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Synthesizers

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RF and Microwave Synthesizers:- About, Types, Application, Design:

A frequency synthesizer is an RF sub-system that can generate a range of frequencies as per the requirement from a frequency reference source.

Frequency synthesizers are used in devices such as radio receivers,  mobile communication systems, walkie-talkies, satellite trans-receivers, and navigation systems as a carrier signal source. A frequency synthesizer is used to generate local oscillator frequencies for the RF systems like frequency multiplier, frequency mixers, phase-locked loops, etc.

To learn more about RF and Microwave systems, we recommend you to check the following article.

To understand the frequency synthesizer in a better way, we recommend understanding more about the following components, which are prime components for the construction of synthesizer modules.

In this RF and Microwave synthesizer article, we will cover,

  • What is an RF and microwave synthesizer?
  • Types of RF synthesizers?
  • Applications of RF and Microwave synthesizer?
  • What are the specifications that need to consider in the selection of an RF synthesizer?
  • RF and microwave synthesizer manufacturers.

The stability and accuracy of the frequency synthesizer's modules are related to the stability and the accuracy of the input reference signal. In general, for stable synthesizers, crystal oscillators are used as stable and accurate reference frequencies.

Prime features of a perfect RF Synthesizer:

RF synthesizers are used to generate a wide range of high frequencies signals from a single clock reference source with an internal PLL. The output frequency from a synthesizer will be a fractional or integer ratio of the input reference signal. Below are the few prime features needed for the RF synthesizer for the suitable operation.

  • low phase noise.
  • low spurious in the output signal.
  • Modulation options.
  • Internal and external LO options.
  • Multiple outputs from the synthesizer with good isolation among the channels.
  • Output frequency tuning from very low MHz to Ka bands.

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Design and Application

RF Synthesizer Design concept?

To design an RF synthesizer, the designer needs to go through the following steps.

Step 1: System analysis.

Frequency synthesizer analysis involves understanding the following features required.

  • Output frequency (frequency bandwidth or tuning frequency range).
  • Step frequency and fine-tune increments requirement.
  • Frequency and phase stability of the output signal.
  • Phase noise performance.
  • Switching time of synthesizer (compare settling time and rise time).
  • Package size and power consumption.

Step 2: Selection of oscillator, VCO, and PLL

The designer needs to select an oscillator and PLL-phase locked loop circuit as per the requirement. A PLL is a feedback control system having two input signals ports and it will produce an error signal which is proportional to the difference in phase of two input signals. Need to select a VCO that can generate the required output frequency based on the control voltage provided to it as per the requirement.

Step 3: Reference and Feedback

The reference signal from the stable crystal oscillator or from the external source is fed to the one input of the PLL and another signal from the VCO (Voltage-controlled oscillator) fed through the power divider ( One port signal to the output port of synthesizer and other to PLL).

  • An error signal will be generated from the PLL with reference to the oscillator frequency and it is used as negative feedback. The generated error signal will pass through the low pass filter and then be fed to drive a VCO output frequency.

If the output frequency drifts are positive in comparison with the reference signal, the phase error signal will increase, and this signal is used to drive the generated frequency from VCO in the positive direction. If the frequency is higher, a negative feedback signal will generate and it leads to VCO output signal frequency to reduce. Hence the output frequency from the synthesizer will be locked to the required frequency.

RF frequency synthesizer types:

There are basically two types of synthesizers on the basis of techniques and RF circuit design approaches used.

Direct:

The direct RF frequency synthesizer generates the waveform directly using an oscillator and mixer without any form of frequency transforming. Direct mode is classified into 2 types as analog and digital.

Direct analog Frequency Synthesis:  

This is the most common type of synthesizer consisting of filter and divider along with the reference source signal. The direct analog frequency synthesizer required critical circuitry and it lead to the generation of spurious signals; the filtering of spurious signals need additional filters and it lead to an increase in the dimension and cost.

Direct Digital Frequency Synthesis:  

Direct digital frequency synthesizers, DDS create a required RF signal by using a digital circuit. In a digital synthesizer, we can easily set the frequency increment steps digitally. The phase advance increments of the DSS determine the signal frequency generated.  

