It is about how frequency changes with respect to the devices.

2. 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.

3. 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.

7. 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.

8. 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.

10. 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.

11. 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.

13. Example of Direct synthesis –Wave tek Model
5135A

14. 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.