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Rat-race phase shifter using PIN diodes
A clever topology to design a wide-band 180 degree phase bit is shown below. The size of this phase shifter is actually large due to the use of rat-race coupler. However, the unique structure allows for fairly wide bandwidth. Here a rat-race 180 degree coupler is used with a pair of switched loads. For more info on rat-race couplers, click here. In order for this scheme to work, the switched loads have to present high reflection coefficients and must remain 180 degrees out of phase in both states. For example, an "anti-parallel" pair of PIN diodes can be used as loads, where one is biased on while the other is biased off. As the signal enters from the left it splits 180 degrees out of phase to the loads ZL and -ZL. The signal pair reflects off the loads in phase, and combines at the output port. In practice the parasitics of the switched diode loads (the off-state capacitance of the diodes) tend to cancel from each other, and a beautiful 180 degree phase shifter results (this is not the case in a quadrature PIN diode 180 degree phase shifter).
The predicted response of a 35 GHz rat-race phase shifter design using 20 femto-Farad, 4.5 ohm diodes is shown below. Note that from 25 to 45 GHz the phase error is less than +/-15 degrees. The size of a rat-race phase shifter is directly related to the frequency band of operation, as the circumference of the rat-race coupler is 1.5 wavelengths at the center frequency. If this topology looks familiar, it is because it is similar to a well-known mixer circuit. Two control signals are required for the rat-race phase shifter, as the loads must be kept in opposite states.
Rat-race phase shifter response