This calculator gives the turns ratio of an RF transformer. Enter the input and output impedance.
Formula
Np/Ns = √(Zp/Zs)
where
- Np = Number of turns in the primary coil
- Ns = Number of turns in the secondary coil
- Zp = Input or Primary impedance
- Zs = Output or Secondary impedance
Background
What is an RF Transformer?
An RF transformer is a passive device used in Radio Frequency (RF) circuits to transfer energy between two circuits. It consists of two or more coils wound around a common magnetic core, which can be made of materials such as ferrite or powdered iron.
The picture below shows a transformer by Mini-circuits.
The schematic for an RF transformer is shown below. This particular one has a center tap on the secondary.
The primary applications of an RF transformer are:
- Impedance Matching between two circuits to achieve maximum power transfer and to minimize or suppress any undesired signal reflection.
- Stepping up or down voltage/current.
- Isolating DC between circuits while enabling the transmission of RF signals
- Interfacing between balanced and unbalanced circuits
- Rejection of common mode signals
RF transformers find numerous applications in areas such as telecommunications, broadcasting, and microwave systems. In telecommunications, for example, they are used to match the impedance between the antenna and the transmitter or receiver. In RF and microwave circuits, RF transformers are utilized at the interface to various circuit components such as Mixers for single-ended to differential conversion.
Application Example
The schematic below shows the differential output from the AD8343 active mixer pins 12 13 being transformer (T3B) coupled to OUTPUT_B.
Overall, the versatility and efficiency of RF transformers make them an essential component in various low and high-frequency circuits and devices.
Transformer Specifications
RF transformers are specified to operate over a frequency range. The turns ratio is a key parameter and refers to the ratio of the secondary to primary impedance Zs/Zp.
Just like any other RF component, transformers have insertion loss, return loss that vary with frequency and a maximum input power level.
Transformer Technologies
In this post we have provided a calculator that’s pertinent to one specific transformer technology namely, core-and-wire. There are however other technologies that can be used to construct RF transformers:
- Transmission line
- Low Temperature Co-fired Ceramic (LTCC)
- Monolithic Microwave Integrated Circuit (MMIC)
The article Demystifying RF Transformers provides an overview of these technologies.