Frequency tunable balun

Cost effective, on-chip integration operating at high frequency


Die picture of the tunable Marchand balun (right) with on-wafer THRU, LOAD, SHORT de-embedding structures (left).

Baluns are common electrical components that covert an unbalanced signal to a balanced signal (or vice versa) and are used to achieve compatibility between systems such as for frequency conversion between communications networks.

Mixed-signal circuits are becoming more common in silicon technology across wider and wider regions of the spectrum. In such cases, broadband response, low loss, magnitude, and phase imbalance are key performance specifications for the balun designer. However, it is a challenge to demonstrate reasonable performance, particularly when implemented in a silicon technology.

Navy scientists have made a significant step in furthering the functionality of baluns implemented in silicon. This invention enables a balun’s operational bandwidth to be tunable around multiple distinct center frequencies by using switches to vary the balun’s dimensions. The result is a tunable, broadband Marchand balun that operates in the K band frequency range.

As an example, the novel balun can be tuned around two frequencies and be implemented using commercially available silicon bipolar complementary metal-oxide semiconductor (BiCMOS) technology that makes use of the process’ native MOS devices to design the required switches. The circuit element uses a pass gate structure for the required switches, and the switches connect additional lengths of line in or out of the balun to change its frequency response. Because different line lengths correspond to different frequencies, the frequency response of the balun can be changed by switching in or out the different lengths of line.

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