a simple approach tyo yig oscil

YIG oscillators have been in use forseveral decades now, mainly in relationto equipment for professionals fromwell-known manufacturers. However,we radio amateurs have been prohib-ited from enjoying their outstandingcharacteristics for many years, owingto the high prices involved. But forsome years now, YIG oscillators havebeen obtainable at reasonable priceson the surplus market. Most of theequipment available has already beentaken out of service, but this poses noproblems as a rule, since YIG oscilla-tors have a very long service life andare of excellent quality. This article isintended to provide some practicalhelp regarding a simple approach tothese high-quality oscillators.

1Advantages of YIG oscillators

YIG oscillators have some advantagesover normal VCO’s. It is above all theirgood signal quality, with a low level ofphase jitter, and their broad band charac-teristics (with a very linear tuning curve)which make them interesting, or evenobligatory, for many measurement appli-cations. Anyone who has become ac-

quainted with the advantages of theseoscillators will no longer want to dowithout them.

The normal frequency ranges are 2 -4GHz, 4 - 8GHz, 8 - 12GHz, 12 - 18GHzand 2 - 8GHz. However, the usablefrequency range usually goes beyond thespecified limit frequencies, so that a YIGwhich is specified for 2 - 4GHz can beused at 1.8 or 1.9GHz, and can frequentlyalso still function at a few 100MHzabove 4GHz, but this varies from type totype.

2YIG resonator

Fig. 1 shows the core of a YIG oscillator.The YIG ball, which sits at the tip of ashort ceramic rod, is positioned in themiddle of a coupling coil (U bolt). ThisYIG resonator is influenced by the mag-netic field that is generated by the tuningcoils. This allows the YIG to be tuned toits frequency.

Bernd Kaa, DG4RBF

A simple approach to YIGoscillators

VHF COMMUNICATIONS 4/2004

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3YIG oscillator connections

The connections of a YIG oscillator areshown in the circuit. They have a stand-ard pin configuration, which is almostalways the same (Fig. 2).

3.1 Operating voltageThese “standard YIG’s” always require +

15V(approximately 150 - 300mA) andfrequently also – 5V as operating volt-ages. But there are also some manufac-turers who depart from this norm, andtheir YIG oscillators need completelydifferent voltages. For example, HewlettPackard, whose YIG’s frequently need +20.5V and – 5.1V, or + 20V and – 10V.

There are also oscillators from Watkin-Jthat require a negative operating voltageof – 14.2V. But the good thing about thisis that these voltage specifications arealmost always printed on the oscillator.Fig. 3 shows three YIG’s with + 15V and– 5V as operating voltages. Fig. 4 showsthree examples from different manufac-turers with an operating voltage of + 15Valone. In spite of differences in shape andsize, the “standard” pin configuration hasbeen retained.

3.2 HeatingThere are usually two connections forheating. These lead to a small PTC plateinside the oscillator, which serves to keepthe YIG element at a uniform tempera-ture. The voltage for heating is notcritical, and is normally about 24V. Arelatively high starting current of severalhundred mA falls off markedly after afew seconds, and then oscillates around avalue of < 100mA. Fig. 5 shows a smallheating plate of this kind on the YIG ball

Fig 1: The core ofa YIG oscillator.

Fig 2: The standard pin connectionsfor a YIG oscillator.


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mounting.

3.3 Main tuning coilThe main tuning coil is usually indicatedby “+ tune” and “– tune”. In contrast to aVCO, where the frequency is controlledusing a tuning voltage, a YIG is “currentcontrolled” i.e. the frequency of the YIGoscillator depends on the current whichflows through the tuning coil. The greatthing about this is that the frequencyresponse curve is very linear in relation

to the current fed in. 20MHz/mA is atypical value.

The main tuning coil consists of thickenamelled copper wire and is very pow-erful. Even currents exceeding 1A can becoped with for a short time. Incidentally,the tuning current should always be fedin to the correct pin. This coil’s resist-ance is approximately 10Ω, but it can liein a range between 5 and 15Ω. The thickmain tuning coil can easily be recognisedin Fig. 6. It runs around the metal plate

Fig 3 Examples of YIG oscillators with operating voltages of +15v and -5v.

Fig 4: Examples of YIG oscillators with operating voltages of +15v.


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on the outside.

3.3 FM coilA second tuning coil is used for finetuning or the FM modulation of theoscillator. It is indicated by “+ FM” and“– FM”.

This small coil consists of very thin wire,has a resistance of only approximately1Ω, and could be destroyed by currentsexceeding 200mA. So care must be exer-cised here.

The small coil on the plate is the FM coiland can be seen in the middle of Fig. 6.But there are also models without an FM

coil. Fig. 7 shows a unit of this type.Although this YIG has no connectionsfor the FM coil, the other pins correspondto the standard layout.

So there’s no need to be scared of YIGoscillators! Using them is really quitesimple and not critical. If you have a YIGthat has the connections laid out in thecircuit as described, everything is reallyclear, even if the pins are not labelled. Asa precaution, you can check the connec-tions for the two tuning coils with anohmmeter. Zener diodes are frequentlyfitted as a protection against an overvoltage on the pins caused by the operat-ing voltages.

