Switched Line Phase Shifters

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A simple switched-line phase shifter is shown in below. The phase shift can easily be computed from the difference in the electrical lengths of the reference arm and the delay arm. The phase of any transmission line is equal to its length times its propagation constant; typically we use electrical degrees for this, not radians.

SPDT switches can be realized in a wide variety of ways, using FET, diode, or MEMS (micro-electro-mechanical systems) switches. The combined isolation of the two switches must exceed 20 dB in the design frequency band, or there will be ripple in the amplitude and phase response due to leakage of the "off" arm, sensitivities to FET parameters, etc.

Switched Line Phase Shifters

Switched line phase shifter topology

It is important to choose a switch technology appropriately for the frequency band of interest. PIN diode switches are often used through 18 GHz for "chip-and wire" construction (this practice is known as MIC, or microwave integrated circuit). In MMIC design (monolithic microwave integrated circuit), the switches are often realized with FETs, up into millimeterwaves. The weird thing here is that a diode is usually a better switch element than a FET, but when employed on a monolithic circuit, FETs can overcome their off-state capacitance by using a shunt inductor trick at very high frequencies (we will cover that topic at a future time in a section about microwave switches… UE). Diodes are almost never employed monolithically (exceptions are offered by TriQuint and M/A-COM), so they have to suffer the variations in wirebond inductance associated with MIC construction, and hence the frequency limitation.

An example of a millimeter-wave MMIC switched-line phase shifter is shown in below. The insertion loss of a switched-line phase shifter is dominated by the switch losses. Typical values are one dB loss per bit through X-band, 2 dB per bit at Ka band and 3 dB or more per bit at W-band. Two complimentary control signals are always required for switched line phase shifters.

Switched Line Phase Shifters

Example of 6-bit MMIC switched-line phase shifter

The figure shows the response of an ideal switched-line phase shifter. The difference in length between the reference arm and the delay arm is one-quarter wavelength at 10 GHz. Such a design would be used to provide a 90-degree phase shift at 10 GHz as shown (see the green trace on the plot). Switched-line phase shifters are often used for the two largest phase bits of a multi-bit phase shifter (180 and 90 degrees). Less complex circuits such as loaded-line elements can be used for 45 degree and lesser bits (see the loaded-line phase shifter discussion below).

Switched Line Phase Shifters

Ideal switched-line phase shifter response, 10 GHz 90 degree bit

The down side of a switched line phase shifter is that it's RMS phase error degrades quickly over frequency. This is truly the poor man's phase shifter. If you want better performance, consider the high-pass/low-pass phase shifter as an upgrade. But with any filter topology, you'll need to consider the variations in performance with component tolerance.



Author : Unknown Editor