FM Combining System

8/13/2019 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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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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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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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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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 bandpass 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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FM Combining System