The RF Signal Balancing Act
As RF, microwave, and millimeter-wave analog signals of ever higher frequencies become more ubiquitous in cellular and radar applications, the need to manage the integrity of these signals becomes increasingly important. Advanced high frequency analog functions are routinely packed onto semiconductor chips that are dense with complex digital functions. But this presents a problem. Semiconductor-based ICs have many layers of active signals and interconnects, but they do not have readily available low impedance paths to ground. This makes it especially difficult to maintain the integrity of microwave and millimeter-wave signals on chip. Special techniques are required.
A common technique for preserving high frequency signal integrity involves the use of balanced signals, in which the reference signal is an inverted version of the original, rather than ground. The chief advantage of balanced signals is common mode rejection, which eliminates noise and undesirable harmonics common to both signals in the balanced pair. Since balanced signals travel together in a tightly coupled pair, induced noise will affect both signals in the same way, allowing the noise to be eliminated by a differential amplifier.
Figure 1: Balanced Signals
Balanced signals are an elegant solution for RF signals on ICs, in which a ground reference is unavailable. But unlike ICs, circuit boards tend to have readily available grounds. Consequently, RF signals on a circuit board are usually treated as single-ended signals. How are these single-ended singles converted into balanced signals internal to the chip? Unless carefully done, this conversion can produce unacceptably noisy balanced signals.
Balun (balanced to unbalanced) circuits are the standard technique for converting unbalanced signals with ground reference into two signals equal in magnitude and opposite in phase. An ideal balun will produce balanced signals in which the reference is an exact inversion of the original. Real world baluns, however, produce signals with imbalance due to lack of perfect grounding at the input, lack of symmetry, transistor offsets, and uneven coupling from the input to the balanced outputs.
There are two categories of imbalance:
- amplitude imbalance, in which the amplitude of the reference signal does not match the amplitude of the original, and
- phase imbalance, in which the phase of the reference signal is not exactly 180 degrees shifted from the original.
Each of these types of imbalance will introduce errors in the signal.
A flux coupled transformer is the most popular balun. It is formed by two separate coil windings around a magnetic core, with one side of the primary winding grounded. The theory of these transformers is explained in many electrical engineering textbooks. These baluns are large and not useful at frequencies above a GHz. The magnetic material is not responsive to high frequency changes. Furthermore, interwinding capacitance between the coils works to nullify the magnetic coupling by providing a direct path across the windings. Consequently, these baluns are not suitable for on chip conversion of RF signals.
Figure 2: Flux Coupled Transformer (Image compliments of www.minicircuits.com)
A more useful realization of a balun for implementation on an IC consists of spiral coupled lines on the semiconductor chip. These can be used to handle RF signals above 1 GHz. An example of an 8-12 GHz balun is shown in Figure 3 with two mutually coupled coils. The balun in this example has a 50 ohm input impedance (to match a typical transmission line), and a 70 ohm output impedance for the balanced signals. One terminal of the input (Pm) is grounded to provide a reference, and the other input (Pp) is connected to the single-ended transmission line.
Figure 3: On Chip Balun With Center Tap on Secondary Coil
Unfortunately, this circuit, as described, will produce errors in the balanced signals. Grounding the Pm terminal creates asymmetry and imbalance due to the unequal terminations as seen by the inter-winding coupling capacitance. Analyzing the circuit with the Method of Moments technique (using a simulation and analysis tool) shows a high degree of both amplitude and phase imbalance, which increases as a function of frequency. This can be seen in the green and purple plots shown in Figure 4.
Figure 4: Amplitude imbalance in dB (on left axis) and phase imbalance in degrees (on right axis), plotted against frequency in GHz.
There is an elegant solution to reduce the imbalance. The midpoint of the differential terminal, labeled “tap” in Figure 3, is a virtual ground. By providing a low impedance path from the center-tap to ground, the Sp/Sm balance can be improved significantly.
Note that the balun topology is designed specifically to provide a short external ground connection. Referring to Figure 3, the tap is placed directly on the midpoint of the outer coil, which feeds the balanced signals Sp and Sm. This location at the midpoint is close to the primary IC inputs Pp and Pm, and only a short distance from an external ground, thereby ensuring a low impedance ground connection.
By grounding the midpoint of the secondary turn, the Sm and Sp signals internal to the chip are forced into better balance, eliminating much of the phase and amplitude imbalance that causes signal error. Using the same Method of Moments analysis technique, the plots (blue and red in Figure 4) of the phase and amplitude imbalance of the differential signals, Sm and Sp, shows much lower imbalance values in the 8 to 12 GHz range.
High frequency RF signals on digital ICs require specialized techniques for maintaining signal integrity. This article has shown, through example, one of the techniques that microwave IC designers at Intrinsix use to ensure the fidelity of high frequency signals as they cross from circuit board onto an IC. Techniques such as this are crucial to the successful production of ICs with microwave and millimeter-wave signals.