Bias T, Band Splitter and Other RF Diplexers

A Simple Bias T allows for a DC voltage to be injected into a circuit without directly altering with the RF signal that is being sent through the same circuit.

• A good benefit to this is the ability to provide power from the injected source location to the desired component that would need power. (Antennas, Amplifier, diodes, transistors and more RF components)
• The reason why being able to inject a DC voltage is important is because it will alter the RF signal in such a way that could help the output. Meaning, if a particular component required a certain voltage to activate another component then the signal would be able to deliver that voltage and then proceed with its RF transmission.

Band Dividers – Rough Broad Band Splitting

For band splitting, it is the idea of taking a single signal and splitting it up multiple times. In order for this to happen, the initial signal needs to have a high enough power to allow for the split to occur because some of the power will be lost in the process. It is fairly safe to say that during the signal split approximately 3dB of loss will occur each time a split takes place, depending on the component used the loss could be lower. Band dividers are an important tool that can be used within an overall system to split signals when necessary.

Naturally, band splitting and power dividers go hand in hand because a signal travels with the power sent through the system. Different power dividers can achieve different levels of splitting and different parameter cutoffs. Some of the most popular ones are versions of Directional Couplers, Wilkinson power dividers, and Waveguide Magic Tee Power dividers. Each of these designs has their trade-offs and depending on which design that is chosen you may receive higher insertion loss versus another or the amount of power it can handle.

Diplexers

In the RF Filter realm, a Diplexer is used to receive one signal and separate the desired frequency ranges so that the receiving end RF components can analyze the data filtered.

Essentially a band splitter, Bias T and a Diplexer are similar due to the multi-purpose singular port that contributes to the overall success of an RF system.

A diplexer can have a multitude of parameters, we can have diplexed bandpass filters, where the first of two output channels can reflect a complete bandpass filter (i.e. passband could be 1088MHz – 1092 MHz) and the second of the two output channels could also be another bandpass filter (i.e. passband 2950 MHz – 3650 MHz)

We can also create a Diplexer where one channel is a lowpass and the other channel is a highpass. This will allow a single package containing two output ports to meet the desired specifications for 2 different types of filters.

Challenges on the manufacturing end

Quality Factor “Q” – Higher frequency specifications desire smaller components, and smaller components can be difficult to work with. Simulating and fabricating at a set “Q” for our lumped element components is extremely important and often times needs to be selected with care and consideration.

• Component size also contributes to the resonant frequency of each lumped element component. If an inductors coil winding or wire gage needs to be altered too much the “Q” of the component is changed and thus the Self Resonance of the component will change.

Size – As the years have progressed Moore’s Law has been proven with the ever growing realization that components must get smaller. Smaller components are more difficult to work with, but that’s something that the entire industry has been able to adjust to and the filters we produce have not succumbed to size problems.

• Some additional factors that affect component size and shape is the limitations to bandwidth and specific power requirements. Bandwidth and power can drive lumped element size changes, which will direct the size of the filter to be larger or smaller depending on the specific requirements.

Parasitics – Parasitic Communication has a pretty significant roll when it comes to diplexers. Due to the nature of the diplexer, there are two different types of filters within an enclosed space. The enclosed space prompts the inductor’s magnetic fields to spread over into nearby inductors causing undesired coupling. At higher frequencies, the filters are more sensitive to this “Parasitic Communication” causing some difficulties when prototyping occurs.

• Part of the problem that occurs here is that shunt capacitors will have some parasitic inductance which will influence the inductors and drive their value away from their simulated values.

The importance of the band dividing, bias T’s, and diplexers is that they are crucial to the success of an RF system in need of taking one source and turning it into two sources. In reality, all three devices require the same thought process, they all require consideration for sizing and shape due to power handling and bandwidth. Due to the fact that all three components divert the path of a single signal, the core decisions for design and fabrication are critical.