Conventional on-chip transmission line structures generally have fixed impedance and fixed delay. Usually, delay and impedance cannot be arbitrarily chosen for a given transmission line. Instead, the delay and impedance are affected by the capacitance and inductance, which vary inversely to one another based upon the distance between the signal line and the ground return line(s). As such, while it is possible to change the delay of a transmission line, changing the delay comes at the cost of increasing signal loss, changing the characteristic impedance, and/or increasing the required area (e.g., footprint) of the transmission line device.
Changing the delay of a transmission line, however, is desirable for a number of applications. For example, delay lines are utilized in signal processing operations for adjusting the time of arrival of one signal relative to that of a second signal. The delay lines may be fabricated for digital circuitry or analog circuitry, and the delay may be fixed or variable. With respect to delaying a signal having a sinusoidal waveform, this being a frequent situation in microwave applications, the effect of the delay line is to impart a phase shift; thus, in this situation the delay line may be regarded as a phase shifter.
A plurality of phase-adjustable lines may be used in a phased array. Generally speaking, a phased array is a group of antennas in which the relative phases of the respective signals feeding the antennas are varied in such a way that the effective radiation pattern of the array is reinforced in a desired direction and suppressed in undesired directions. The relative amplitudes of, and constructive and destructive interference effects among, the signals radiated by the individual antennas determine the effective radiation pattern of the array. Phased arrays are used to electronically steer the direction of maximum sensitivity of a receiver, providing spatial selectivity or equivalently higher antenna gain. Phased arrays find use in many different wireless applications, including but not limited to RADAR and data communications. Beam steering is achieved by first shifting the phase of each received signal by progressive amounts to compensate for the successive differences amongst arrival phases. These signals are then combined, where the signals add constructively for the desired direction and destructively for other directions.
A conventional way of controlling the phase of each element in a phased array is to provide each element with a plurality of transmission lines, each of the transmission lines having a known delay. A switch in the signal path of each element is used to select a particular transmission line for that element, thereby imparting a known delay to the element. However, such systems suffer from numerous drawbacks. For example, providing each element with a plurality of transmission lines is costly in terms of space used (e.g., footprint), manufacturing, etc. Also, the switch in the signal path of each element causes signal attenuation, which is undesirable in such applications.
Moreover, as described above, conventional systems are incapable of changing the delay of a transmission line without either increasing signal loss, changing the characteristic impedance, and/or increasing the required area (e.g., footprint) of the transmission line device. Thus, systems that utilize delays (e.g., phased-array antenna systems) suffer from these drawbacks.
Accordingly, there exists a need in the art to overcome the deficiencies and limitations described hereinabove.