I am reading this time about a frequency synthesizer with a VCO. What i am trying to understand is the use of a programmable counter. In the book it says that it divides the VCO frequency down to the reference frequency. Which is the ref. frequency?
Hi,
The reference frequency is the one used to compare the output from the divided or mixed-down VCO against, so as to control a synthesizer's output frequency.
At the heart of many a synthesiser is the phase-locked loop (PLL) which employs a 'phase comparator'. This is a device that acts to make its two inputs, the reference and the comparator, be in phase with each other. Remember, if two signals are in phase at all times, it must be that they have the same frequency which is the whole point of the circuit.
If one of those signals is obtained from a source called the 'reference' and the other from a VCO, then the output of the phase comparator (a DC voltage when locked) may be used to drive the VCO frequency so that the system locks making F(ref) and F(comp) the same.
The reference can be from a crystal oscillator source in the case of a straight synthesizer or, when the PLL is being used as a demodulator or filter, another signal of varying frequency which may contain some modulation. But that is another story.
Cheers - Joe
Hi,
That should read -
.......................... Remember, if the two signals differ by the same phase angle at all times,...............
Cheers - Joe
ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.