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 out-

put 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 National’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 func-

tional. 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 re-

quires 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 feed-

back network provides the gain as well as one, or two, poles

for low pass filtering. Figure 5 illustrates a typical active loop

filter.

Certain performance characteristics are essential for an op

amp if it is to be used in a PLL loop filter. Low input referred

voltage and current noise are essential, as they directly

affect the noise of the filter and hence the phase noise of the

PLL. Low input bias current is also important, as bias current

affects the level of ‘reference spurs’, artifacts in the fre-

quency spectrum of the PLL caused by mismatch or leakage

at the output of the phase detector. A large input and output

swing is beneficial in terms of increasing the flexibility in

biasing the op amp. The op amp can then be biased such

that the output range of the PLL is mapped efficiently onto

the input range of the VCO.

With a CMOS input, ultra low input bias currents (2 pA) and

low input referred voltage noise (5.5 nV/

), the LM6211

is an ideal op amp for using in a PLL active loop filter. The

LM6211 has a ground sensing input stage, a rail-to-rail out-

put stage, and an operating supply range of 5V - 24V, which

makes it a versatile choice for the design of a wide variety of

active loop filters.

Figure 7 shows the LM6211 used with the LMX2430 to

create an RF frequency synthesizer. The LMX2430 detects

the PLL output, compares it with its internal reference clock

and outputs the phase error in terms of current spikes. The

LM6211 is used to create a loop filter which averages the

error and provides a DC voltage to the VCO. The VCO

generates a sine wave at a frequency determined by the DC

voltage at its input. This circuit can provide output signal

frequencies as high as 2 GHz, much higher than a compara-

tive passive loop filter. Compared to a similar passive loop

filter, the LM6211 doesn’t add significantly to the phase noise

of the PLL, except at the edge of the loop bandwidth, as

shown in Figure 6. A peaking of loop gain is expected, since

the loop filter is deliberately designed to have a wide band-

width and a low phase margin so as to minimize locking time.

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FIGURE 5. A Typical Active Loop Filter

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FIGURE 6. Effect of LM6211 on Phase Noise of PLL

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