Digital PLL Frequency Synthesizers: what they are, how they work 

Digital PLL synthesizers are a form of frequency synthesizer that are used in many radio frequency designs from broadcast radios to high performance professional radios and signal generators to spectrum analyzers and many more forms of electronic equipment.
As the name suggests, these synthesizers are based around the use of a phase locked loop or PLL.
PLLs work by using a phase comparator to compare the phase of RF signals from a fixed frequency reference and a voltage controlled oscillator, VCO. The resulting signal from the phase detector can be used as an error voltage to be fed back via a loop filter to the control terminal of the VCO. This has the effect of reducing the phase difference between the reference and VCO signals to a point where there is a fixed phase difference betweenth e two signals and the PLL is in lock and the VCO and reference are on exactly the same frequency.
This can be used in a frequency synthesizer by introducing a digital divider into the loop between the VCO and phase detector. As the loop tries to reduce the phase difference and hence the frequency difference between the signals at the input of the phase detector, if a digital divider is placed into the circuit, then the VCO will run at a multiple of the phase comparison frequency determined by the division ratio of the divider - a division ratio of two will mean the VCO runs at twice the phase comparison frequency and so forth.
To reduce the phase comparison frequency and gain smaller step sizes, the reference oscillator frequency can be divided down to give the required phase comparison frequency.
The final stage in the realisation of the digital PLL frequency synthesizer is to make the divider within the loop itself programmable. In this way the division ratio can be set and the output frequency can be determined.
More details about digital PLL frequency synthesizers can be found: www.electronics-notes.com/art...
More details about the different types of frequency synthesizer: www.electronics-notes.com/art...
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