Loop filter of a phase-locked loop

Details Loop filter of a phase-locked loop Loop filter of a phase-locked loop

1996-12-04

1998-07-21

Grimm, Siegfried H.

Oscillators

Automatic frequency stabilization using a phase or frequency...

Particular error voltage control

331 25, H03L 7089, H03L 8093

Patent

active

057839719

DESCRIPTION:

BRIEF SUMMARY
This application is the national phase of international application PCT/ Fl96/00150, filed Mar. 12, 1996 which designated the U.S.
The present invention relates to a phase-locked loop and especially to a loop filter used therein.
A phase-locked loop used as a frequency synthesizer contains the parts shown in FIG. 1. Input frequencies are compared in a digital phase/frequency detector 2, one of which frequencies is a reference frequency f.sub.ref divided by a factor R in a reference divider 1 and the other is an output frequency f.sub.vco which is divided in a loop divider 6 by a factor N. A phase comparator 2 generates U (up) and D (down) pulses, the widths of which are proportional to the frequency and/or phase difference of the input signals. A charge pump 3 converts the U and D pulses into bipolar pulses, the polarity of which indicates the direction of the frequency and/or phase difference and the width of which indicates the magnitude of the difference. The pulses provided by the charge pump are integrated in a loop-filter 4, the direct voltage V.sub.t obtained from which is the control voltage of a voltage-controlled oscillator VCO 5 and it controls the VCO so that the frequency and/or phase difference of the input signals of the phase comparator is minimized.
First generation charge pumps had a voltage output, whereas nowadays most charge pumps of IC circuits containing parts of the PLL circuit have a current output. The first mentioned known charge pumps have a three-state voltage output: the output is connected to the positive supply voltage V.sub.DD, to the negative supply voltage V.sub.ss (which was often ground) or was left open. A bias resistor is used in the filter (integrator) for converting the output voltage into a current excitation for the integrator (filter). When the phase difference of the input signals of the phase comparator is small, the output pulses of the charge pump are narrow and the charge pump is open for most of the time. Therefore, it appears that the integrator is supplied via a resistor much larger than the bias resistor, that is, from a current source. However, the value of the supply current depends on the output voltage of the filter, that is, the VCO tuning voltage V.sub.t and the supply current is symmetrical only if V.sub.t is exactly V.sub.DD /2, provided of course that Vss=0. Otherwise, it depends on the sign of the phase difference.
A typical passive loop filter is shown in FIG. 2. A filter 21 comprises an integration capacitor C2, a damping resistor R2 and a smoothing capacitor C3. The pulses provided by the charge pump 3 (FIG. 1 and 2) are the output of the filter. Often an additional, passive low-pass filter is used for reducing noise and a reference frequency signal possibly penetrated the filter 21. A resistor R1 is a bias resistor. A disadvantage of the first generation charge pump and the passive filter shown above is that the output current of the charge pump is dependent on the output voltage V.sub.t of the filter.
In order that the input current of the integrator would be independent of its output voltage V.sub.t and the sign of the phase difference, it is known to use instead of a passive filter an active filter (integrator) provided with an operational amplifier as shown in FIG. 3. Ideally, an operational amplifier 31 would keep the voltages on its input ports equal. Therefore, if the voltage affecting in its positive input pole is V.sub.DD /2, the same voltage would affect in its negative input pole and the current through the bias resistor R1 would be independent of the sign of the phase difference.
A general problem with active filters is that an operational amplifier is slow as such and it is not intended to be supplied by fast pulses generated by digital circuits. In practice, the pulses provided by the charge pump go straight through a capacitor C33 of a feedback path, FIG. 3, to the input of the operational amplifier causing it to overdrive. Also when the loop is in a steady state, the charge pump generates pulses since noise constantly causes

REFERENCES:
patent: 4970472 (1990-11-01), Kennedy et al.
patent: 5121085 (1992-06-01), Brown
patent: 5382923 (1995-01-01), Shimada et al.
patent: 5414741 (1995-05-01), Johnson

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