Updated May 13,
to go to our main page on microwave filters
New for May 2010! We
now have our own example of a YIG filter!
YIG stands for yttrium iron garnet.
Yttrium is quite often misspelled, "yittrium" would be
a phonetically correct spelling.
Yttrium is atomic symbol Y
atomic number 39.
Ytterbium is atomic symbol
Yb, and atomic number 70. Don't confuse the two!
The following quick lesson was
contributed by Richard from Tasmania:
YIG is actually a synthetic
form of garnet. It has interesting magnetic properties which allow
it to perform as a tunable microwave filter for example.
This additional material on YIG
technology just in from James, a semiconductor guy with some RF
YIG is a ferrite with very
high resistivity and a sharp ferrimagnetic resonance. These properties
allow YIG resonator oscillators to achieve DRO-like phase noise
performance and very wide tuning ranges: 2-20 GHz tunable oscillators
are available. High performance, with unloaded Q's >1000, demands
a resonator that is a highly polished sphere or ellipsoid made
from a single crystal of YIG. Frequency tuning of the YIG resonator
is accomplished by varying the currents in electromagnets that
are an integral part of the YIG oscillator (or filter) module.
Filters can be built that are reciprocal or non-reciprocal depending
on coupling structures.
There are some prices to
be paid for this performance. The YIG sphere is not cheap, and
needs to be oriented carefully. The magnetic fields required are
large, so YIG oscillator modules need heavy pole pieces, are power
hungry and tune slowly. Further, temperature control of the resonator
is required, and the resonator coupling loops are generally 3D
structures that further increase the cost of production.
If you want to reuse a YIG
oscillator that you got cheap on ebay, then this article:
could be very helpful. Along
with guidance on the care and feeding of YTO's, there are some
very clear photos of the inner workings.
This YIG lesson was contributed
by David Gilmartin... thanks! Here's some comments on the filter
by David, feel free to chime
in with more information.
If I recall correctly, this
YIG tuned filter is from an 8563 spectrum analyser. You may notice
it has three ports. This is because of the internal switch. The
switch is used to switch the input to the low band. Above a couple
of GHz the switch is turned off and the RF goes to the high band
via the coupling loops and YIG spheres. Below the switch is a
small substrate which has connections for the switch bias and
coax for the low band output.
Having the switch integrated
into the YTF allows full span sweeps, which earlier analysers
couldn't do since they used mechanical switches, e.g. 8566.
YIG filter, top
YIG filter, cover
Here's two views of the filter,
with and without the gold mesh removed. Gold mesh serves to provide
an RF ground plane above the YIG spheres, kind of like a diaper
for stray RF radiation. There's a bad mental image, a mesh diaper...
the two images can be viewed in higher resolution if you click on
Here's a close-up view of the
SPDT switch. It is a series shunt/shunt FET
design, which gives wide bandwidth and high isolation.