CIRCULATORS AND DIPLEXERS
microwave circulator is a nonreciprocal ferrite device which contains three or
more ports. The input from port n will come out at port n + 1 but not out at any
other port. A three-port ferrite junction circulator, usually called the
Y-junction circulator, is most commonly used. They are available in either
rectangular waveguide or strip- line forms. The signal flow in the three-port
circulator is assumed as 1 to 2, 2 to 3, and 3 to 1 as shown in Figure 1.
If port 1 is the input, then the signal will come out of port 2; in an ideal
situation, no signal should come out of port 3 which is called the isolated
port. The insertion loss of the circulator is the loss from 1 to 2, while the
loss from 1 to 3 is referred to as isolation. A typical circulator will have a
few tenths of a dB insertion loss from port 1 to 2 and 20 dB of isolation from
port 1 to 3 for coaxial circulators (30 dB or more for waveguide circulators).
When the input is port 2, the signal will come out of port 3 and port 1 is the
isolated port. Similar discussions can be applied to port 3.
|Since circulators contain magnets, they should not be mounted near
ferrous metals since the close proximity of metals like iron can change
the frequency response.
|As shown in Figure 2, if one port of a circulator is loaded, it
becomes an isolator, i.e. power will pass from ports one to two, but
power reflected back from port two will go to the load at port three
versus going back to port one.
Another useful device is the 4-port Faraday Rotator Circulator shown
symbolically in Figure 4. These waveguide devices handle very high power and
provide excellent isolation properties. It is useful when measurements must be
made during high power application as shown. A water load is used to absorb the
high power reflections so that a reasonable power level is reflected to the
receiver or measurement port.
|As shown in Figure 3 this circulator is made into a diplexer by adding
a high pass filter to port two. Frequencies from port one that are below
10 GHz will be reflected by port two. Frequencies above 10 GHz will pass
through port two. At the 10 GHz crossover frequency of the diplexer, a
10 GHz signal will be passed to both ports two and three but will be
half power at each port. Diplexers or triplexers (one input and three
output bands), must be specifically designed for the application.|
The Maximum Input Power to a Measurement Device - The ideal input to a
measurement device is in the 0 to 10 dBm ( 1 to 10 mW) range. Check
manufacturer's specification for specific maximum value.
If the RF transmission lines and their components (antenna, hybrid, etc.) can
support the wider frequency range, circulators could be used to increase the
number of interconnecting RF ports from two as shown in Figure 5, to four as
shown in Figure 6.
Figure 7 shows an alternate configuration using diplexers which could
actually be made from circulators as shown previously in Figure 3.