Updated July 6,
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here to go to our page on switchable circulators
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and isolators on EverythingRF.com
Content on isolators has been
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Why are circulators and isolators
relatively expensive in the world of cheap microelectronics? Because
for the most part they are hand assembled, tuned and tested. Tolerances
on material properties of the ferrite and magnet as well as mechanical
tolerances mean that invariably someone must make at least minimum
wage tweaking the product. Tuning methods are different at different
manufacturers. One method is to design the part so that the ports
are all greater than 50 ohms, then tweak the impedance down by squeezing
RTV over the traces to increase their capacitance while watching
the result in real time on a network analyzer.
A circulator is a ferrite device
(ferrite is a class of materials with strange magnetic properties)
with usually three ports. The beautiful thing about circulators
is that they are non-reciprocal.
That is, energy into port 1 predominantly exits port 2, energy into
port 2 exits port 3, and energy into port 3 exits port 1. In a reciprocal
device the same fraction of energy that flows from port 1 to port
2 would occur to energy flowing the opposite direction, from port
2 to port 1.
CCW and CW circulators
The selection of ports is arbitrary,
and circulators can be made to "circulate" either clockwise
(CW) or counterclockwise (CCW). The above symbols are available
in a free download called Electronic_Symbols.doc, you can find it
in our download area.
In addition to the ferrite substrate,
a magnet is required to make a circulator.
Circulators come in waveguide,
coax, and "drop-in" microstrip varieties. Microstrip circulators
are often used in T/R
modules to duplex the antenna to the power amp and LNA. Waveguide
always provides the best loss and power handling. Here's a WR-42
(Ku-band) waveguide circulator we found on Ebay:
is sometimes called a "duplexer", meaning that
is duplexes two signals into one channel (e.g. transmit and receive
into an antenna). This is not to be confused with the term "diplexer"
which is refers to a filter arrangement where two frequency bands
are separated into two channels from a single three-terminal device.
A lot of people mix up these terms. You can remember the correct
definitions because "filter" and "diplexer"
both have an "i" in them, and "circulator" and
"duplexer" both have a "u".
What are circulators good for?
The make a great antenna interface for a transmit/receive system.
Energy can be made to flow from the transmitter (port 1) to the
antenna (port 2) during transmit, and from the antenna (port 2)
to the receiver (port 3) during receive. Circulators have low electrical
losses and can be made to handle huge powers, well into kilowatts.
They usually operate over no more than an octave bandwidth, and
are purely an RF component (they don't work at DC).
rule of thumb!
A circulator's isolation is roughly equal to its return loss, and
should always be specified to the same requirement. A circulator
with 20 dB isolation will need to have a return loss of 20 dB. Think
about it, if you terminate the third arm in a perfect 50 ohms, the
clockwise isolation you will measure in a CCW circulator won't be
better than the stray signal that is bouncing off the loaded port
due to the reflected signal due to its mismatch to 50 ohms.
Circulators and isolators can
be made from 100's of MHz to through W-band (110 GHz). They can
be packaged as planar microstrip components, coaxial components
or as waveguide components. Waveguide circulators and isolators
have by far the best electrical characteristics. You can specify
insertion loss down to less than 0.2 dB in some cases! Microstrip
and coax circulators and isolators might have losses between 0.5
and 1.0 dB. Note that the more bandwidth you ask for, the crummier
the insertion loss and isolation will be.
A really cool type of circulator
is a switchable circulator,
in which an electrical signal is used to switch the orientation
of the circulator from CW to CCW and vice versa. The way the circulator
is constructed it latches into a particular orientation and will
stay there in the absence of the electrical signal, say, for instance
your power supply goes off. The means for switching the orientation
is a single high-current DC pulse that is provided by the driver
circuit. This in an expensive technology, but it makes an unbelievably
low-loss RF switch with high power handling.
Got any good material on circulators
and isolators? drop us a line, we want to expand this page into
a more useful tutorial!