| 摘要 |
The invention relates to the locking of a phase-locked loop when the frequency setting (an) of the loop is changed. The locking speed of the loop is improved at the expense of noise characteristics so that these are momentarily degraded. When changing the frequency, the difference between the new frequency set for the VCO (430) and the actual frequency (fVCO) is measured and the VCO is immediately controlled according to this difference. To that end, counters (441, 444) dividing a reference frequency (fref) and the VCO frequency are made to simultaneously start counting from zero. Thus the length of the pulse issued by a phase difference detector (410) corresponds to the said frequency difference. After the setting of the new frequency value the loop filter (420) is turned into a purely capacitive circuit the output voltage (vc) of which changes proportionally to the length of the pulse from the phase difference detector. The loop parameters are arranged such that the resulting voltage difference causes a change in the VCO frequency such that the magnitude of that change is immediately correct. The loop according to the invention locks onto the new frequency faster than in the prior art. Moreover, the number of additional circuits required by the invention is relatively small.
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