But how can you compare the phases of two signals if their frequencies are different? From my signal courses I remember that in order to talk about the phase difference of two signals their magnitude spectrum must be same. I understand that it is related with the operation of the IC. Can you explain it please?
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But how can you compare the phases of two signals if their frequencies are different? From my signal courses I remember that in order to talk about the phase difference of two signals their magnitude spectrum must be same. I understand that it is related with the operation of the IC. Can you explain it please? If both frequencies are not yet synchronized a so called "pull-in" action takes place, which simply means that the VCO frequency moves towards the reference frequency - under the condition, that the frequency difference is nor too large this defines the "pull-in-range".
However, this is a relatively complicated procedure and there is no proportional relationship between this frequency difference and the control signal for the VCO. This is PLL theory and I cannot explain it here in detail. During this pull-in-process there are no phases which could be compared with each other. Only when both frequencies are nearly equal when the PLL has locked the phase difference mainly determines the control voltage and can cause synchronization. It is important to know that the above mentioned applies to a product detector, which you are going to use.
There are other detectors Phase-Frequency Detector, PFD which are able to react upon frequency differences with a proportional control voltage. Added after 12 minutes: Perhaps I should explain the pull-in process in more detail: If you multiply two different frequencies a term exists which contains the frequency difference and the phase difference.
Thus, the control signal is modulated with a signal which roughly is identical to the frequency difference, but it is NOT sinusoidal important! This modulating signal is unsymmetric and has a mean value which moves the VCO towards the reference. As a consequence, the frequency of this modulating signal decreases. It is a highly non-linear process. And therefore, the linear PLL model applies only when this process has come to an end when the PLL has locked , and only then the phase difference comes into the play and keeps the PLL in this synchronized condition 1 members found this post helpful.
Phase Lock loop (PLL) LM565 Circuit
It achieves this through a closed loop feedback mechanism that compares the input signal with the output and makes the necessary corrections so that the phase remains synchronous. The values of the components may have changed during design, so please use the full schematic in the final draft of the circuit diagram. The job of a PLL is to track an incoming frequency and match the phase precisely. However, in this circuit the feedback loop has a divided-by counter, which returns the feedback signal that is 16 fold less. As a result, the phase lock will attempt to compensate and multiply the incoming frequency 16 fold. If the inputs signal changes, the phase detector will recognise the change in frequency and force the VCO to change the output accordingly, such that the output is equal to the new input frequency, thereby eliminating the error value from the phase comparator. The range of frequencies over which the PLL will track an input signal and remain locked is the lock frequency.
The device being cheap can be used in applications where cost is considered. In order to understand let us simplify this block diagram further to get the following. In the device pin 2 and pin3 are inputs where we can connect the input analog signal but usually pin 3 will be grounded and pin2 is used as input. The input signal goes in to the phase detector along with VCO feedback and this phase detector compares whether both signal are in same phase or frequency. If they are in phase or frequency the PD provides zero voltage output and if phase or frequency is present the PD provides positive output voltage. This output voltage of PD is given to amplifier to amplify the voltage signal and the amplified voltage is given to VCO, which generates waveform whose frequency depends on magnitude of the given input voltage.