So far, this series has provided a survey of RF transformer theory and the technologies behind the various types of RF transformers commonly used in RF system design. The previous sections dove into baluns & ununs and provided a deeper understanding of RF transformer performance parameters. This section concludes the series with considerations to keep in mind when selecting RF transformers for your application. We will also include a description of Mini-Circuits’ current RF transformer technologies and methods for selecting a transformer configuration that best meets your specific requirements.
Questions to Ask When Selecting A Transformer
Selecting a component for any RF application requires a complex decision-making process, from modeling and simulation to determining the performance criteria for a signal chain. Since the functions of RF transformers are essential in some applications, the performance and capability of a transformer can have significant impacts on the gain and power budgets for the rest of the signal chain. Selecting the appropriate transformer for a given application is therefore very important for optimal RF design. 

For example, it is important to know the DC and power requirements of a given RF circuit, since some RF transformer configurations either isolate, pass, or allow injection of DC current up to a nominal maximum power rating. The selection process includes several other factors as well, and can be broken down into steps with the aid of the RF transformer configuration guide included later in this section.

Step 1: What frequency range is required of the transformer?
Filtering a selection of transformers down to the desired frequency range for your application can narrow down the candidate pool significantly. Most applications have an operating frequency range determined by the 3 dB bandwidth. RF transformer frequency range is typically defined by the 3 dB bandwidth, not the full frequency range (bathtub curve), and it is relatively easy to match the system frequency range with a transformer frequency range. Because RF transformer technology greatly influences operating frequency range, the frequency requirements may predetermine the technology needed. This is especially true for applications beyond several gigahertz where there are fewer core & wire transformers that support these frequencies. This means that transmission line, LTCC, and MMIC transformer designs are more common.
Step 2: What impedance ratio is needed to ensure proper match?
If impedance matching or transformation is required in a circuit, this is likely how a transformer will be used. Characteristic impedances are typically decided by system designers during the modeling and simulation phase of the design process. It is important to remember that the impedance ratio of a transformer directly impacts the return loss when inserted in a system, and there may be performance limitations that influence ratio requirements.
Step 3: What kind of mounting and connector interface is needed?
Depending on the transformer technology and typical applications for different transformer types, the transformer may be packaged in a connectorized assembly, in a surface mount package, or as a bare die. It is important to consider the type of parasitics, loss, reflections, and other real-world effects inherent to different device housings before deciding on a particular RF transformer package and interface type.

For surface mount applications, there are core & wire, LTCC and MMIC devices available. Some applications may require wire bonding of bare die into compact assemblies.

Step 4: Is a balun configuration needed?
Not all RF transformer configurations can be used as baluns. For instance, Configuration D transformers are autotransformers, and are not suited to balun use. Configurations A, C, G, J, R, and K are commonly used as baluns. (Refer to Table 2 below).
Step 5: Is there a need for DC isolation or DC injection?
Only certain transformer configurations support DC isolation and DC injection. For DC isolation, configurations A, B, C, and E are viable, while for DC injection, configurations A, B, and K can be used. (Refer to Table 2 below).
Step 6: Are there footprint or height constraints?
For some applications, there may be strict constraints on footprint size and device height that limit the selection to more compact technologies such as LTCC, MMIC or surface mount core & wire. Because the size of an RF transformer impacts the frequency range and other performance variables, size constraints may be directly related to electrical performance constraints. For example, the need for an extremely small RF transformer could limit low frequency performance, minimum insertion loss, power handling, DC isolation/injection capability, and impedance ratio.
Step 7: Are there specific RF electrical performance requirements?
Lastly, the remaining RF electrical performance parameters among a manageable selection of RF transformers can be compared and an ideal RF transformer can be identified. These RF parameters include, minimum insertion loss or insertion loss at specific frequencies, amplitude unbalance/balance, phase unbalance/balance, and return loss.
Step 8: Are there other considerations?
There may be additional considerations that are not readily determined during the early simulation and prototyping phase. Some experimentation may be required for which easy sampling options can be crucial in deciding on a specific RF transformer model.
Breakdown of RF Transformer Parameters and Features
Table 1 provides a breakdown of all key RF transformer parameters and features that may need to be considered when selecting an RF transformer for a given application.
Table 1: Breakdown of key RF transformer parameters and features