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Microwave Phase Shifters


A microwave phase shifter is a device which alters the phase of its output signal in the microwave region, in response to an external electrical command.  Therefore, if an input signal is of the form sin WT, a phase shifter takes this input signal and produces an output signal SIN ( WT+ NULL), where NULL is the phase shift.

Alternatively, the output can be written A(N)SIN(wt)- (N), where upon a programmed phase shift(N), a(N)is the insertion loss (= the loss due to inserting another component in the circuit), and-(N)is the phase shift due to propagation. The difference between the phase of the input signal and the phase of the output signal is the sum of the phase shift due to the propagation through the phase shifter, plus the programmed phase shift.

Any reactance placed in series or shunt with a transmission line will produce a phase shift. This has given rise to many different circuits for use as phase shifters at microwave frequencies. Some uses include: 

• Phase modulators—Phase shifters can be used to mix a carrier signal with a modulating signal, changing the phase of the carrier by a fixed amount.

• Frequency converters—A frequency converter either generates harmonics on top of an input signal, or is used in a sound mixer, sending a second signal to mix with the original input signal.

• Frequency translators-- A frequency translator uses a phase shifter which is actively controlled to change the phase of a signal periodically. For example, a frequency translator will add another 10 degrees of phase every 10 nanoseconds. Since phase is the first derivative of frequency, if you change phase at a constant slope, you are adding a frequency component to the signal; thus the name "frequency translator". 
Phase shifters are employed in:

• Military applications/ECM (=Electronic Countermeasures): Doppler shifting of a reflected radar signal will be translated in frequency in an amount proportional to the velocity of the target. Phase shifting the signal emitted by the object can show up as an intentional deception as to the velocity of the target object. By suppressing spurious signals and harmonics, the velocity deception can be concealed.

• Broadband applications

• Testing instruments

• WLAN

• Satellite communications

• Phased array radar--Phased array radar systems can allow a warship to use a single radar system for finding ships and aircraft simultaneously, as well as directing missiles. This is because the array of radar antennas can focus on various targets simultaneously.

• Smart antennas-- Instead of having an antenna send a signal in all directions, a phase shifter can point the antenna electronically in microseconds, without physically realigning the antenna. When the antenna is receiving a signal, beam steering can filter out unwanted sources of interference.  


•  Circuit and power matching.

Phase shifters come in analog and digital forms:

Analog phase shifters are devices whose phase shift changes continuously as their control inputs are varied. Analog phase shifters are readily convertible to digital control by the addition of suitable D/A converters and appropriate linearizing circuits.

Digital phase shifters provide a discrete set of phase shifts or time delays that are controlled by two-state "phase bits." The highest-order bit is 180 degrees, the next highest is 90 degrees, then 45 degrees, etc., as 360 degrees is divided into smaller and smaller binary steps. A three-bit phase shifter would have a 45- degree least significant bit (LSB), while a six-bit phase shifter would have a 5.6-degree (technically, a 5.625) least significant bit.   The shortest phase length is the reference or "off" state, and the longest path or phase length is the "on" state. Thus a 90-degree phase shifter actually provides minus ninety degrees of phase shift in its "on" state.

 

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