YIG Filters and Oscillators

By William Richardson, W3IMG 3333 Harness Creek Rd Annapolis, MD 21403

The YIG device is commonly used as a filter or oscillator in electronic equipment that operates in a frequency range from 0.5 to 40 GHz. The device possesses remarkable characteristics: It has a wide frequency coverage from 2:1 to 18:1, offers electronic tuning with linearity of 0.2% or better, has a Q of 1000 or more and its size is typically 2x2x2 inches.

One drawback of the YIG device is its high cost (in the $500 to $2000 range). YIG filters were marketed in the early 1960s, followed closely by the YIG oscillator. Many of these devices are found on the surplus market in aged equipment such as microwave receivers, signal converters, signal generators and spectrum analyzers.

What is a YIG?

YIG is an acronym for Ytrrium-lron-Garnet, a ferrite material. YIG crystals are grown and formed into spheres. When

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Fig 1—A YIG oscillator is shown at A, shows a 4-pole bandpass filter and C illustrates a 4-pole band-reject filter.

YIG SPHERE

YIG SPHERE

Yig Sphere

BERYLLIUM -

OXIDE MOUNTING RODS

MAGNETIC STRUCTURE

Fig 2—YIG spheres are placed within a single magnetic field. See the text for an explanation of how this is done.

BERYLLIUM -

OXIDE MOUNTING RODS

MAGNETIC STRUCTURE

Fig 2—YIG spheres are placed within a single magnetic field. See the text for an explanation of how this is done.

the sphere is placed in a magnetic field, it resonates, creating a condition where the resonant frequency is directly proportional to the strength of the magnetic field. If the magnetic field is generated by an electric current, the resonant frequency is directly proportional to this tuning current. There is a linear relationship, then, between the tuning current and resonant frequency.

A practical resonant circuit is created by placing a YIG sphere within a loop of wire having one turn or less. Fig 1 shows three configurations: an oscillator, a bandpass filter and a band-reject filter. YIG discriminators are available, but rare and of little use to the amateur. In each of the three cases, the YIG spheres are placed within a single magnetic field produced internally in the YIG device as shown in Fig 2. The sphere is glued to the tip of a beryllium-oxide rod which in turn is mounted to the case, rotated and aligned linearly to orient the sphere correctly in the magnetic field. The berryllium-oxide rod has excellent thermal conductivity and a self-regulating heater heats the rod to keep the YIG sphere at a constant temperature above ambient. Fig 3 shows the internal workings of a YIG device.

Filters and Oscillators

As mentioned, YIG devices are expensive and the amateur will have to search for them on the surplus market. Older equipment will contain filters; oscillators were developed at a later date. The first YIG devices covered octave bandwidths. Later units covered wider ranges. Bandpass filters are available in 2, 3 or 4 stages. Some 4-stage filters have connectors to allow for an amplifier to be inserted between the second and third stage. Filter bandwidths of 15 to 50 MHz are commonly available with bandwidths up to 300 MHz. Oscillator power output is in the order of 10 to 50 mW. Older oscillators above 4 GHz used Gunn diodes instead of transistors; these require more power. A YIG oscillator is shown in Fig 4.

Never take a YIG device apart. The case is usually a part of the magnetic structure and it might not be reassembled correctly.

YIG Design Considerations

RF Connections

Designing equipment that uses YIG

Yig Tuned Gunn Oscillator

Fig 3—A close-up view of the internal parts of a YIG device. The YIG sphere is exposed at the top of the mounting rod (center). The metallic band is the coupling loop. An oscillator substrate is mounted in the upper right-hand corner.

Fig 4—A cylindrical YIG oscillator. (Photo courtesy of Ferretec, Inc)

Fig 3—A close-up view of the internal parts of a YIG device. The YIG sphere is exposed at the top of the mounting rod (center). The metallic band is the coupling loop. An oscillator substrate is mounted in the upper right-hand corner.

