YIG filters (and oscillators): How do they work?
 

Just curious... how do tuneable filters, oscillators, phase shifters, etc.
made from Yttrium Iron Garnet (YIG) materials work? Does an applied DC
magnetic field alter the effective permeability of the material? Or is
something more complex going on?

Also... what tends to limit YIG devices to not typically be used much below
some "many hundreds" MHz? Is it just loss in the material starts to become
prohibitive? Or something else?

I've done a little Googling, and although there's a lot about YIGs out there,
it's usually at either a much lower level or much higher level than what I'm
after here!

Thanks,
---Joel Kolstad

Joel Kolstad wrote:

Just curious... how do tuneable filters, oscillators, phase shifters, etc.
made from Yttrium Iron Garnet (YIG) materials work? Does an applied DC
magnetic field alter the effective permeability of the material? Or is
something more complex going on?

Also... what tends to limit YIG devices to not typically be used much below
some "many hundreds" MHz? Is it just loss in the material starts to become
prohibitive? Or something else?

I've done a little Googling, and although there's a lot about YIGs out there,
it's usually at either a much lower level or much higher level than what I'm
after here!

Thanks,
---Joel Kolstad


I had a professor in a laser electronics class make an offhand comment
on how the ground energy state of ruby (ruby = YIG, here) lasers
seperates into a number of seperate states under bias, and this can be
used to make a microwave oscillator. Translated, that means that the
electrons in the material are what's actually resonating. I suspect
that this is the effect that's used for YIG oscillators.

As for the rest of the whys and wherefores -- I just don't know.

--
-------------------------------------------
Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

A few good links on YIGs from Watkins-Johnson he
http://www.wj.com/technotes/index.asp

On Wed, 22 Jun 2005 12:10:10 -0700, "Joel Kolstad"
wrote:

Just curious... how do tuneable filters, oscillators, phase shifters, etc.
made from Yttrium Iron Garnet (YIG) materials work? Does an applied DC
magnetic field alter the effective permeability of the material? Or is
something more complex going on?

Also... what tends to limit YIG devices to not typically be used much below
some "many hundreds" MHz? Is it just loss in the material starts to become
prohibitive? Or something else?

I've done a little Googling, and although there's a lot about YIGs out there,
it's usually at either a much lower level or much higher level than what I'm
after here!

Thanks,
---Joel Kolstad

I was curious about that recently, and _somewhere_ I found a really
nice explanation. Wish I could point you to it; I won't do it justice.
But the basic idea is that magnetic dipoles (from the electrons in
material) align with the DC magnetic field. If something perturbs
them, they will oscillate with a natural frequency depending on their
mass and the strength of the restoring force: the externally applied
field. Think of a (tiny) bar magnet on a pivot, like a compass needle.
If there is no damping, it will oscillate. The strength of the
externally applied field determines the restoring force, and therefore
the oscillation frequency. The reason given for making the YIG
resonator a ball is to get a very uniform magnetic field through the
whole ball so that all the atomic dipoles have as nearly as possible
the same resonant frequency. The high Q resonance, of course, is what
lets it be a filter or the tuning element of an oscillator. It's very
handy that it's relatively easy to tune over an octave or more range.
However, since the tuning involves changing a fairly large magnetic
field, it's much slower than tuning with a varicap diode.

OK, now that I've written all that, of course it becomes easy to find
the reference: http://pw1.netcom.com/~dstraigh/yig.html Oh, well.

Cheers,
Tom

About the freq limit: consider that the resonant freq depends on the
applied magnetic field, and you can't perfectly shield the osc from ext
fields: power line frequency things, Earth's field, etc. Since f is
linearly proportional to the field strength, things are percentage-wise
more immune to external fields if the field strength is high, and thus
the resonant frequency is high. Also, other designs are relatively
more practical at lower frequencies; it's no great trick to make a
lumped LC oscillator at a couple hundred MHz (though electronic tuning
over an octave range becomes a bit of a chore because of all the stray
capacitance). So it's a matter of the frequency range over which they
are relatively more practical and economical than the alternatives.
And of course, there are many factors to the "practical" part of that.

Cheers,
Tom

Thanks, Tim, that helps a lot! For what you've said, if I were to apply a
mass-spring analogy, the electrons would end up as the mass and the DC field
strength would correspond to the spring constant.

I'll read up at the link you provided. It would appear that with no DC field
applied, you'd get the lowest resonant frequency... at until not too long ago,
that appeared to be in the ballpark of 1GHz. Now it's not uncommon to see
500MHz YIG filters... I wonder what changed?

The application I had in the back of my mind was for nice, sharp filters in
the high VHF/low UHF range. I saw a circulator recently for 432MHz and, while
it was large at about 4" square (but flat -- only about 1/4" thick), it still
struck me as a pretty nice alternative to trying to build a transformer-based
isolator at such frequencies. Hence the thought of perhaps being able to use
YIG filters down there as well...

---Joel

They use the spin of the electrons. If you apply a magnetic field to the
YIG,the spinning electrons precess at a rate proportional to the
magnetic field. This rate happens to fall in the microwave region and a
typical range is from 8GHz to 18GHz.
The sphere is coupled to the oscillator circuitry by various means - a
loop of wire for example - and the spinning electrons look like a
resonator to the circuitry.
Alan