6 Phase Balance with Amplifiers and Baluns

Returning back to the topic of amplifier and balun trade-offs, baluns come in many forms, packages and designs. Classic ferris-type baluns are usually prone to phase imbalance, the recommendation is to consult the data sheet before making your final selection and not base your selection only on insertion loss or the bandwidth that the balun can cover for your particular application. Smaller packages that use lithography structures have tighter tolerances and better repeatability, which typically means there is a possibility to improve the phase imbalance. However, this comes at the cost of smaller or more narrow bandwidth choices, which is not always good if the design calls for lower frequencies in the UHF bands near DC.

Module baluns produce some of the best phase imbalance results, but they are big, bulky, and costly—as much as $2,500 for just one modular balun. These really expensive baluns provide some of the widest bandwidth-hitting DC frequencies on the low BW-end and maintain good phase flatness into the multi-gigahertz regions. Figure 6-1 compares the phase imbalance of some types of baluns available on the market today.

If your design calls for a wide bandwidth but there are also cost constraints, one neat trick is to put two baluns or transformers back-to-back to improve the phase imbalance. See Figure 6-2 and Figure 6-3. The only downside is the doubled amount of PCB area to implement this type of front-end structure.

Using either balun configuration A or balun configuration B from Figure 6-2, or the red and blue curves in Figure 6-3, you can see that a phase imbalance of 5 degrees or less can be extended out to 3GHz over the original single-balun configuration, over the green curve. If using one of these balun configurations, note that each balun combo can have varying degrees of improvement.

Phase balance is also prevalent on the amplifier side. Because low-noise amplifiers and gain blocks have single-ended inputs and outputs, you can assume that these types of amplifiers do not have good phase imbalance and have high even-order distortion, which is why HD2 is not specified in these types of amplifier data sheets.

Fully differential amplifiers (FDAs) are the classic amplifier input interface to the ADC with differential inputs and outputs. Even though the FDA enables single-ended signal to a differential signal conversion, tying one input pin to ground in some fashion, the FDA inputs are sensitive to this reference shift, and therefore, the FDA can exhibit more even-order distortion in this configuration.

FDAs are typically characterized with wide-band modular baluns to capture the published performance metrics. However, the TRF1208 amplifier uses a compensated input structure, allowing for a single-ended input interface by default and removing the dependency of the balun on the inputs like a traditional FDA. TRF1208 input structure fits well when interfacing to RF analog receiver cards, which are classically single-ended.

Using the ADC12DJ5200RF ADC, Figure 6-4, directly compares a typical wide-band balun interface, the TRF1208 interface, a low-noise amplifier plus wideband-balun interface, and an FDA with and without a balun on the differential inputs.

Notice that in all of these configurations below 1GHz, the performance is fairly equal. As the analog input frequency climbs past 2GHz, however, there is a distinct growth in the even order distortion for all combinations except the TRF1208 (red curve).