In this article we introduce dividers and couplers as part of the "Inductor Applied Products" series.
What are dividers and couplers?
Dividers and couplers are distributors (circuits) used to split high-frequency signals into two separate systems. They are also called splitters, and are used in familiar devices such as DVD recorders, TV sets, and CATV set top boxes (STB). Recorders have an RF OUT connector used to connect the antenna cable to the TV, and this is a divider output. In addition, devices with two tuners enabling recording of one TV program while watching another or viewing of two TV programs simultaneously have built-in distributors.
Couplers are required when a CATV STB is equipped with a dedicated Internet or OOB (Out of Band) data tuner.
[Block configuration: Example of a double tuner]
[Block configuration: Example of a CATV tuner]
Features and structures of dividers and couplers
The basic function of dividers and couplers is to input a single signal and distribute it to two outputs. Although there is no clear general definition, Murata considers dividers as having two distributed outputs with a distribution ratio of 1:1 (equal distribution), and couplers as having two different distributed outputs (distribution ratio 1:n). As described previously, dividers are used when two sets of the same reception system (tuner) are required. Conversely, couplers are used when splitting into systems with different data transfer rates, such as video and data systems. This enables a design that increases the distribution loss of the data system, which has a slow transfer rate, while at the same time minimizing the distribution loss of the video system in order to increase the reception sensitivity and provide high-quality images.
Important characteristics of dividers and couplers
Three important characteristics can be listed.
First is insertion loss. Dividers are often located at the front end of circuits, so insertion loss is an important characteristic. In the case of 1:1 equal distribution, the calculated ideal loss value is -3 dB, so the closeness to this value is used as the judgment standard. Poor insertion loss adversely affects the reception sensitivity.
The loss levels of the two outputs of a coupler vary according to the distribution ratio. Therefore, the distribution ratio and loss should be determined in consideration of the respective tuner reception sensitivities.
The second characteristic is isolation. Isolation represents the degree of interference between two outputs. Poor isolation means that undesired noise components leaking from one tuner may flow back to and interfere with the other tuner. Various judgment standards are used according to the internal circuitry of the tuner to be configured, but an isolation level of 20 dB or more is often sufficient to pose no problems for practical use.
The third characteristic is return loss. Return loss is often discussed for the input side. For example, in case of CATV cable connection, some CATV standards specify that the return loss must be 6 dB or more. Therefore, the device must be able to clear this condition with sufficient margin. When multiple CATV cables are connected to a device with poor return loss, the reflected wave from each cable flows back into the CATV cables, generating a component called "ingress noise" that is transmitted along the entire cable and disrupts the signal quality of the overall service. Therefore, dividers must also satisfy the required characteristics in consideration of this condition.
Introduction of products
First, we will introduce couplers. The DXP18CN series commercialized by Murata has a lineup of two models with respective coupler output side loss of -10 dB and -15dB.
Select the appropriate model to match the video tuner and data tuner reception sensitivities.
On the other hand, Murata does not have a lineup of standalone divider products, and instead proposes circuits such as shown in the figure below. This circuit applies an existing common mode choke coil, and connects a resistor (R) and inductor (L) in series between the output terminals to secure isolation. The aim of this proposal is to enable realization of the required characteristics at low cost by configuring a simple circuit. In this manner, Murata is unable to guarantee divider characteristics at the parts level, but we can provide technical information to facilitate design.
[Recommended divider circuit]
[Example of characteristics data]
Dividers and couplers are essential parts for incorporating multiple tuners in TV sets, recorders and CATV reception systems. Dividers and couplers proposed by Murata Manufacturing can realize high performance in a compact package.
In the future, we intend to continue developing new technology and parts, and will strive to propose the best solutions for our customers.
Person in charge: Murata Manufacturing Co., Ltd. DTV GUY