What is the relationship between input phase and output phase of a pll(phase locked loop)?
Why is it called phase locked? Does it keeps/locks output phase to phase of input meaning output has same phase as input?
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4\$\begingroup\$ This reads like a homework question. What have you tried so far? \$\endgroup\$– BB ONNov 4, 2019 at 18:13
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3\$\begingroup\$ Otherwise: what did a search on internet reveal? \$\endgroup\$– HuismanNov 4, 2019 at 18:13
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\$\begingroup\$ No it is not home work question. Please believe me \$\endgroup\$– DSPCSNov 4, 2019 at 18:18
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2\$\begingroup\$ As much as we would like to say we have god-like insight into everything, we can't verify whether homework questions are indeed homework. Which is why the necessary condition is that they sound like one, not that they are one. \$\endgroup\$– BB ONNov 4, 2019 at 18:35
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2 Answers
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"Phase locked" means that the fundamental feedback mechanism is comparing the phase of the output with the phase of the reference signal, maintaining a constant relationship between them.
The phase offset is not necessarily zero — it depends on the design of the phase detector. Some enforce an offset of 90° or 180°.
answered Nov 4, 2019 at 18:36
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\$\begingroup\$ How it maintains a constant relationship between phase of output and phase of refrence?by using any controller? \$\endgroup\$– DSPCSNov 5, 2019 at 7:37
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\$\begingroup\$ What do you mean by constant relationship? Does you mean that it tries to keep phase of output same as input refrence ? Or you mean something else by constant relationship? \$\endgroup\$– DSPCSNov 5, 2019 at 7:39
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1\$\begingroup\$ maybe more accurate would be to say "constant difference", i.e. the difference of phases is constant (this constant might be 0° but not neccessarily) \$\endgroup\$– CurdNov 5, 2019 at 10:49
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Following up on Dave Tweed answer, the PLL will have leakage currents, thus the UP and the Down pulses will be imbalanced to correct for the leakages.
Leakage currents, such as base-currents into NPN bipolar integrators to compare the UP with Down pulses, will cause large Reference Frequency spurs.
When I diagnosed this, I changed to a Darlington integrator.
answered Nov 5, 2019 at 3:28