S Transmission Methods in Telecommunication Systems (4 cr) Carrier Wave Modulation Systems

2 Helsinki University of Technology,Communications Laboratory, Timo O. Korhonen Phase-locked loops (PLLs) n Phase-locked loop is a feedback arrangement capable to synchronize itself to a noisy external reference n The output signals of the loop can be used to produce for instance multitude of locked frequencies n PLL application areas include... –modulators –demodulators –frequency synthesis –multiplexers –signal processors

3 Helsinki University of Technology,Communications Laboratory, Timo O. Korhonen The PLL principle n The PLL circuit consists of –phase comparator (in the figure below the multiplier) –lowpass filter –feedback amplifier –VCO (voltage controlled oscillator), whose output frequency is linearly proportional to input amplitude n Principle: phase difference of X c (t) and v(t) adjusts VCO Phase comparator output is comparable to phase difference of input signals

4 Helsinki University of Technology,Communications Laboratory, Timo O. Korhonen PLL phase comparator realizations n Circuits: (a) analog and (b) digital phase comparator circuit n Note that for (a) output is proportional to –input signal phase difference –input signal amplitudes (unintended AM thus harmful) n In (b) AM effects are compensated and response is more linear XOR-circuit ideal pulse ratio: 50/50

5 Helsinki University of Technology,Communications Laboratory, Timo O. Korhonen FM detection by PLL time domain phase domain frequency domain

6 Helsinki University of Technology,Communications Laboratory, Timo O. Korhonen PLL FM-demodulator: the feedback analysis Solve transfer function with feedback: This is applied to the linearized PLL yielding relationship between the input phase and output voltage:

7 Helsinki University of Technology,Communications Laboratory, Timo O. Korhonen Applying the FM signal to the linearized PLL model n Remember the FM wave: where the modulating signal is denoted by x(t). The input FM phase to the system is thus n This is in frequency domain: assuming no DC component or V(0) = 0, or

8 Helsinki University of Technology,Communications Laboratory, Timo O. Korhonen Applying FM signal to the detector... (cont.) n Thus the input is and the output is where the loop equivalent transfer function is n Assume that the first order LP function is used or Y(f)

9 Helsinki University of Technology,Communications Laboratory, Timo O. Korhonen PLL based frequency synthesizer VCO Filt. Phase detector Phase detector Divide by 10 By adjusting the divider different frequencies can be produced whose phase is locked into f in Reference signal f in is locked for instance to the fundamental frequency of a crystal oscillator

10 Helsinki University of Technology,Communications Laboratory, Timo O. Korhonen Detecting DSB using PLL-principle n An important application for PLLs is in synchronization of receiver local oscillator in synchronous detection n In the Costas PLL (below) two phase discriminators are used to: –cancel out DSB modulation x(t) in the driving signal –synchronize the output frequency to the center frequency of the DSB spectra (the suppressed carrier) –to detect the DSB signal Costas PLL detector for DSB PD: phase detector (=multiply+LPF) Loop drives phase error to zero LPF yields constant (zero) output when loop is locked to carrier