Datasheet

10k 100k 1M

OFFSET FREQUENCY (Hz)

-1.0

0.0

1.0

2.0

3.0

4.0

ADDED PHASE NOISE (dB)

ACTIVE LOOP FILTER

WITH LM6211

PASSIVE LOOP FILTER

LM6211

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SNOSAH2C –FEBRUARY 2006–REVISED MARCH 2013

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 frequency 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/√Hz), 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 output 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 41 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 comparative 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 40. A peaking of loop gain is expected, since the

loop filter is deliberately designed to have a wide bandwidth and a low phase margin so as to minimize locking

time.

Figure 40. Effect of LM6211 on Phase Noise of PLL

Product Folder Links: LM6211