week 5 ppt - The Basic Phase Locked Loop The purpose of a...

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The Basic Phase Locked LoopThe purpose of a phase locked loop (PLL) is to recreate an input signal. A PLL will generate an input signal and lock in-phase with an external signal. What makes the PLL especially useful in contemporary electronics is its method of lock-in and the internal signals generated.The PLL below is a diagram of a generic, unconfigured circuit.
Notice how this PLL circuit forms a feedback loop. The two inputs (i.e. the external input and the output of the voltage-controlled oscillator) and the phase comparator are locked during the PLL's operation.A phase comparator functions much like the mixer in a superheterodyne receiver. Recall from Week 3 that the mixer is a non-linear block that produces sum and difference frequencies. With mixers, you want the RF carrier frequency to have a difference of 455Khz so that it can pass through the IF amps.A PLL works in a similar fashion. The output of the phase comparator -- or, the difference between the input and the voltage controlled oscillator (VCO) frequency -- must be zero. This is a key point for PLL circuits, and it will happen only when the VCO is exactly the same frequency as the input signal. Between these two signals, there is a phase shift. This phase shift generates a DC voltage that passes straight through the low-pass filter and is subsequently applied to the input of the VCO.If the VCO frequency does not match the input frequency, the difference frequency at the output of the phase comparator is non-zero. This might happen when the circuit is first turned on or when the input frequency is changed. In this case, a low-pass filter reduces the voltage before cycling it back to the input of the VCO. The voltage produced by the low-pass filter is called the error voltage

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