Oscillators – Automatic frequency stabilization using a phase or frequency... – Particular error voltage control
Patent
1991-10-30
1992-12-29
Grimm, Siegfried H.
Oscillators
Automatic frequency stabilization using a phase or frequency...
Particular error voltage control
331 25, H03L 7093, H03H 1112
Patent
active
051755106
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The object of the present invention is a loop filter situated in the feeback channel of a frequency synthesizer and consisting of an integrator with a zero in the frequency response in order to achieve stability, and of a low pass filter. Such a frequency synthesizer comprising a loop filter known from FIG. 1 is used for example in radio telephones as a local oscillator. In the synthesizer, frequency is formed in a known way according to FIG. 2 in a voltage-controlled oscillator, the frequency of which has been locked to the frequency of a crystal oscillator used as a comparison frequency. The frequencies of the voltage-controlled oscillator and the crystal oscillator are conveyed via dividers to a phase comparator, from the output of which the signal based on the phase or frequency difference is obtained. Generally, at least one of the dividers is then programmable. From the output of the phase comparator, the signal goes to the loop filter, the control voltage of the oscillator being obtainable from the filter.
In a known loop filter a problem is caused by the filtering of sidebands brought about by the comparison frequency while striving for as fast a loop as possible. The reason for the problems is that the poles (on the frequency level) of the low pass filter near the natural frequency of the loop reduce the phase margin, decreasing thus the damping factor.
Attempts have been made to solve the problems by using a low enough natural loop frequency, or by using a separate acceleration coupling. However, in that case, the dimensioning of the components has to be made so precise that even a small tolerance variation in the components causes the setting time to grow too long, or that the sidebands rise too high. A further drawback is the complex structure of the acceleration coupling.
In these known cases, the loop filter has been formed by using an integrator, as can be seen in connection with the operational amplifier V of FIG. 1. In the feedback channel, resistor R1 and condensators C1 and C2 together with input resistor Ri form an integrating factor. Further, an RC-filter is often coupled after the integrator.
SUMMARY OF THE INVENTION
The object of the present invention is to move or essentially reduce the problems and drawbacks mentioned above. In order to achieve this, the invention is characterized in that another zero has been added to the frequency response causing the phase margin to grow; further, a pole has been added at a frequency higher than the zero point, when the impact of the zero on the amplitude response is fundamentally overruled. For example, a bipolar active low pass filter is previously known from an article of D. Kesner: "Take the guesswork out of phase locked loop design," EDN, Jan. 1973, pages 54-60. According to the article, the purpose of the active filter is to replace the RC-filtering following the integrator in order to remove sidebands. In the present invention, the frequency response of the integrator is changed so that the phase margin grows and the sideband filtering can be achieved by RC-filtering.
A multipolar loop filter is also known from a publication of J. Gorski-Popiel: "Phase-locked Loop Frequency Synthesizer," IEEE Press, 1975, Frequency Synthesis: Techniques and Applications, pages 111-118. This filter with no zero in the frequency response contains a block (T1), the purpose of which is to totally overrule the effect of RC-circuits in the filter output.
According to the present invention, the impact of the RC-circuits on the phase is reduced at the loop cut-off frequency. The loop of a frequency synthesizer, including the loop filter of the present invention, becomes faster, and at the same time, it is still possible to effectively filter sidebands. Additionally, the loop can be planned as less critical in relation to the tolerance of the components.
BRIEF DESCRIPTION OF THE DRAWINGS
The present invention will now be described by way of an exemplary embodiment and with reference to the accompanying drawings of which:
FIG. 1 i
REFERENCES:
patent: 4514706 (1985-04-01), Thompson
patent: 4742566 (1988-05-01), Nordholt et al.
patent: 4855689 (1989-08-01), Kinkel
Grimm Siegfried H.
Nokia Mobile Phones Ltd.
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