The following article / app note titled, "Bias T, Band Splitter and Other RF Diplexers," was submitted to RF Cafe by
Bree Engineering. It is a brief
introduction to and explanation of the theory and application of the named frequency
selective devices. A Bias-T is frequency dependent just as much so as a band splitter
or diplexer; it differentiates between DC (0 Hz) and the RF frequency. Bree
Engineering Corporation was founded in 1999 and is a manufacturer of custom
electronic filters, multiplexers, filter banks and other related types of components
in the frequency range of 0.1 MHz to 40 GHz. Designs include Chebychev,
Bessel, Butterworth, Gaussian, transitional, elliptic-function and
pseudo-elliptic-function filters in lumped element, cavity, combline, interdigital,
and ceramic resonator formats.
By Bree Engineering
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 cut offs. 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.
- 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
- 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 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.
- 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.
- 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 and . At higher frequencies the filters
are more sensitive to this "Parasitic Communication" causing some difficulties when prototyping
- 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
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
About Bree Engineering
Bree Engineering Corporation
was founded in 1999 and is a manufacturer of custom electronic filters, multiplexers,
filter banks and other related types of components in the frequency range of 0.1 MHz
to 40 GHz. Contact Bree Engineering for specific applications, Bree Engineering
designs include Chebychev, Bessel, Butterworth, Gaussian, Transitional,
Elliptic-Function and Pseudo-Elliptic-Function filters. In addition to Lumped Element
designs, Bree Engineering also produces Cavity designs (Combline or Interdigital) and
Ceramic Resonator designs.
Bree Engineering is known for our responsiveness to the needs of our customers,
our agility and our consistent on-time delivery.
All products are built to your specifications, so if you need a surface mount
device or one with connectors, high or low power handling, or optimized for price or
any specification parameter you need, we will tell you what is achievable, and
explain all of the trade-offs involved so you can make very well-informed decisions.
Posted October 23, 2018