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How is the loop bandwidth of a PLL calculated? Is it the area under the Transfer Function vs. Frequency plot?

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It's the frequency at which the open loop gain crosses through unity.

Below this frequency, the output spectrum is substantially that of the reference frequency, degraded by divider and phase detector noise.

Above, the output spectrum is substantially that of the oscillator.

Around that frequency, within on octave, depending on the order and damping of the loop, the noise may be a dB or so worse than either the reference or the oscillator, even in a properly designed loop. If it's many dBs worse there however, that's a sign of a poorly designed loop that is approaching instability.

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