Table 2: Guide to Mini-Circuits’ RF transformer configurations.

Configurations

Type

Min Freq.

(MHz)

Max Freq.

(MHz)

Characteristics

Configuration Differences & Additional Details

A, B

DC isolated primary and secondary with center tap

0.01

1400

• Exhibits very high DC isolation.

• Center tap enables perfect grounding on both sides and provides better unbalance.

Config A and Config B are very similar, except B has center tap available on the both sides.

C, E, E1

DC isolated primary and secondary

0.01

800

• No center tap.

• Unbalance is affected due to virtual ground.

• Config C provides DC isolation.

• Config E and E1 are used for balun operation as primary connected to ground.

• Config E1 is implemented using LTCC technology and can operate from 12 to 18 GHz.

D, D1

Autotransformer

0.05

2500

• Used for impedance transformation.

• Allows for any impedance ratio based on number of turns.

• Config D1 is modified version of config D.

• Additional capacitor decouples the signal from ground and allows DC to pass.

F

Tri-filar

0.01

200

• User customizable configuration.

• This configuration consists of 3 lines all coupled to each other and can function differently depending on how the 3 lines are interconnected.

G, K

Transmission line transformer

4.5

9000

• Available as core & wire or LTCC.

• At lower frequency, coupling between lines is achieved through the magnetic core.

• At higher frequency, the coupling is achieved through capacitive coupling.

• The LTCC transmission lines are achieved through coupled transmission lines, and are limited in performance at lower frequencies.

• Both configurations allow DC passing between primary and secondary windings.

• Config G is a simple transmission line transformer, i.e. a bi-filar transformer.

• Config K has an additional feedback winding which allows for DC current to be sourced to the Secondary ports without saturating the core, but requires external capacitors are for DC current to be sourced.

 

Configurations

Type

Min Freq.

(MHz)

Max Freq.

(MHz)

Characteristics

Configuration Differences & Additional Details

H

Guanella Transformer

10

4500

• 4 winding transmission line transformer.

• Typically provides 1:4 impedance ratio.

• Commonly used as a balun.

• Allows for current to be sourced equally to the outputs.

 

J, R

Marchand Balun

390

13500

• Planar Marchan Baluns.

• Available as LTCC or MMIC.

• Operate at much higher and wider frequency bands compared to transmission line transformers.

• Provide DC isolation between primary and secondary windings, and the potential to allow for DC bias.

• Config J is for a standard Marchand Balun and the secondary winding is DC grounded.

• Config R has the secondary winding decoupled from DC ground, which allows for DC biasing to be applied to the next stage without requiring a decoupling capacitor.

Q

Impedance matching

DC

2500

• 50 to 75 Ohm impedance matching transmission line transformer.

 

 

Transformer Technology & Guidelines

As discussed in the previous sections of this series, there are four main RF transformer technologies, each with distinct electrical and mechanical properties. The limitations or advantages of each RF transformer technology may make it ideal for some applications while precluding its use in others. The following is a brief description of the various RF transformer technologies in terms of capabilities and application-specific considerations.
Core & Wire
Core & wire transformers are most often used for applications below 6 GHz, although some models can operate to nearly 10 GHz. Some of the reasons for this frequency limitation are grounded in the physics surrounding the ferromagnetic core of the transformer. The coils wrapped around the core create a strong magnetic coupling, which is mainly confined between the core and loops. Losses, as well as the impact of the interwinding capacitances tend to increase for this type of transformer at higher frequencies.

Beyond 1 GHz, capacitive coupling is much greater than magnetic induction in terms of signal transmission. As this effect is a function of size, there are frequency limitations based on the physical size of a core & wire transformer. The size of the wire and core are also major factors in determining the power handling of these transformers. Power handling is also largely limited by the saturation of the ferrite core, which is why these types of transformers are typically limited to less than 1W of power.

Core & wire transformers are typically made of heavy metals and are relatively bulky compared to planar RF transformer technologies. Therefore, applications that require compact size devices may be better served using LTCC or MMIC transformers.

Quarter-Wave Transmission Line Transformers
Quarter-wave transmission line transformers are typically fabricated using microstriplines or coaxial transmission lines with specific impedances. They are called quarter-wave transformers because they are sized to be ¼ of the wavelength of the desired operating frequency. As a result, the frequency performance of these transformers is directly related to the physical length of the transmission line and are therefore also band-limited. Quarter-wave transmission line transformers can be made to operate at frequencies over 13 GHz. These transformers are often assembled in coaxial connectorized packages, which are fabricated using the coaxial cable transmission lines that makeup the transformer.
 
LTCC
Low temperature co-fired ceramic (LTCC) RF transformers are designed using a planar construction method that is extremely repeatable and produces a robust and reliable device. Hence, LTCC transformers are often used in Hi-Rel applications such as aerospace, space, military communications and radar. These devices come in models that operate to nearly 20 GHz. Like other transformer technologies, the physical construction of these devices impacts the frequency performance limiting low frequency performance to several hundred MHz. However, high frequency LTCC RF transformers are extremely compact and ideal for low-profile surface mount applications.
 
MMIC
Monolithic Microwave Integrated Circuit (MMIC) technology can be used to fabricate extremely small RF transformers with good high-frequency performance beyond 13 GHz. Due to their small size, these types of transformers are typically limited to low frequency performance down to several GHz. These devices are either delivered in surface mount compatible packages or as bare die for wire bonded assemblies.
 
Conclusion
RF transformers are a key component of many RF systems, and will likely continue to be increasingly useful as RF and digital systems are more tightly integrated and the bandwidth requirements for communications systems continues to grow. Therefore, it is highly valuable to have a clear approach to selecting RF transformers for an application, which includes an understanding of RF transformer theory, performance parameters, technology, and applications.