True Time Delay in Phased Array Antennas

Key Takeaways

• True time delays are one of the key elements of wide-band phased-array antennas.

• True time delays eliminate the beam squint phenomenon by applying variable phase shifting across the signal spectrum. 

• Time delay units, or boards, are used in phased-arrays to provide beam steering and phase shifting. 

Using true time delays, wideband communication systems provide speedier communication connections.

There is an increasing need for faster, more reliable communication networks, and wideband communication systems are working to fulfill this need. The higher the bandwidth, the faster the communication data rates. However, in wideband communication, it is difficult to transmit and receive signals due to the distribution of signals across a wide spectrum.  

Antenna technology has been extensively modified to address over-arching spectrum problems. Among the list of solutions are phased array antennas, as they are highly efficient, with advantageous characteristics including electronic beamforming, spatial diversity features, and high signal-to-noise-ratio (SNR). 

Phase shifters are used for directing the beams in phased-array antennas and are used to improve efficiency in narrowband communication systems. A phenomenon called beam squint limits phase shifter performance in wideband phased-array antennas. True time delays eliminate this beam squint phenomenon by applying variable phase shifting across the signal spectrum, making them a key element of wide-band phased-array antennas. 

The Beam Squint Phenomenon

Beam squint is a frequency-dependent distortion of the beam steering angle in phased-array antennas. The use of the same phase shift for all array components in a phased-array antenna creates the beam squint phenomenon, where the difference in phase shift at the low and high end of the spectrum points the beam differently from one end to another. Variable phase-shifting provided by true time delay circuits can reduce beam squint and help to achieve high resolution. 

Reducing Beam Squint

Selective beam steering can be introduced with a set of time delay lines. Arranging the time delay lines from shortest to longest provides the necessary steering granularity and required beam steering.  By carefully setting the true time delay, it is possible to introduce suitable phase shifting that matches the signal spectrum in phased-array antennas. 

If the requirement is to azimuth steer a vertical oblong beampattern in a one-dimensional array antenna, place true time delays between each column of sub-arrays. In two-dimensional array antennas, true time delays can be introduced. For a single steering solution, a fixed time delay line between the elements can be used. The optical method of true time delays is also available for variable phase shifting. 

True Time Delay Units

True time delay units, or boards, are used in phased-arrays to provide beam steering and phase shifting. Traditional time delay units were switched delay lines with quantized delays. When placed in the signal paths on array elements or subarrays, these time delay lines introduced specific time delays. However, the switched true time delays generated insertion loss between the reference and true time delay units, and the loss increased with frequency. 

Today, true time delays can be achieved in several ways:

• Switched lines

• Complementary Metal-Oxide Semiconductor (CMOS)

• Microelectromechanical (MEMS) 

• Gallium Arsenide (GaAs) 

MEMS, CMOS, and GaAs based true time delay units fall under the trombone or active distributed configuration class of time delays. Additionally, monolithic microwave integrated circuit chips (MMICs) are available in the market, which can introduce specific or programmed time delays. 

Generally, a time delay unit is a phase shifter with special features used at the subarray level. Its basic function is to provide a specific time delay using a multi-path structure. Compared to phase shifters, a true delay unit is capable of providing many wavelengths of phase shift and the phase shift is frequency dependent. This makes the group delay difference between spectrum ends formed by time delay units to be flat over the required bandwidth. This flattening of the group delay reduces the beam squint and improves the bandwidth. 

A CMOS-based true time delay for wideband phased-array antennas consists of a true time delay circuit (TTD), digital step attenuator (ATT) circuit, wideband distributed gain amplifiers (WDGA), and a serial peripheral interface (SPI). Artificial transmission lines, single-pole-double-throw switches (SPDTs), and double-pole-double-throw switches (DPDTs) are used in attenuator and time delay blocks. A wideband gain amplifier with positive gain compensates for the insertion loss in the system. The compact design of the CMOS based true time delay units is low cost and suitable for wideband phased-array antennas.

Beam squinting degrades the transmission and reception of a wideband communication spectrum and is a serious concern when trying to establish faster communication networks. True time delays can reduce beam squinting by providing variable phase shifting, and they are a crucial element in wideband phased-array antennas. Introduce precise time delays using true time delay units in phased-array antennas to improve the system bandwidth and beam steering, enabling faster communication networks. 

For more information on phased-array antenna design and simulation, check out this webinar.