Datasheet

VCO

INPUT

CHARGE

PUMP

OUTPUT

S_PLL

OUT

LM6211

SNOSAH2C –FEBRUARY 2006–REVISED MARCH 2013

www.ti.com

Figure 38. Compensating for Input Capacitance

This peaking can be eliminated by adding a feedback capacitance, C

, as shown in Figure 38. This introduces a

zero in the feedback network, and hence a pole in the closed loop response, and thus maintains stability. An

optimal value of C

is given by Equation 2. A simpler approach is to select C

= (R

for a 90° phase

margin. This approach, however, limits the bandwidth excessively.

Typical Applications

ACTIVE LOOP FILTER FOR PLLs

A typical phase locked loop, or PLL, functions by creating a negative feedback loop in terms of the phase of a

signal. A simple PLL consists of three main components: a phase detector, a loop filter and a voltage controlled

oscillator (VCO). The phase detector compares the phase of the output of the PLL with that of a reference signal,

and feeds the error signal into the loop filter, thus performing negative feedback. The loop filter performs the

important function of averaging (or low-pass filtering) the error and providing the VCO with a DC voltage, which

allows the VCO to modify its frequency such that the error is minimized. The performance of the loop filter affects

a number of specifications of the PLL, like its frequency range, locking time and phase noise.

Since a loop filter is a very noise sensitive application, it is usually suggested that only passive components be

used in its design. Any active devices, like discrete transistors or op amps, would add significantly to the noise of

the circuit and would hence worsen the in-band phase noise of the PLL. But newer and faster PLLs, like TI’s

LMX2430, have a power supply voltage of less than 3V, which limits the phase-detector output of the PLL. If a

passive loop filter is used with such circuits, then the DC voltage that can be provided to the VCO is limited to

couple of volts. This limits the range of frequencies for which the VCO, and hence the PLL, is functional. In

certain applications requiring a wider operating range of frequencies for the PLL, like set-top boxes or base

stations, this level of performance is not adequate and requires active amplification, hence the need for active

loop filters.

An active loop filter typically consists of an op amp, which provides the gain, accompanied by a three or four pole

RC filter. The non-inverting input of the op amp is biased to a fixed value, usually the mid-supply of the PLL,

while a feedback network provides the gain as well as one, or two, poles for low pass filtering. Figure 39

illustrates a typical active loop filter.

Figure 39. A Typical Active Loop Filter

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