Fig 4—A cylindrical YIG oscillator. (Photo courtesy of Ferretec, Inc)

devices is easy, but three things must be considered when doing so: RF connections, device power and tuning current. RF connections are the simplest (see Fig 5). The coaxial connectors, usually type SMA, may be difficult to obtain. It is best to mount the filter and/or oscillator directly to the mixer using coaxial barrels and adaptors as necessary to reduce SWR effects. Any microwave mixer having a 50-ohm impedance and covering the desired frequency range is suitable; a doubly-balanced mixer is preferred. Fig 5 shows both a YIG filter and oscillator connection; either may be used with a variety of devices.

The usual IF for a YIG device is 160 to 250 MHz, but using a 144-MHz receiver to complete the microwave receiver is an easier design. Insertion loss of the YIG filter is about 6 dB. With the mixer and IF, the overall system noise figure (NF) is about 20 dB. A preamplier placed in front of the filter will give the best NF, but the preamplifier is susceptible to intermodulation from out-of-band signals. With the preamplifier placed after the filter, the mixer sees amplifier noise at both its normal and image response—the amplifier's NF is effectively increased by 3 dB, Dual 2-stage YIG filters have connections for inserting a preamplifier between the second and third stages and are specially designed for use with such a unit. This type of filter, however, is scarce on the surplus market.

Device Power

YIG filters require power only for their heater. This power requirement is usually 20 to 28 volts at 5 watts or less. The initial surge may be higher. YIG oscillators have heaters and require power for the oscillator circuit. Sometimes two voltages are required for the oscillator circuit. Most older oscillators above 4 GHz use Gunn diodes that require currents up to 1.5 A. Other oscillators require that their voltage be programmed with frequency. Do not operate any YIG device without first making sure of its power requirements or the unit will fail.

Tuning Current

To design a tuning current generator (YIG controller) great care must be exercised because of its required current stability. A typical YIG device has a tuning sensitivity of 20 MHz/mA, ie, if the tuning current changes by 1 mA, the YIG resonant frequency changes by 20 MHz. Typical tuning currents vary from 100 to 1000 mA, Some fortunate experimenters ocassionally find a YIG device with an integral tuning generator. If you see a YIG in a unit, continue to look for the YIG controller circuitry as well.

Fig 6 shows a YIG controller. It is essentially a regulated, variable-voltage

Fig 5—YIG tuner RF connections.

TUNING COIL

Fig 6—A typical YIG controller is essentially a regulated variable voltage power supply.

power supply. As the arm of the tune potentiometer is moved, the voltage across the sense resistor Rs follows and equals the voltage at the arm. The current through Rs and the YIG tuning coil varies in the same manner, with the current value dependent on the value of Rs. The sense resistor must be of sufficient wattage and stability so that there is no significant change in its resistance over different operating conditions. The supply voltage across the tune potentiometer must also be stable. A multi-turn potentiometer works best in this application. The YIG tuning current times the value of Rs must equal the range of the voltage at the arm of the tune potentiometer. Q1 's supply voltage must exceed the sum of the voltage drops across Rs, the YIG tuning coil and Q1. The current capacity

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of the supply must exceed the maximum YIG tuning current. However, the supply need not be regulated.

YIG Device Operation

Most YIG oscillators have an auxiliary tuning coil often labeled fm coil. It consists of a few turns around the YIG sphere itself and not the magnetic core material. This coil is used to frequency modulate the oscillator over small deviations up to 200 kHz. The inductance of the main tuning coil is so great that fast current changes through it generate excessive voltages across the coil because of the basic relationship E = L x di/dt.

YIG devices exhibit drawbacks when swept-tuned because the magnetic flux intensity requires a ferromagnetic core. Tuning speed is slow compared to a varactor. The maximum tuning rate is about 0.01 seconds across its entire range. Faster tuning is being developed by using laminated cores. Like all ferromagnetic devices, there is hysteresis; the tuning curve differs slightly when tuning up in frequency as compared to when tuning down. YIG devices are superior to varactor-tuned devices so far as stability and linearity are concerned.

Conclusion

Recent information disseminated from the manufacturing scene is about a small cube-shaped device called a Micro YIG.1

1C. Maker, "Cubic yig Components Save Time and Space," Microwaves & RF, Apr 1986, vol 25, no 4, p 115.

