Pll ppt

phase locked loop systems

1. Phase locked loop basics PLL  stands for  'Phase-Locked Loop'  and is basically a closed loop frequency control system, which functioning is based on the phase sensitive detection of phase difference between the input and output signals of the controlled oscillator (CO ) The Phase Locked Loop method of frequency synthesis is now the most commonly used method of producing high frequency oscillations in modern communications equipment PLL circuits are now frequently being used to demodulate FM signals, making obsolete the Foster- Seerly and radio detectors of the early years. Other applications for PLL circuits include AM demodulators, FSK decoders, two-tone decoders and motor speed controls. 4/30/2018 Pavithran Puthiyapurayil ,EE, MNU 1

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3. A basic phase locked loop, PLL, consists of three basic elements: Phase comparator / detector:    As the name implies, this circuit block within the PLL compares the phase of two signals and generates a voltage according to the phase difference between the two signals .  4/30/2018 pavithran ,EE, MNU 3

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5. Phase Detector A PD is a circuit that senses two periodic inputs and produces an output whose average value is proportional to the difference between the phases of the inputs The input/output characteristic of the PD is ideally a straight line, with a slope called the “gain” and denoted by K PD

6. How is the PD Implemented? We seek a circuit whose average output is proportional to the input phase difference. An Exclusive-OR (XOR) gate can serve this purpose. It generates pulses whose width is equal to Δϕ

7. Loop filter:    This filter is used to filter the output from the phase comparator in the PLL. It is used to remove any components of the signals of which the phase is being compared from the VCO line. It also governs many of the characteristics of the loop and its stability .  Voltage controlled oscillator (VCO):     The voltage controlled oscillator is the circuit block that generates the output radio frequency signal. Its frequency can be controlled and swung over the operational frequency band for the loop.   4/30/2018 pavithran ,EE, MNU 7

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9. Simple PLL and Loop Filter Negative feedback loop: if the “loop gain” is sufficiently high, the circuit minimizes the input error. The PD produces repetitive pulses at its output, modulating the VCO frequency and generating large sidebands. Interpose a low-pass filter between the PD and the VCO to suppress these pulses. A student reasons that the negative feedback loop must force the phase error to zero, in which case the PD generates no pulses and the VCO is not disturbed. Thus, a low-pass filter is not necessary. As explained later, this feedback system suffers from a finite loop gain, exhibiting a finite phase error in the steady state. Even PLLs having an infinite loop gain contain nonidealities that disturb V cont

10. Simple PLL: Phase Locking We say the loop is “locked” if ϕ out (t)- ϕ in (t) is constant with time. An important and unique consequence of phase locking is that the input and output frequencies of the PLL are exactly equal.

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13. A  voltage-controlled oscillator  or  VCO  is an  electronic oscillator  whose  oscillation   frequency  is controlled by a  voltage  input. The applied input voltage determines the instantaneous oscillation frequency. Consequently,  modulating  signals applied to control input may cause  frequency modulation  (FM) or  phase modulation  (PM). A VCO may also be part of a  phase-locked loop . 4/30/2018 pavithran ,EE, MNU 13

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15. What is a PLL? Control System Representation Parts of a PLL PLL in Simulink 4/30/2018 pavithran ,EE, MNU 15

16. Digital frequency control system Generate high speed oscillations Acquire and track signals Radio Frequency Demodulation DX- ing RF communications 4/30/2018 pavithran ,EE, MNU 16

17. 4/30/2018 pavithran ,EE, MNU 17 Control system representation

18. 4/30/2018 pavithran ,EE, MNU 18 Modeling a PLL

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20. Phase Detector Filter Voltage Controlled Oscillator Programmable Counter 4/30/2018 pavithran ,EE, MNU 20 Parts of a PLL

21. Phase Detector Acts as comparator Produces a voltage proportional to the phase difference between input and output signal Voltage becomes a control signal 4/30/2018 pavithran ,EE, MNU 21

22. Parts of a PLL Filter Determines dynamic characteristics of PLL Specify Capture Range (bandwidth) Specify Tracking Range Receives signal from Phase Detector and filters accordingly 4/30/2018 pavithran ,EE, MNU 22

23. Parts of a PLL Voltage Controlled Oscillator Set tuning range Set noise margin Creates low noise clock oscillation 4/30/2018 pavithran ,EE, MNU 23

24. Parts of a PLL Divider Divides the VCO output by the degree of the open loop gain Feedback loop allows phase comparison 4/30/2018 pavithran ,EE, MNU 24

25. ( mō´dem )  :-  Short for  mo dulator- dem odulator.  A modem is a device  or  program  that enables a  computer  to transmit  data  over, for example, telephone or cable lines. Computer information is stored   digitally , whereas information transmitted over telephone lines is transmitted in the form of  analog  waves. A modem  converts between these two forms. 4/30/2018 pavithran ,EE, MNU 25

26. Standard Modem Interface Fortunately, there is one  standard   interface  for connecting  external modems  to computers called  RS-232 . Consequently , any external modem can be attached to any computer that has an RS-232  port , which almost all personal computers  have. There are also modems that come as an  expansion board  that you can insert into a vacant  expansion slot . These are sometimes called  onboard  or  internal modems . 4/30/2018 pavithran ,EE, MNU 26

27. bps   :  How fast the modem can transmit and receive data. At slow rates, modems are measured in terms of  baud  rates. The slowest rate is 300 baud (about 25  cps ). At higher speeds, modems are measured in terms of bits per second (bps). The fastest modems  run  at 57,600 bps, although they can achieve even higher  data transfer rates by compressing the data. 4/30/2018 pavithran ,EE, MNU 27

28. Obviously, the faster the transmission rate, the faster you can send and receive data. Note, however, that you cannot receive data any faster than it is being sent. If, for example, the device sending data to your computer is sending it at 2,400 bps, you must receive it at 2,400 bps. It does not always pay, therefore, to have a very fast modem. In addition, some telephone lines are unable to transmit data reliably at very high rates. 4/30/2018 pavithran ,EE, MNU 28

29. voice/data:  Many modems support a  switch  to change between voice and data  modes . In data mode, the modem acts like a regular modem. In voice mode, the modem acts like a regular telephone. Modems that support a voice/data switch have a built-in loudspeaker and microphone for voice communication. 4/30/2018 pavithran ,EE, MNU 29

30. auto-answer   : An auto-answer modem enables your computer to receive calls in your absence. This is only necessary if you are offering some type of computer service that people can call in to use. data compression   : Some modems perform data compression, which enables them to send data at faster rates. However, the modem at the receiving end must be able to decompress the data using the same compression technique. 4/30/2018 pavithran ,EE, MNU 30

31. flash memory   :  Some modems come with  flash memory  rather than conventional  ROM , which means that the  communications protocols can be easily updated if necessary. Fax capability:  Most modern modems are  fax modems , which means that they can send and receive faxes . 4/30/2018 pavithran ,EE, MNU 31