[Inductor Applied Products] Chapter 2: Ultra-small Balun for TV Tuners
In this article we introduce ultra-small balun for TV tuner applications as part of the "Inductor Applied Products" series.
What are Balun?
"Balun" is a word created from "balance to unbalance," and refers to a "balanced (signal) - unbalanced (signal) converter."
Unbalanced here refers to the signal transfer mode in which the signal send and return paths are clearly differentiated, and multiple signal lines use a common ground as the return path. In contrast, balanced refers to the signal transfer mode in which the signal send and return paths form pairs, and these send and return signal lines have an electrically symmetrical design.
Balun are used between the unbalanced line leading from the antenna and the balanced terminals of the silicon tuner, and function to convert signals from unbalanced to balanced, and to match the impedance. The basic balun structure consists of two same-polarity coils located facing each other.
The basic balun circuit configuration is typically 1:1 or 1:4 according to the conversion ratio. For example, expressed in terms of the impedance conversion ratio, this is 50 Ω:50 Ω, 50 Ω:200 Ω, or 75 Ω:75 Ω, 75 Ω:300 Ω. The appropriate balun is selected according to the characteristic impedance of the antenna input side and the silicon tuner side. In addition, 1:1 configurations typically include transformer types and floating types. Murata's balun use the floating-type design described below, which is advantageous in terms of characteristics and miniaturization.
[Main types of balun]
Although it may be somewhat difficult to understand, the correct balun must be selected according to the impedance conditions in order to achieve the fullest performance of the silicon tuner. Specifically, signal reflection due to improper impedance matching in the connections increases the signal loss, which may make it difficult to achieve the desired reception sensitivity, increase susceptibility to specific noise and interference signals, or result in other problems. Therefore, thorough checks should be made during design.
Balun have actually been used in various wireless communication and other reception circuits since long before the advent of silicon tuners. Typical older balun were approximately 10 mm square in size, and had custom specifications with lead wires wound in a complex manner through a spectacle-shaped ferrite core. The resulting high cost and large high-frequency characteristic variance made it difficult to incorporate balun into circuit design. Murata developed and commercialized the DXP series and DXW series as ultra-small surface-mount types with specifications suitable for silicon tuners, and are becoming ever more compact and low-profile.
1. Introduction of DXP series and DXW series
Murata has commercialized the DXP series that uses original thin-film technology and the DXW series that utilizes winding technology as ultra-small balun suitable for silicon tuners. The supported frequencies cover the 50 MHz to 870 MHz range that includes the full terrestrial broadcasting band and the 470 MHz to 870 MHz range for mobile applications. The lineup also includes a model that uses a 1:4 internal structure to support a pseudo 1:6 impedance conversion ratio, and a model that can support satellite broadcasts (BS/CS).
Each product is compact and has a low profile, achieving a vast reduction in the mounting area compared to conventional spectacle-shaped balun. In particular, the DXP18BN series uses an internal wiring design that simplifies the connection wiring with the aim of further reducing the required mounting space.
Points when selecting balun
(1) Check the characteristic impedance of the antenna side (input side), and select 50 Ω or 75 Ω. 75 Ω is generally used for terrestrial wave systems, and 50 Ω for CATV and mobile systems.
(2) Check the characteristic impedance of the silicon tuner IC side circuit, or check if balun specifications are recommended as IC application circuit reference information, and use this to select an impedance conversion ratio of 1:1 or 1:4/1:6.
(3) Select a DXP series balun when the impedance conversion ratio is 1:4 or 1:6, or a DXW series balun when 1:1.
Characteristic impedance matching is important in order to achieve the fullest balun characteristics. However, the desired characteristics may not be achieved even when the balun selected as described above is incorporated into an actual circuit and evaluated. This is because the actual impedance on the IC side does not exactly match the ideal impedance (50 Ω/75 Ω/200 Ω/300 Ω). In these cases, the impedance must be matched, or the balun must be reselected. Feel free to contact Murata for details and solutions.
2. Characteristics required of balun in tuner circuits
The balun is located at the front end of silicon tuner IC, so the balun insertion loss directly affects the reception sensitivity. In case of digital broadcasts, reception may suddenly become impossible when the strength of the radio wave (loss = reception sensitivity level) in particular falls below a certain threshold, and this loss difference tends to be more distinct compared to analog broadcasts. With the switch to digital broadcasting, this means that insertion loss must be strictly taken into account, so low loss over a wide frequency band is also required of balun to ensure stable reception of all channels over the widest broadcast area possible.
[Image drawings of reception states]
In addition, CMRR (*1) is also important. CMRR represents the ratio of in-phase signal components included in a differential (unbalanced) signal, and is also used to express the phase error and amplitude error between balanced terminals. For example, when CMRR is 30 dB, this indicates that the phase error is 3° relative to 180°, and the amplitude error is approximately 0.2 dB.
Both the DXP series and the DXW series simultaneously achieve low loss and a high CMRR without variance over a wide frequency band.
(*1) CMRR: Common mode rejection ratio
When a signal with a poor CMRR is amplified by an RF amplifier, a distortion component is generated, resulting in reduced tuner sensitivity. Therefore, a high CMRR is also demanded.
[Example of characteristics data]
The DXP series uses original photolithography micro fabrication technology and highly coupled coil circuit and structure design technology developed for film-type common mode choke coils. In addition, technology for designing small, high-performance balun circuits has also been newly established, enabling realization of the characteristics required of balun noted above in a small, low-profile size.
On the other hand, the DXW series uses high-precision winding control technology developed for winding-type common mode choke coils, and realizes high-performance balun characteristics in a small package. Due to structural limitations, there is no 1:4 model in the DXW series lineup, but the low loss that is a feature of winding types enables realization of even higher-performance 1:1 conversion ratio balun.
With the switch to the digital broadcast era, TV tuners are also converting to silicon tuners. Amidst this trend, balun are playing an increasingly important role. Murata Manufacturing intends to make use of its original balun circuit design technology, photolithography micro fabrication technology and winding control technology to develop new technologies and new products to further meet various user demands.
Person in charge: Murata Manufacturing Co., Ltd. DTV GUY
The information presented in this article was current as of the date of publication. Please note that it may differ from the latest information.