There actually is DC feedback provided by the VCO/Phase detector.
Often there is a 'bleeder' resistor included to ensure cap starts out at zero volts.
Well, you can think what you wish, but that is the dc feedback path. The op amp is an integrator, and is a very common PLL loop filter circuit. Without feedback, it would simply go to one of the supply rails a millisecond after you turn on the power supply.
Imho the phase detector maybe a simple EXOR gate. The Opamp acts as a lowpass/integrator.
The DC feedback is the charge collected on C1.
A bleeder resistor may be placed parallel to C1, but will increase phase jitter...
The phase detector is usually actually a phase/frequency detector. It has the capability to determine if the frequency of the VCO is higher or lower than the input reference frequency. It also, once they are at the same frequency, to output a voltage proportional (+/-) to the phase difference between the VCO and the reference input.
So you turn on the power supply. The VCO starts oscillating. The VCO feeds one input to the phase detector, the reference oscillator feeds the other input. The phase detector then tells the loop filter if the VCO frequency is too high or too low, and outputs an appropriate voltage. That voltage either charges UP or DOWN the integrator. As the integrator output moves either up or down, the VCO frequency does the same. Eventually the loop filter forces the VCO frequency, and then its phase, to "lock" to the reference input. That is the "DC feedback"
Rich
The phase detector outputs not an analog voltage, but a steady PWM signal, which is lowpassed to generate the VCO voltage. This circuit is very common, but has some disadvantages (steady phase deviation, see **broken link removed**)However, for OP-am a little off the voltage will drive the op-am into supply rail and misfunction. So how could you guarantee that your DC-offset is low enough to not drive the op-am into supply rails?
The phase detector outputs not an analog voltage, but a steady PWM signal, which is lowpassed to generate the VCO voltage. This circuit is very common, but has some disadvantages (steady phase deviation, see **broken link removed**)
In an active lowpass, there is no need for a dc feedback.
The phase detector is usually actually a phase/frequency detector. It has the capability to determine if the frequency of the VCO is higher or lower than the input reference frequency. It also, once they are at the same frequency, to output a voltage proportional (+/-) to the phase difference between the VCO and the reference input.
So you turn on the power supply. The VCO starts oscillating. The VCO feeds one input to the phase detector, the reference oscillator feeds the other input. The phase detector then tells the loop filter if the VCO frequency is too high or too low, and outputs an appropriate voltage. That voltage either charges UP or DOWN the integrator. As the integrator output moves either up or down, the VCO frequency does the same. Eventually the loop filter forces the VCO frequency, and then its phase, to "lock" to the reference input. That is the "DC feedback"
Rich
Using OpAmp integrator in PLL is very common, and the integrator is dynamic against the VCO drifting duing to the temperature, voltage drifting, etc.
That circuit is much better than DC feedback.
There actually is DC feedback provided by the VCO/Phase detector.
Often there is a 'bleeder' resistor included to ensure cap starts out at zero volts.
that is not correct. It should be biased about 0.7V.the nagative input for the amplifier is zero
that is not correct. It should be biased about 0.7V.
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