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15

HMC701LP6CE

v09.1112

8 GHz 16-BIT FRACTIONAL-N PLL

Charge Pump Tri-State

De-asserting both upout_en and dnout_en effectively tri-states the charge pump while leaving all other functions

operating internally.

PFD Jitter & Lock Detect Background

in normal phase locked operation the divided Vco signal arrives at the phase detector in phase with the divided

crystal signal, known as the reference signal. Despite the fact that the device is in lock, the phase of the Vco signal

and the reference signal vary in time due to the phase noise of the crystal and Vco oscillators, the loop bandwidth

used and the presence of fractional modulation or not. the total integrated noise on the Vco path normally dominates

the variations in the two arrival times at the phase detector if fractional modulation is turned off.

if we wish to detect if the Vco is in lock or not we need to distinguish between normal phase jitter when in lock and

phase jitter when not in lock.

First, we need to understand what is the jitter of the synthesizer, measured at the phase detector in integer or fractional

modes.

the standard deviation of the arrival time of the Vco signal, or the jitter, in integer mode may be estimated with a

the loop 3 dB bandwidth and a 20 dB per decade roll off at greater offsets. the simple locked Vco phase noise

approximation is shown on the left of Figure 10.

Figure 10. Synthesizer Phase Noise & Jitter

(EQ 9)

where

the closed loop PLL and n is the division ratio of the prescaler

the rms phase jitter of the Vco in rads, Ф , results from the power sum of the two sidebands:

2

SSB

(EQ 10)

Since the simple integral of (EQ 9) is just a product of constants, we can easily do the integral in the log domain.

For example if the Vco phase noise inside the loop is -100 dBc/Hz at 10 kHz offset and the loop bandwidth is

100 kHz, and the division ratio n=100, then the integrated single sideband phase noise at the phase detector in dB is

3.2 milli-degrees rms.

While the phase noise reduces by a factor of 20logn after division to the reference, the jitter is a constant.

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