Q. what is frequency synthesized signal generator? Describe its circuit in detail.
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Give the circuit details of frequency synthesizer also explain its working.
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Explain the working of frequency signal generates with neat sketch.
Frequency generators are of two types
(i) Free Running: In this the output can be tuned continuously, over a frequency range buy mechanical or electronic methods.
(ii) Synthesizer: it has its output derived from a fixed frequency, highly stable oscillator and covers the range in a series of steps. The synthesizer technique provides a generator with excellent frequency accuracy and stability, but careful design is needed to overcome the problem of spurious signals in the output.
Frequency Synthesizers are of two types
(i) Direct Frequency synthesizer: Uses stable crystal oscillator, followed by a series of harmonic multipliers and mixers to provide the range of different output frequencies.
(ii) Indirect frequency synthesize: Uses a phase locked loop to give an output which is a fraction of that of a stable crystal oscillator. Given shows than basic indirect frequency synthesizer. The output from the phase detector is a signal proportional to the low pass filter selects the signal, which is then amplified and fed to the voltage controlled oscillator (VCO). The output from the VCO is fed to a programmable divider and then back to the phase detector. The system drives towards until they are equal and at this stage there is still a phase difference between the two frequencies which produces sufficient d.c. signal to keep locked to the output frequency is and this can be changed by varying usually through a keyboard from the front panel of the instrument.
The switching speed between frequency settings, of the frequency generator depends on the time needed for to lock to and on the setting time. For large frequency differences the lock time can be long. It is minimized by setting the VCO close to its final value, usually under microprocessor control and then letting the phase lock loop pull it to its exact value. The setting time is the time after lock, for the output to reach within a given range of the final output settings; it is typically a few milliseconds.
Disadvantage: for a wide frequency range, with small incremental steps, a low reference and a VCO which can cover a wide range are required the programmable divider also needs a large divider ratio and this produces noise within the loop band width.
A multiloop frequency synthesizer: The low frequency synthesizer has a small range and can operate with small steps, whilst the high frequency synthesizer operates over a wide range with wide steps. Combining these two signals, in a mixer, gives a system which can work over a wide frequency range, with small steps.