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Uses of Microwave Synthesizers


A microwave synthesizer takes an input reference frequency and translates it into a number of output frequencies in the microwave range. A synthesizer may involve voltage-controlled oscillators, phase detectors, mixers, and amplifiers to perform this translation function.


The following parameters are major considerations of a frequency synthesizer:
·         Frequency Coverage (also called Range or Bandwidth)
·         Frequency Resolution (or Step Size)
 
Test and measurement devices require a wide range of frequencies and a fine step size, whereas other devices, such as those used in a resonant cavity, may require a narrow bandwidth and a coarse step size.
 
Synthesizers come in the following varieties: Direct Analog Synthesizers, Direct Digital Synthesizers, and Indirect Synthesizers.
 
The direct analog synthesizer employs an oscillator to create base frequencies, which are then run through a mixer. The mixed signal is then put through frequency filters.
 
Advantages of an Analog Synthesizer:
·         Fast switching speed (in the micro to nanosecond range),
·         Has low output phase noise, since it uses components which have low residual noise.
 
Disadvantages of an Analog Synthesizer:
·         Limited frequency coverage and limited step size (can be improved by means of adding more hardware stages, which means greater cost)    
·         Need to filter out undesirable products from the mixer.
 
A direct digital synthesizer (or DDS) takes a base clock frequency as the input. This is put into a phase accumulator, which at each clock cycle sums its previous output with a tuning word. This is then compared with a look-up table, and run through a Digital Analog Converter (DAC) to reconstruct the signal to a sinusoidal shape.
 
Advantages of a DDS:
·         Fast Switching Speed (depending only on the digital circuitry)
·         Low phase noise, that can be better than the phase noise of the clock source input
·         Fine frequency resolution, determined by the phase accumulator.
Disadvantages of a DDS:
·         Limited usable bandwidth/doesn't operate at high frequencies—the highest frequency at which it operates is up to one-half of the input clock frequency.
·         Needs a lowpass filter in the DAC to reconstruct the waveform. This limits the highest operation frequency to 40% of the input clock frequency.
·         Spurious content from quantization errors/DAC conversion errors.
The disadvantages of the DDS make it most useful for fine frequency tuning, coupled with a direct or indirect analog synthesizer, instead of a stand-alone synthesizer device.
 
Indirect frequency synthesizer
The indirect frequency synthesizer has a reference frequency, usually from a crystal oscillator, as its input. A Phase Linked Loop (PLL) is employed, which compares a feedback signal from the output to the reference frequency. This is put through a Low Pass Filter, and then a tunable voltage-controlled oscillator (VCO). This generates a signal that is fed back to a phase detector, along with the reference signal. The phase detector compares the signals at both inputs and focuses on a lock frequency—the desired frequency of the synthesizer.
 
Advantages of an indirect synthesizer:
·         Reduced levels of spurious signals, due to the low-pass filter action of the feedback loop
·         Smaller step size
·         Wide bandwidth
·         Less complexity compared to the direct analog synthesizers.
Disadvantages of an indirect synthesizer:
·         Longer frequency switching time (inversely proportional to the step size)
·         Higher phase noise
·         Coarse resolution.
Modern instruments combine all three types: direct analog, direct digital, and indirect frequency synthesizers, to get the best characteristics of all three.
 
Uses of microwave synthesizers:
·         In chemistry: By making a resonant cavity with focused microwave energy, chemical reaction rates can be sped up 1000 times.
·         Testing equipment for communications systems: Microwave synthesizers can be used to test receiver antenna performance.
·         Computer architecture: Microwave synthesizers are employed in device drivers for mainframe computers.
·         Military purposes: Microwave synthesizers can be used as signal generators, to simulate a flying object's Doppler shift, and then another synthesizer can be used to track the changes in the object's Doppler shift.

 

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