MAKE IN INDIAFrequency Synthesized Signal

Generator

Desh Deepak Shaurya Jain Varun Kumar

Pankaj Pandey

Frequency Synthesized Signal Generator

About Frequency Synthesized Signal Generator

Frequency Synthesizers are basically RF signal sources with variable output frequencies which are directly derived and generated from a precise reference frequency (internal or external) with no additional frequency error. The ideal output signal is a single spectrum line with no other unwanted frequencies. This can't be realized in practice and all RF-signals have a so-called spectral purity. This is a combination of harmonics, non-harmonics (discrete spurious), SSB phase noise, noise floor and residual FM

Frequency Synthesized Signal Generator

Methods of Frequency Synthesis 1. Indirect Method (Phase locked Loop)

2. Direct Synthesis

Indirect Method component of indirect method

1. Voltage controlled Oscillator (VCO)2.The Programmable Divider 3.The Phase Detector4.The Phase reference5. The Loop filter

Voltage controlled Oscillator (VCO)

It is the source of the output frequency and has the ability to be tuned electronically usually by applying a variable voltage.

Some oscillators are electronically tuned using a current especially in the higher frequencies , but for the general discussion of the indirect synthesizer the signal source will be considered as a Voltage controlled device

Programmable Divider

It is a logic element that divides a frequency of the VCO by an integer that can be entered via programmable switches a microprocessor or the other method.

Phase Detector

It provides an analog output that is a function of the phase angle between the two inputs which in the case of a frequency synthesizer is the reference source and the output of the programmable divider.

Reference Source

It is a very accurate and stable frequency source which is typically a quartz crystal oscillator. The accuracy of entire synthesizer is dependent on the accuracy of the reference source

The crystal oscillator operates in the region of 1-10 MHz and that frequency is divided down using digital counters to provide the necessary clock and reference frequencies for the synthesizer

Loop FilterIt is an analog filter and is required to assure

stable and noise free operations of the synthesizer.

Working of the PLL

The programmable divider divides the frequency of the VCO by N. The output frequency of the programmable divider is fv/N or fr/N

fv- desired frequency of the VCO fr- reference frequency applied to the Phase detectorN- integer entered in to the programmable divider

The output of the programmable divider is fed to the phase detector and compared to the phase of the reference frequency.

If output of the phase detector were returned to the VCO, any variation in the phase could be corrected so that the frequency of the VCO would be exactly N times the reference frequency.However the phase determination can be made only once in each reference frequency period, and thus the frequency of the VCO can only be corrected at this rate, this would cause the frequency of VCO to be modulated and creates spurious sidebands called the reference side bands, in such a fashion as to make the VCO output useless for precision measurements.A filter is inserted between the phase detector and the VCO so that the periodic frequency changes are smoothed and then the frequency modulation is reduced .

Block Diagram of PLL

Disadvantages

1. It is not capable of removing all the side band energy and something must retain. The sideband level required for the critical testing is very low and can make the PLL synthesizer practically unsuitable.

2. The loop filter affects the frequency slewing the characteristics of the synthesizer

3. When it is necessary to change the frequency of the signal generator by large amount the time required for the change can be long.

DIRECT SYNTHESIS METHOD

In this method a single 18 MHz reference frequency is used which is divided, mixed multiplied, to provide 10 outputs in 100KHz steps from 2 to 2.9 MHz, as well as 16 MHz output.

Rather than stabilizing the frequency of VCO by comparing the phase of fraction of the VCO’s frequency to reference frequency, this method generates the desired frequency from reference frequency.

A selected frequency from the 2 to 2.9 MHz set is heterodyned with 18 MHz reference, and the sum is filtered to produce 10 selectable frequencies from 20 to 20.9 MHz in 100 KHz steps

Block diagram of Direct Synthesis

This frequency is divided by 10 and mixed with 16MHz and so on.

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

Although the synthesizer circuits appear complex and expensive, each decade is identical with the previous decade. Because there are no low frequency circuits, the frequency can be changed practically instantaneously in addition because the entire synthesizer operates from a single frequency source ,the close-in frequency spectrum can be quite pure without any undesired modulation

APPLICATIONSThey are found in many modern devices, including radio receiversmobile telephonesRadiotelephonesWalkie-talkiessatellite receiversGPS systems, etc.

A frequency synthesizer can combine frequency multiplication, frequency division, and frequency mixing (the frequency mixing process generates sum and difference frequencies) operations to produce the desired output signal.

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