Indirect:  

In the Indirect frequency synthesis, an additional phase-locked loop circuit is used to generate the fine-tune frequencies. In an indirect synthesizer final signal is generated with the help of PLL by an oscillator.

Analog Indirect Frequency Synthesis:  

This is a basic indirect frequency synthesizer that uses phase-locked loop (PLL) technology with a voltage-controlled oscillator and phase detector to tune and generate the required frequency.

Digital Indirect Frequency Synthesis:  

The digital indirect frequency synthesis uses a digital circuit to control the PLL between the voltage-controlled oscillator and the phase detector. The VCO of the synthesizer runs at a frequency equal to the phase comparison division ratio. By altering the division ratio using the digital circuit, the frequency of the output signal varies. It is smaller in dimension and mainly used for general radio applications.

Applications of RF Synthesizers:

RF synthesizers are used in any RF circuits were need of high-frequency signal source. Two main applications are reference signals to RF systems and clock signals to the high-speed Digital RF circuits.

Reference signal to RF systems.

RF synthesizers are used as a reference signal to the mixers of RF systems like up/down converters, TLT, etc. As in the RF synthesizers, we can set the required frequency and will get a phase-stable signal, we can use it directly to the input to the mixer as a reference signal.

Clock reference to high-speed RF converters (ADC/DAC).

Signals generated from a frequency synthesizer can use as a digital counter, which in turn acts as a clock signal in a digital ADC/ DAC.

The counter is first preset to some initial value of count and this counter counts down at each cycle of the clock signal once it reaches zero counts, the state of the counter output changes, and the count value is reloaded. This circuit will work like a flip-flop, and it is very easy to implement and interface to any microprocessor or a digital system.

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Basics & Selection Feature

Main specifications need to consider in the selection of RF Synthesizers: 

When selecting an RF synthesizer for a system design, it is necessary to consider the following specifications for the perfect design of the overall system.

Frequency range: 

The frequency of an RF synthesizer is the actual range of frequency which the RF synthesizer can generate. If the required range of frequency is small, then better to select a digital PLL synthesizer, for the wider band frequency requirements analog RF synthesizer will be an ideal selection.

Step size:  

One of the significant considerations in the selection of RF synthesizers is the tuning step size. If the number of tuning is more it is ideal to select a Digital PLL synthesizer, where we can get the fine-tuning options, but the price will be more. If the requirement of tuning and step size is minimum, better to select cost-effective analog RF synthesizers.

Phase noise:  

As the phase noise of the system will affect both data and analog radio communications systems, and the synthesizer will be the prime source of the signal, the phase noise of the synthesizer needs to consider as a prime factor. Higher phase noise may lead to the degradation of the performance of the entire system.

Agility:  

The agility of the RF synthesizer indicates how fast it can switch to a new frequency (oscillator to change frequency quickly) once it is set in the controller. This specification will be a prime consideration where the quick set of the new frequency from the current frequency is required.

Local and Remote interface:

Now the RF synthesizers are available with the option of a remote interface for setting the frequency and controlling the gain of the output signal. For the application where the synthesizer needs to use inside an assemble like TLT or up-down converter, the remote interface option will be an ideal choice.

Output Power and Gain control:

Now RF synthesizers are coming with the option for the output signal level control. Need to select an RF synthesizer that can provide the required signal strength, which can be directly fed to other systems like mixer without having any additional amplifiers externally and meet the overall system requirements. The gain control option will help to control the output power level.

Package and RF connectors:

The package of the RF synthesizer needs to select as per the size requirement of the board where it needs to fit in. RF synthesizers will be available with various connector options for the output port and the LO port so that designers can select one as per the need.

Conclusion:

RF synthesizers are widely used within all most all RF systems as a prime source of the reference signal. Now there are many manufactures are available that can offer RF synthesizers with different advanced options like remote interface, very fine-tuning step size, etc. Designers can select suitable manufacturers as per the design requirement.

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RFMWC-RF & Microwave Community

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