Page 8 - Phase-Locked Loops Design, Simulation, and Applications

INTRODUCTION TO PLLS Ronald E. Best 6
few veterans who started their career with building ham radios from electron tubes. The tube
on the right side of the figure in combination with the LC tank circuit forms an oscillator, and
as we will recognize soon, it is even a voltage-controlled oscillator. The output signal of the
oscillator [labeled H and corresponding to u (t) in Fig. 1.1a] is capacitively coupled to the grid
2
of the tube on the left. The reference signal [labeled S and corresponding to u (t) in Fig. 1.1a]
1
is also fed via another capacitor to that grid. Because the grid voltage–anode current
characteristic of electron tubes is nonlinear, the anode current contains a product term that is, a
signal proportional to S · H or u (t) · u (t). As will be shown in Sec. 2.4.1, the circuit
1 2
around the left tube is a multiplier type phase detector. When the circuit is locked, this product
is a measure of phase error—in other words, of the phase difference between the signals S and
H. The parallel RC circuit in the anode is the loop filter. The voltage drop across that filter is
therefore proportional to the phase error. That voltage applied to the anode of the right tube is
now the difference of the battery voltage (e) and the voltage drop across resistor R—that is,
the phase error modulates the anode voltage of the oscillator. Because the frequency generated
by the oscillator is an almost linear function of anode voltage, the oscillator is a VCO indeed!
This brilliant invention was widely ignored by most engineers for about 20 years. One of
the first large-scale industrial applications of the PLL (back in the 1950s) was the color
subcarrier recovery in color TV receivers. PLL-like circuits were also used in TV for line and
frame synchronization. Somewhat later frequency synthesizers built from PLLs were used to
generate a raster of frequencies in the local oscillator of FM receivers. The real breakthrough
of the PLL came with desktop computers and with the PC, where PLLs are used for many
types of data synchronization—for instance, reading digital data to and from floppy disks,
hard disks, modems, tape drives, and the like. One of the largest applications today is probably
the mobile phone, where the PLL is used again for frequency synthesis.

Classification of PLL Types

The very first phase-locked loops (PLLs) were built from discrete components, including
electron tubes and, later, discrete transistors. All these circuits were linear circuits. The first
PLL ICs appeared around 1965 and were also purely analog devices. An analog multiplier
(four-quadrant multiplier) was used as the phase detector, the loop filter was built from a
passive or active RC filter, and the well-known voltage-controlled oscillator (VCO) was used
to generate the output signal of the PLL. This type of PLL is referred to as the linear PLL
(LPLL) today. In the years that followed, the PLL drifted slowly but steadily into digital
territory. The very first digital PLL (DPLL), which appeared around 1970, was in effect a
hybrid device: only the phase detector was built from a digital circuit (for instance, from an
EXOR gate or a JK-flipflop), but the remaining blocks were still analog. A few years later, the
“all-digital” PLL (ADPLL) was invented. The ADPLL is exclusively built from digital
function blocks; hence, it doesn’t contain any passive components like resistors and
capacitors.

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