Understanding YIG Oscillators: Principles, Operation, and Applications

YIG stands for Yttrium Iron Garnet. It’s a crystal known for its exceptionally high Q factor. This characteristic contributes to very low phase noise and the ability to tune across multiple octaves of frequency. YIG crystals are grown in a manner similar to silicon crystals. These crystals are then sliced to create small YIG cubes.

YIG spheres are made by tumbling these YIG cubes. These spheres typically range in size from 10 to 30 mils. A YIG sphere is typically mounted on the end of a conductive rod, usually made of beryllium.

There are two primary reasons for this mounting:

• The rod acts as a “tuning stick” to orient the YIG sphere within the resonant circuit.
• Maintaining a constant temperature allows the YIG to deliver optimal performance. The rod serves as a thermal conductor to and from the YIG sphere and can also incorporate a proportional heater.

YIG Tuned Oscillator Circuit Operation

This circuit is a type of electronic oscillator whose frequency is tuned by a YIG sphere.

YIG spheres, as mentioned, are made from yttrium iron garnet, a magnetic material exhibiting the YIG resonance effect. This allows the sphere’s resonant frequency to be adjusted by applying an external magnetic field.

Let’s understand how a YIG tuned oscillator works. It utilizes a ferrite material that resonates at microwave frequencies when placed within a DC magnetic field. The resonant frequency is directly proportional to the strength of the applied magnetic field. This DC magnetic field is generated by an electromagnet, a permanent magnet, or a combination of both. The magnetic field of the electromagnet is controlled by varying the current flowing through it.

At a molecular level, YIG resonance involves the alignment of electron paths, which creates a combined magnetic dipole.

As we know, electric current generates a magnetic field, and conversely, changing magnetic fields induce current in conductive loops. This principle is used in YIG oscillator design. Small conductive loops couple to and from the resonant magnetic field of the YIG sphere.

There are three coupling methods: oscillation feedback, signal transfer, and rejection. Oscillation feedback is used to generate variable capacitance and inductance in different circuit configurations. These configurations include common-gate, common-base, and common-source oscillator tank circuits. These different YIG oscillator circuit topologies are depicted in the figure above.

Manufacturers and Vendors

Teledyne Microwave Solutions is a leading company in the design, development, and manufacturing of YIG oscillators known for their low phase noise characteristics. Teledyne offers FM coils for phase-locking applications across a frequency range of 0.5 to 26 GHz.

When choosing a YIG tuned oscillator, consider these specifications:

• 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

Some popular YIG tuned oscillator manufacturers and vendors include:

• Giga-tronics
• Teledyne Microwave Solutions
• Stellex
• MICRO LAMBDA WIRELESS, INC.
• Ditom Microwave Inc.

Advantages of YIG Oscillators

Here are some key benefits of YIG oscillators:

• They offer good signal quality with low phase jitter compared to VCOs (Voltage-Controlled Oscillators).
• They exhibit superior broadband characteristics.
• They have a linear tuning curve.
• The YIG acts like a tank circuit when placed in the air gap of electromagnets.
• Low phase noise can be achieved thanks to magnetic resonance.
• A wide range of frequency options are available, including 2-4GHz, 4-8GHz, 8-12GHz, 12-18GHz, and 2-8GHz.

Disadvantages

• YIG oscillators consume more power and have slower tuning speeds compared to varactor diode-based oscillators.
• They are more complex and expensive to implement.

Conclusion

YIG tuned oscillators are valued for their wide tuning range, low phase noise, and stability. These features make them well-suited for applications such as radar systems, microwave instrumentation, and communication devices.