to synchronize with the input frequency, and the PLL will be locked.

Dr. Blanton - ENTC More PLL 10 The range of frequencies over which the PLL can acquire lock with the incoming signal is called the capture range or (acquisition range). The capture range is always smaller than the lock range.

Dr. Blanton - ENTC More PLL 11 When the PLL is locked, the VCO output provides a periodic waveform which is at the same exact frequency as the input signal, except for a finite difference  o, which is the phase difference necessary to generate V e to keep the PLL in lock.

Dr. Blanton - ENTC More PLL 12 If the input signal includes many frequency components and noise and other disturbances, the PLL will be locked on one component (closest to  o ). The frequency output of PLL will regenerate this particular component eliminating the other undesired frequencies.

Dr. Blanton - ENTC More PLL 13 Assume that the PLL is opened between the loop filter output and the VCO control input. This would cause the error voltage to be artificially reduced to zero, and the VCO will continue to oscillate at the free-running frequency, f o.

Dr. Blanton - ENTC More PLL 14 Let the input signal of fs (f s > f o, yet close to f o ) be applied. Since the phase detector normally functions as a mixer, the output of the phase detector will be two frequency components, a sum frequency (f sum = f o + f s ) and a difference frequency (  f = f o – f s ). Any mixer can be used as a phase detector.

Dr. Blanton - ENTC More PLL 15 The loop filter is a narrow-band low pass filter that filters f sum leaving the difference frequency (  f = f o – f s ). If f s is close to f o, the difference frequency will appear at the output of the loop filters as a sinusoidal beat note.

Dr. Blanton - ENTC More PLL 16 If the loop is closed, the VCO frequency will be modulated by the beat note.  f becomes a function of time.

Dr. Blanton - ENTC More PLL 17 The portion of the beat note that modulates the VCO closer to the input signal appears more rounded, and the portion that modulates the VCO away from the input signal appears more peaked. Because of the asymmetry, the beat note contains a finite DC voltage which steadily pushes the VCO frequency toward the input signal.

Dr. Blanton - ENTC More PLL 18 As the VCO drifts toward f s, the beat note frequency rapidly decreases, the asymmetry increases, and the transient rapidly converges to a steady-state DC value, corresponding to the lock condition where the VCO frequency is exactly equal to f s.

Dr. Blanton - ENTC More PLL 19 Once the system is locked, the difference  f is identically equal to zero, and only a DC voltage generated by the phase difference between the VCO output and the input signal, remains at the loop filter output. It is assumed that the detector characteristic has a gain of K d (V/rad), and the filter has unity gain at DC.

Dr. Blanton - ENTC More PLL 20 The total time taken by the PLL to establish lock is called pull-in time.

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