Phase-Locked Loop (PLL)

lektronik
abor
Laboratory
Phase-Locked Loop (PLL)
M. Schubert
Versuch 4: Phasengerastete Schleife (PLL)
Hochschule Regensbur
A phase-locked loop (PLL) gains information about an external oscillation by driving a local oscillator (LO)
synchronous (phase-locked) to the external oscillation and taking phase- and frequency information from the
LO. The LO comes typically as voltage-, current- or digitally-controlled oscillator, abbreviated as VCO,
CCO or DCO, respectively.
in
P(F)D
ud
Filter
uf
in
PD
ud F0
R1
uf, out
R2
out
C
VCO
LO
Fig. 1: (a) Basic structure as FM demodulator,
Building Blocks:
PD: phase detector:
PFD: phase-frequency detector:
Filter: typically lead-lag
LO: local oscillator
uc
(b) Using a lead-lag element as loop filter
gives information about the phase error that the control loop tries to zero
gives information about the phase error and frequency before locking
optional, smoothes the output of the PD to reduce phase noise
oscillator with controllable output frequency, typ. VCO, CCO or DCO.
M. Schubert
Versuch 4: Phasengerastete Schleife (PLL)
Hochschule Regensbur
Using the PLL for Demodulation of Amplitude Modulated (AM) Signals.
AM signals in Germany are found for example in Langwelle, Kurzwelle and Mittelwelle broadcasting. The
information of the sender is encoded within the signal’s envelope while its frequency remains constant.
Figure 2:
(a)
x(t)
(a) A good PLL allows to picking
the maxima and reconstruct
the envelope
(b) Green: Phase-lock obtained.
Red: No phase lock. The
receiver outputs the difference
frequency as misinformation.
(c) Sampling the tops obtains
high amplitudes and low error
due to PLL phase noise.
(d) Uncertainty by phase noise
t
(b)
(c)
e1
e2
t
t
(d)
LO out
Quadrature amplitude modulation (QAM) is a more sophisticated type of amplitude modulation. QAM is
used in mobile phones and WLAN (wireless local area network) systems.
M. Schubert
Versuch 4: Phasengerastete Schleife (PLL)
Hochschule Regensbur
Using the PLL for Demodulation of Frequency Modulated (FM) Signals.
FM signals in Germany are found for example in Ultrakurzwelle (UKW) broadcasting. The information of
the sender is encoded within the signal’s frequency while its amplitude remains constant.
Figure 3:
Matlab simulation.
(a) Sinusoidal signal
to be transmitted.
(b) Signal frequency
modulated on
carrier signal.
The desired signal information after demodulation by PLL is the signal driving the local oscillator, that runs
synchronous (phase-locked) with the received oscillation.