RF Wireless World

What is YIG Oscillator ?

YIG is the short form of Yttrium Iron Garnet. It is a crystal which has very high Q. This high Q characteristics provides very low phase noise and multi-octave frequency tuning.

YIG crystals are developed like silicon crystals. From this small YIG cubes are made after slicing it. YIG sphere is made from YIG cubes after inserting them into a "tumbler". YIG spheres are available in 10 to 30 mils size.

Typically, YIG sphere is mounted on end of conductive rod usually made of beryllium . There are two reasons for doing this.
• Rod functions as tuning stick here in order to orient YIG sphere in resonant circuit.
• When temperature is kept constant, YIG yields best performance. The rod basically is a thermal conductor to and from YIG sphere and a proportional heater.

YIG tuned oscillator circuit working

This circuit is a type of electronic oscillator that uses a YIG sphere to tune its frequency. As mentioned, YIG spheres are made from yttrium iron garnet, a magnetic material that exhibits the YIG resonance effect. This effect allows the sphere's resonant frequency to be adjusted by applying an external magnetic field.

Let us understand how YIG tuned oscillator works. It is a ferrite material which resonates at microwave frequencies when inserted into a DC magnetic field. This resonance is directly proportional to strength of applied magnetic field.

DC magnetic field is generated with the use of electromagnet, permanent magnet or combinations of both of these. The magnetic field of electromagnet is tuned with the use of variable current. At molecular level, YIG resonance is basically alignment of electron paths. This will create combined magnetic dipole.

As we have learnt current generated magneticfield while magnetic fields generate current when these are coupled to a conductive loop. This concept is used in the design of YIG based oscillators. Here small conductive loops are used which allow coupling to/from resonant magnetic field of YIG spheres. There are three methods by which coupling can be applied viz. oscillation feedback, signal transfer and rejection.

The oscillation feedback is employed to generate variable capacitance and inductance in different configurations. The configurations are common gate, common base and common source oscillator tank circuits. These different YIG oscillator circuit topologies are depicted in the figure-1 above.

Manufacturers or vendors

Teledyne Microwave Solutions is pioneer in design, development and manufacturing of YIG oscillators having lowest phase noise. Teledyne has FM coils for phase locking from 0.5 to 26 GHz frequency range. Following are the generic specifications to be considered before purchasing YIG tuned oscillators:
• Tuning range (GHz)
• Power output (dBm)
• Power variation(dB max)
• Harmonics (dBc max)
• Temp. Drift (MHz max)
• Tuning linearity (% max)
• Hysteresis (MHz max)
• Phase noise (dBc/Hz)
• Frequency Pushing (MHz/V)
• Frequency Pulling

Following are the popular YIG tuned oscillator manufacturers or vendors.
• Giga-tronics
• Teledyne Microwave Solutions
• Stellex
• MICRO LAMBDA WIRELESS, INC.
• Ditom Microwave Inc.

Advantages of YIG oscillator

Following are the benefits or advantages of YIG oscillators:
• They offer good signal quality at low phase jitter compare to VCO.
• Better broadband characteristics.
• Linear tuning curve
• YIG works like tank circuit when it has been placed in the air gap of electro-magnets.
• Low phase noise can be achieved with YIG due to magnetic resonance.
• Available is wide variety of frequency ranges which include 2-4GH, 4-8GHz, 8-12GHz and 12-18GHz and 2-8GHz.

Disadvantages

• It has more power consumption and slow tuning compare to varactor diode based oscillators.
• They are more complex and expensive to implement compared to other types of oscillators.

Conclusion: YIG tuned oscillators are known for their wide tuning range, low phase noise and stability. These features make them suitable for various applications including radar systems, microwave instrumentation and communication devices.

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