O A frequency synthesizer is an electronic system for generating any of a range of frequencies from a single fixed time base or oscillator.

O They are found in many modern devices, including radio receivers, mobile telephones, radiotelephones, walkie-talkies, CB radios, satellite receivers, GPS systems, etc.

O Most frequency synthesizers are based around a phase locked loop or PLL . The PLL uses the idea of phase comparison as the basis of its operation. From the block diagram of a basic loop shown below, it can be seen that there are three basic circuit blocks, a phase comparator, voltage controlled oscillator, and loop filter. A reference oscillator is sometimes included in the block diagram, although this is not strictly part of the loop itself even though a reference signal is required for its operation.

O A phase locked loop, PLL, needs some additional circuitry if it is to be converted into a frequency synthesizer.

O The loop is broken and additional blocks added to provide the frequency synthesizer action. These blocks add a frequency offset into the loop in one way or another.

O The basic action of the loop remains. The phase detector produces an error voltage proportional to the phase difference between its two input signals. This means that the voltage controlled oscillator will run at a different frequency to that of the phase detector or comparison frequency.

O Digital PLL synthesizer:   This is the concept that is at the root of most single loop synthesizers. It involves placing a digital divider in the loop between the voltage controlled oscillator. This means that the voltage controlled oscillator frequency will be divided by the division ratio of the divider, e.g. n, and the VCO will run at n times the phase comparison frequency. By changing the division ratio of the divider, the output frequency of the oscillator can be changed. This makes the frequency synthesizer programmable.

O There is the second method of frequency synthesis , not using the phase-locked loop and thus immune to some of the problems inherent in the phase-locked loop , called direct synthesis.

O Figure shows an example of the direct frequency synthesis. In this example a single 18-MHz frequency is used , which is divided , mixed multiplied , etc.to provide 10 outputs in 100-KHz steps from 2.0 to 2.9 MHz, as well as a 16-MHz output.

O A selected frequency from the 2.0 to 2.9 MHz set is heterodyned with the 18-MHz reference , and the sum is filtered to produce 10 selectable frequencies from 20.0 to 20.9 MHz in the 100 KHz steps . This frequency is divided by 10 and mixed with 16 MHz to produce 10 selected frequencies from 18.0 to 18.09 MHz, but with 10-KHz steps . This is mixed with another selected frequency from 2.0 to 2.9 MHz set to produce a sum from 20.0 to20.99 MHz. This frequency is divided by 10 and mixed with 16 MHz , and so on.

O It can be seen from the block diagram that the circuits are repeated for each switch, and the repeated circuits are required whenever a decade is added.

Example of Direct synthesis –Wave tek Model 5135A

O Direct:   The direct forms of frequency synthesizer, are as the name suggests implemented by creating a waveform directly without any form of frequency transforming element. Direct techniques including forms of oscillator and mixer are used. 

O Indirect:   Indirect frequency synthesis is based around phase locked loop technology. Here the output signal is generated indirectly. In other words the final signal is generated by an oscillator that is controlled by other signals. In this way the signals used in creating the output are indirectly replicated by the output oscillator, thereby giving the name to this technique.