6. instantaneous frequency & pll

Instantaneous Frequency & PLL (Phase-Locked Loop or Phase Lock Loop)

Reference Book: B. P. Lathi, Chapter-4 & 5 4th Edition

Prepared By: Md. Ahsanul Hoque, Lecturer, EEE, IIUC

Instantaneous Frequency

Reference Book: B. P. Lathi, Chapter-5, Page: 203-204

Inst. Freq. (Contd.)


Inst. Freq. (Contd.-Extra For help)

PLL

Note: This Slide just for reviewing the basic control idea about PLL

PLL (Contd.)


A PLL is a negative feedback system where an oscillator-generated signal is phase and frequency locked to a reference signal.

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Consider VCO operating without input signal at free-running frequency fr and input signal of frequency fINincreasing from zero is applied to the PC. If the input frequency is less than f IN1, then the error voltage Ve is zero as illustrated in figure and VCO operates at a frequency fr. When the input signal frequency fIN reaches a frequency fin1 (the lower edge of the capture range), then the output or error voltage Ve jumps from zero to some negative voltage with beat notes of frequency (difference between input signal frequency and actual VCO output signal frequency; fIN – fOUT. The error voltage Vg is then filtered, amplified and amplified voltage Vd is applied to the control terminals of the VCO. The instantaneous frequency of VCO decreases because fOUTfalls for nega tive values of Vd and increases for positive values of Vrf. At some instant of time, the decreasing frequency of the VCO equals fIN l (lower edge of the capture range), then lock results-in, and the output signal frequency of the


VCO may be equal to the input signal frequency (that is,fOUT = fIN). The VCO frequency locks with input signal frequency up to fIN2 (the upper end of the lock range). If the input signal frequency exceeds fIN2 then error voltage Vg will fall to zero and the VCO will operate at the free running frequency fr, as illustrated in figure. If the input signal frequency is now slowly swept back and it attains the value of fd1 then the loop (VCO frequency) locks with the input signal frequency, causing a positive jump of the error voltage Ve. So the VCO output frequency increases from fr continuously till fOUT becomes equal to fIN. The VCO frequency fOUTlocks with the input signal frequency fIN upto fd2 (the lower edge of the lock range) as shown in figure by dotted lines. Now if the frequency of the input signal falls below fd2, then the error voltage Ve will fall to zero and the VCO will operate at the free running frequency.


PLL (Contd.)


Locking the Loop

Reference Book: Electronic Communication, Paul H. Young Chapter-10, Page: 393-397

Mathematical Example: Static Behavior

6. instantaneous frequency & pll

Documents

error voltage vg

error voltage ve

input signal frequency

input signal

instantaneous frequency

lock range

lower edge

capture range