Reflection phase shifters

Updated April 13, 2006

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Quadrature phase shifter

The quadrature phase shifter is shown below. Here an equal-split quadrature coupler divides the input signal into two signals 90 degrees out of phase (the definition of "quadrature" is two signals 90 degrees apart, click here for more info on quad couplers). These signals reflect from a pair of switched loads, and combine in phase at the phase shifter output, as long as the loads are identical in reflection coefficient (both magnitude and phase). Unlike the loaded line structure discussed previously, the quadrature phase shifter can be used to provide any desired phase shift. Ideally, the loads should present purely reactive impedances, which can range from a short circuit to an open circuit or anything in between. This structure provides a bandwidth of up to an octave, depending on the bandwidth of the quadrature coupler itself. The coupler can be a Lange or a box hybrid on microstrip, or an overlay coupler in a stripline circuit. The size of a quadrature phase shifter is directly related to the frequency band of operation, as the coupler typically uses one or more quarter-wave sections. Only one control signal is required for a quadrature phase shifter, since the loads can be biased simultaneously.

Quadrature phase shifter

Below are the two states that the diode provides to effect a reflection phase shifter (the same two states that are used in PIN diode switches). Note that both states have high reflection coefficients (poor return loss). Ideally they would be perfect open or short circuits, and reflect 100% of the incident power, at phase angle 180 degrees apart (left and right side of the Smith chart).

Rat-race phase shifter

A clever topology for a 180 degree phase bit is shown below. 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 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 port 3. 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 phase shifter). The predicted response of a 35 GHz rat-race phase shifter design using 20 femto-Farad, 4.5 ohm diodes is shown in Figure 9. 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 (180 degree bit)

Rat-race phase shifter response