Care must be exercised with YIG oscilla-tors from HP, as most have unlabelledconnections which do not correspond tothe “standard” and often also need decid-edly unusual voltages. The connectionshere should be known, or you shouldobtain a connection diagram. Figs. 8 and9 show two examples from HewlettPackard.

3.4 Output of YIG oscillatorsThe output to be expected lies in therange between + 10dBm and + 15dBm.For many examples, it can even extend to+ 20dBm. Depending on the frequencyrange, the output can vary by a few dB’s.

Fig 5: The PTCheating plate in aYIG oscillator.

Fig 6: The tuning coils.


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4Simple putting into operation ofYIG oscillators

What’s a simple way of putting a YIGoscillator into operation?

A mW meter with a suitable attenuator,or better, a spectrum analyser, is con-nected to the RF output. After feeding inthe correct operating voltage, connect anadjustable power supply (approximately0 – 20V / 1A) via an output resistance(Fig. 10).

Then slowly increase the tuning currentuntil a measurable signal level is ob-tained at the output (lower frequencylimit). Carry on increasing the currentuntil the output signal fades again. This isthe upper frequency limit of the oscilla-tor. The operating range can therefore bedetermined using a frequency counter.

It should be taken into account that thisfrequency limit is somewhat dependenton temperature. So heating should beconnected up if possible. It should alsobe taken into account that a considerabletuning current sometimes has to flowthrough oscillators for relatively highfrequencies before oscillation sets in.Thus, for example, in a YIG which isspecified for up to 18GHz, 420mA isalready flowing through the main tuningcoil before it begins to operate at ap-proximately 8GHz.

Fig 7: A YIG oscillator with no FMmode.

Fig 8: An example of a HP YIGoscillator with non standardoperating voltages.

Fig 9: Another example of an HP YIGoscillator.


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4.1 Triggering of YIG oscillatorsTo be able to trigger a YIG at a voltage,you need a YIG driver, a voltage/currenttransformer. An appropriately wired op-erational amplifier with an output transis-tor normally carries out this task. Asuitable circuit can be seen in [1].

5Other models

Square format units can also pop up.Figs. 11 and 12 show two specimens ofthis type. Luckily, the connections are

mostly labelled here as well.

5.1 YIG oscillators with integrateddriverThere are also types of equipment thatalready incorporate the YIG driver. Figs.13 and 14 show oscillators like this.These are then usually tuned with atuning voltage of between 0 and 10V. Aconnection diagram should acquired forthese types, since no rules are applicablehere. Finally, I’d like to mention twoother types, although they are rare.

Fig 10: Circuit for operating a YIGoscillator.

Fig 11: An example of a squareformat YIG oscillator.

Fig 12: A second example of a squareformat YIG oscillator.

Fig 13: An example of a YIGoscillator with built in YIG driver.


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5.2 YIG oscillators with variableoperating voltageIn these types, the operating voltage mustalso be varied if they are to be able tooperate in all possible frequency ranges.

Here, for example, is an oscillator for 8.0– 12.0GHz, which has an operating volt-age of 15.5 – 9.7V. At the lowestfrequency of 8.0GHz, an operating volt-

age of 15.5V should be set, and at thehighest frequency of 12.0GHz the operat-ing voltage should be only 9.7V. So forthese types the operating voltage must bealtered inversely to the tuning current.But it can also suffice to switch theoperating voltage in two ranges. Thusthis oscillator can be operated at 15.5Vfrom 8 to 10GHz and at 10.5V from 10 to12GHz. Fig. 15 shows this oscillator.

Fig 14: A second example of a YIGoscillator with a built in driver.

Fig 15: An example of a YIGoscillator requiring variableoperating voltages.

Fig 16: An exampleof o YIG oscillatorwith a permanentmagnet.


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5.3 YIG oscillators with permanentmagnetYIG oscillators with permanent magnetsrepresent a decidedly special and veryrecent development. These YIG’s oscil-late even without a tuning current, andthey do this at their centre frequency, the“free run frequency”.

Thus a YIG for 2 - 4GHz operates at3GHz and can, depending on the polarityof the tuning current, be tuned to 1GHzup or down. Fig. 16 shows such anoscillator.

6Cooling

However strange the idea may seem atfirst, a YIG should be heated and cooled.Heated so that the YIG pellet reaches itsoperating temperature rapidly, andcooled so that the heat building up in themain tuning coil can be used to heat theYIG up really well. Cooling is necessary,above all, at relatively high operatingfrequencies, since here there can be aconsiderable current of up to 1A, and soa considerable heat loss is generated.

Fig. 17 shows a miniature YIG oscillator,which operates at up to 10GHz andbecomes very hot. This YIG was experi-mentally mounted on this heat sink, but itwas somewhat too small.

7Conclusion

I couldn’t find much on the Internet onthe subject of YIG oscillators and theirconnections. So I just hope that theseremarks (without a lot of theory) help tomake it easier to use these interestingcomponents.

8Literature references

[1] Synthesiser signal generator for 10 to1,800MHz Bernd Kaa, DG4RBF, VHFCommunications 2/2004 pp 66 – 94.

Fig 17: An exampleof a miniature YIGoscillator thatoperates up to10GHz.


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a simple approach tyo yig oscil

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