Several varieties of the Micro YIG exist: a 1 cubic inch package to house components that operate at a frequency of 0.5 to 8 GHz and a 1.25 cubic inch package for applications up to 24 GHz. The device is designed to fit into tighter equipment spaces and to offer significant performance benefits in hysteresis and tuning speed. All internal connections are placed on a single surface to reduce production costs and simplify repairs. Because this development is relatively new to the marketplace, it is a matter of time before they will be available on the surplus market for a reasonable price. In the meantime, if you are considering experimentation and operation at the microwave frequencies, give serious thought to using a larger YIG device in your next project.

Bits

Antennas—A Four Day Short Course

On March 25-28, 1987, the Southeastern Center for Electrical Engineering Education at the Central Florida Facility in St Cloud, FL, will host a four-day study course on Antennas: Principles, Design and Measurements. The registration fee is $695 and this includes course materials and refreshment breaks. Early registration is encouraged. Contact SCEEE Central Florida Facility—Management Office, 1101 Massachusetts Ave, St Cloud, FL 32769, Attn: Ann Beekman, Registrar; tel 305-892-6146 for course details. —KA1DYZ

Call For Papers

The 1987 IEEE Vehicular Technology Conference will be held at the Holiday Inn Hotel and Convention Center in Tampa, FL during June 1-3. Papers are sought covering the full range of electronics in vehicular technology. Six copies of a 500-word summary should be submitted by December 15, 1986 to: Professor Gerard Lachs, VTC Technical Coordinator, Dept of Electrical Engineering, Univ of Southern Florida, Tampa, FL 33620. Authors will be notified of acceptance by January 30, 1987. Author's kits will be sent to the accepted authors with further instructions and deadlines. These papers will be published in the 37th Vehicular Technology Conference Record, which will be available at the conference and from the IEEE Publications Office. Write to the above address for a list of topics to be covered— KA1DYZ

Artech House Announces New Electronic Books

Each year the Artech House, Inc publishes a catalog of books related to the electronics and communication industry. During 1986, a number of new and inviting topics were covered in 10 new publications. A short sketch is offered on each:

• Solid-State Radar Transmitters by Edward D. Ostroff, et al; 272 pages about background and technical explanations to fully understand this technology. Practical design guidance and comprehensive theoretical discussions. Price: $60.

• Shipboard Antennas by Preston E. Law, Jr; 575 pages of information explaining the whats, whys and hows of over 250 individual naval and commercial antennas. Illustrations cover what is needed to integrate naval antennas into the complex electromagnetic environment of a modern ship. Price: $61.

• Electric Filters by Martin Hasler, PhD, and Jacques Neirynck, PhD; discover how to go beyond simple filter designs and understand the theoretical background of filters in this 355-page book that sells for $60.

• Microwave Mixers by Stephen A. Maas, PhD; this 500 page book deals with the theory, analysis, design and system applications of microwave and millimeter-wave mixers and mixer circuits. Price: $60.

• Linear Active Circuits: Design and Analysis by William Rynone, PhD; the newest and most comprehensive approach to linear circuit analysis is available in these 540 pages. Price: $66.

• Microwave Tubes by A. S. Gilmour, PhD; a comprehensive and balanced review of the operating principles of microwave tubes is covered in 600 pages. Price: $60.

• Dielectric Resonators by Darko Kaifez, PhD and Pierre Guillon, PhD; a 500-page compilation of the currently available information on dielectric resonantors and their applications in microwave circuits. Price: $60.

• Distributed Processing and Database Systems, Vol I & II by Wesley W. Chu, PhD; a two-volume book featuring an in-depth review of the latest advances in both distributed data processing and distributed database systems. The two-volume set sells for $112; vol I consists of 300 pages and vol II has 500 pages; each costs $56.

• Signal Theory and Processing by Frederic de Couion; 550 pages worth of signal generation, detection and interpretation techniques. Price: $60.

• Telecommunication Systems by Pierre-Girard Fontolliet; these 450 pages provide a global, macroscopic view of telecommunication networks and systems. Detailed background of the practical aspects of transmission methods, modulation, system operation and network planning is included. Price: $60.

Prices are subject to change without notice. It is best to contact the Artech House, Inc, prior to purchase. Write to 685 Canton St, Norwood, MA 02062 for further information on the mentioned publications.—KA1DYZ

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