fm combining system
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B Y : B A L A S J R . M A R L O N S A R I O
I E C E P M E M B E R
FM Combining Systems
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WHY COMBINERS ARE USED
As populations migrate to suburban areas, it hasbecome more desirable to construct large broadcast-ing facilities that can reach these heavily populated
areas from more central locations. Of course, theseprime locations have become more valuable, so itimportant to use each location to its fullest potential.
This can best be done by sharing a transmitter site and
a common antenna among several users. Toaccomplish this, the broadcast industry uses combin-ers of various types and sizes.
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WHY COMBINERS ARE USED
Shortage of Prime Locations
Group ownership of FM stations in a market has led toproliferation of combined stations, and, with the
implementation of DTV systems, FM stations are beingforced off existing towers, making it even moreimperative that they share tower space, whichincreases the demand for combined systems
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COMBINER CHARACTERISTICS
Frequency Response
is the fundamental property that enables a filter cavityto sort frequencies.
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COMBINER CHARACTERISTICS
Insertion Loss
The loss of energy at the resonant frequency.
The lost energy is converted to heat and dissipated in
the metal surfaces of the cavity.A cavity that is larger in size is more efficient than asmaller sized cavity in that it will provide a lowerinsertion loss at the resonant frequency with
comparable frequency response.
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COMBINER CHARACTERISTICS
Group Delay
as the frequency changes further away from f0, thesignal takes more time to pass through the cavity.
Excessive group delay within the pass band can resultin signal distortion.
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COMBINER CHARACTERISTICS
Impedance
Current flow in any RF circuit must overcome resis-tance, capacitive reactance, and inductive reactance.
The vector sum of these is termed impedance.
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COMBINER CHARACTERISTICS
Physical Size
The physical size of the cavity is established for thepurpose of power capacity and electrical performance.
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COMPONENTS OF COMBINERS
Tee or Star-Point Junction
is a coaxial component that allows two RF signals toflow into a common path, a star-point junction is a tee
with more than two input paths.
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Quadrature Hybrid (Hybrid)
The heart of the modern balanced combiner system.
Is a complex broadband device that has the ability tooperate in various modes, either singly orsimultaneously.
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Hybrid as Signal Splitter
the hybrid is acting as a power splitter and phaseshifter.
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Hybrid as Signal Combiner
A second mode uses a hybrid in reverse, for combiningtransmitters.
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Hybrid as Signal Reflector
When two identical devices with high impedance,such as bandpass filters tuned to another frequency or
band reject filters tuned to the incoming frequency, areattached to ports 3 and 4 of the hybrid, the signalentering at port 1 is reflected and exits the hybridthrough port
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Hybrid Ring
Used when two hybrids are used in a ringconfiguration to both split and combine a single inputsignal, virtually 100% of the signal exits the ringthrough the hybrid leg opposite the input.
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FILTERS
Band Reject or Notch Filter
There are several ways to design a band reject or notch filter,
but they all accomplish the same purpose. In one form, acavity with only an input coupling loop is mounted off thetransmission line by means of a matched tee. This provides apath that removes the tuned frequency from the system,allowing other frequencies to pass with minimum loss.
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Interdigital Filters
Interdigital filters have only recently been introducedas an alternative to loop- and iris-coupled filters at FM
frequencies. Interdigital filters do not employ individ-ual cavities that must be coupled together.
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ISOLATORS
An isolator is comprised of a circulator and a load.
Signals move between legs in only one circulardirection, giving the device its name.
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DIRECTIONAL COUPLERS
are commonly found on each broadband output of acombiner system and are convenient ports for takingFCC-required test measurements, enablingdiagnostics, and serving as a port for any protectionand monitoring system the combiner may employ.
Directional couplers located on the inputs to each
module further enhance the versatility of the system.
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GROUP DELAY EQUALIZER
consists of a quadrature hybrid and two identicalbandpass filters that have only one coupling loop, sothe energy is coupled in and out of the cavity by thesame loop
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TYPES OF COMBINERS
Branched or Star-Point Combiners
A branched combiner is a simple combination of a teejunction and the required number of filters to ensure a
sufficient amount of isolation to prevent spurs
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Balanced Combiners
based on a hybrid ring. Each leg of the ring containsan identical set of either band- pass or band reject
filters, hence the term balanced.
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Notch Filter Balanced Combiners
In the notch-filter balanced combiner, both notchfilters within the hybrid ring are tuned to reject TX1s
frequency, which enters the combiner at port 1. Thatsignal is reflected by the filters and exits at port 2.
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Bandpass Filter Balanced Combiners
In a bandpass balanced combiner system, bandpass filters
are used within the hybrid ring. The basic system layout issimilar to that of a notch combiner.
In the bandpass system TX1 also enters port 1 but passesthrough the hybrid ringsbandpass filters and out port 4,while signal TX2, entering at port 3, is reflected by the filters
and exits at port 4.
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COMBINING DIGITAL AND ANALOG SIGNALS
High-Level Combining
uses a directional coupler that has been mechanicallyenlarged to handle power levels in the kilowatt range, witha nominal coupling factor of 10 dB, and can be used in
reverse to inject the digital signal into the analog RFstream.
Mid-Level Combining
was developed by incorporating a standard 3 dBquadrature hybrid and using two analog transmitters: onestandard analog transmitter and one linearized transmitterequipped to transmit digital along with the analog.
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Combining Using Bandpass Balanced Combiners
used to minimize the size and cost of the digitaltransmitter and reduce the energy wasted.
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Back-feeding is used for low level combining ofanalog and digital signals with minimal loss.
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Cross-feed or Split-feed is a further extension of back-feeding. Rather than segregating digital and analog signalsinto separate transmission lines, it combines the analogsignals of some stations with the digital signals of others.
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Thanks for Listening ;3
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