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Seminar on:

Seminar on:. PHASE LOCKED LOOP. Presentation by : Alekh Pradhan Adm.No. 3 I&E/99. BASICS. It’s an electronic circuit consisting of a Phase Detector, a Low-pass Filter, and a VCO.

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  1. Seminar on: PHASE LOCKED LOOP Presentation by : Alekh Pradhan Adm.No. 3 I&E/99

  2. BASICS • It’s an electronic circuit consisting of a Phase Detector, a Low-pass Filter, and a VCO. • The PLL evolved in the early 1930s, but with the rapid development of IC technology however, it has emerged as one of the fundamental building blocks in electronics technology.

  3. 565 PLL Integrated Circuit

  4. BLOCK VIEW OF THE PLL

  5. IC view of PLL

  6. PLL Operating Principle • Before the input signal is applied, the PLL is in the free-running state. Once the input frequency is applied, the VCO frequency starts to change and the PLL is said to be in the capture mode. The VCO frequency continues to change until it equals the input frequency, and the PLL is then in the phase-locked state. When phase-locked, the loop tracks any change in the input frequency through its repetitive action.

  7. PLL phasor diagram • When i/p signal has frequency greater than the max. lock frequency Fin2 then Ve is zero. At this point o/p frequency is Fr. Similarly for min. lock frequency too.

  8. The Phase detector • Out of the two inputs one is the input signal and the other is the output signal from the VCO. • It compares the two pulse trains and produces a DC voltage which is proportional to the phase difference between the two frequencies. This voltage is known as the error voltage. • Without the input signal, Ve and the VCO operates at a set frequency Fr which is also called free-running frequency of the VCO.

  9. Why use Low-pass Filter? • It controls the capture and lock ranges, bandwidth, and transient response. • It removes the high-frequency components in the o/p of the phase detector. • It removes the high-frequency noise.

  10. Filter: active or passive? • Active filter includes the gain, while for the case of passive filter an amplifier is used to provide the gain.

  11. Purpose of VCO • Provides an oscillating o/p signal whose frequency can be adjusted over a range by a dc voltage. • The frequency of oscillation is set by an external resistor and a capacitor and then varied by an applied dc voltage.

  12. 8-Pin VoltageControlledOscillator

  13. Voltage supply to the PLL: • 6V to 12V range.

  14. Operating frequency range: • 0.001Hz to 500kHz.

  15. Just one example:- • i/p frequency range = 40 – 60 kHz. • o/p frequency range =58 – 52 kHz. • Lock range = 20 kHz. • Capture range = 4 kHz. • Centre freq. (free running freq.) =50 kHz.

  16. COMMON APPLICATIONS • Frequency synthesizers. • FM demodulation networks. • FSK decoders. • Modems. • Telemetry receivers and transmitters. • Tone decoders. • AM detectors. • Tracking filters.

  17. Frequency Synthesis • Frequency divider is inserted between the o/p of the VCO and the Phase comparator. • The VCO o/p is Nxf where f is the loop signal to the Phase comparator. • Frequency multiplication holds as long as the loop is in lock.

  18. Frequency Demodulation • The FM carrier frequency is made the centre frequency of the PLL.

  19. Frequency Translation • A mixer stage outputs the difference frequency (Fo-Fr)to the phase comparator so that the reference frequency is translated by a small factor equal to (Fo-Fr).

  20. Thank you!

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