Tài liệu Circulat doc

1 2 3 1 3 2 1 3 2 10 to 12 GHz 8 to 10 GHz 8 to 12 GHz FILTER Filter could be a piece of waveguide which passes above 10 GHz OUTPUT INPUT HIGH PASS OUTPUT 6-7.1 Figure 1. Symbolic Expression for a Y-Junction Circulator Figure 2. Isolator From A Circulator Figure 3. Diplexer From A Circulator CIRCULATORS AND DIPLEXERS A 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 1v2, 2v3, and 3v1 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. 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. 10 kW 1 kW Water Load Receiver/Measurment Device (9 mW) ANTENNA VSWR 2:1 Reflected power down 10 dB POWER INPUT SOURCE CW Reflected power now down 9 kW 40 dB attenuator 1 kW * 0.9 kW 100 W * 10 W * 100 W * All loads and the antenna have a 2:1 VSWR 20 dB from power input ** If reverse leakage is not attenuated by at least 20 dB, this leakage path dominates at the measurement port. Normally, a coaxial circulator will have at least 20 dB of reverse attenuation and a waveguide circulator will have at least 30 dB of reverse attenuation. ** AFT FWD Low Rx Low Tx Hybrid AFT FWD Low Rx Low Tx Hybrid High Rx High Tx * High power device * * AFT FWD Low Rx Low Tx Hybrid High Rx High Tx High power device Low power device L H L H 6-7.2 Figure 4. Faraday Rotator Circulator Figure 5. Low Band Configuration Figure 6. Low/High Band Configuration Figure 7. Alternate Low/High Band Configuration 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. 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. . for a Y-Junction Circulator Figure 2. Isolator From A Circulator Figure 3. Diplexer From A Circulator CIRCULATORS AND DIPLEXERS A microwave circulator is. 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