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Ghost Cancellation P P PPPP PPP

P A S S I V E P O W E R P R O D U C T SP A S S I V E P O W E R P R O D U C T SP A S S I V E P O W E R P R O D U C T S

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Ghost Cancellation

TV Switchless CombinerSystems

in

27 October 1997

by

Scott B DurginEditor: Technical Publications


The advent of high power TV transmittersystems that has lead to the development of

Switchless Combiners for parallel signalamplification, has lead to potential problemscaused by ghosting signals that result from

antenna system VSWR. In a two-transmitterparallel amplification TV system which

employs a switchless combiner, the method offeeding the two transmitters in phase quadra-ture, coupled with a corresponding RF system

configuration, can be an effective way ofeliminating the ghosting signals.

Abstract


Schematic and Mode Table - Switchless CombinerFigure 1

INPUTB

INPUTA

output

phaseshifters

H1 H2

0°-90°

to load

0°-90°P1

P2

MODE P1 P2A+B TO AIR

A TO AIR, B TO LOAD

B TO AIR, A TO LOAD

0°

-90°

-90°

0°

0°

0°

The requirement for parallel amplificationof TV transmitters can be satisfied in a fewdifferent ways, the two most popular of whichare through the use of either a switched hybridcombiner or a switchless combiner. From practi-cal considerations, the latter of these is the moredesirable of the two due to the simple functionaladvantage that the transmitters do not need to beshut down during mode changing. (i.e.; 'LiveMode Transfer).

1 Moreover, if proper phase

control of the signals is exercised, the use of theSwitchless Combiner can provide an effectivemeans of ghost cancellation. This paper dis-cusses the operation of TV Switchless Combin-

ers which, together with the transmitters, areconfigured to provide effective cancellation ofghost signals originating through reflections atthe antenna system.

Figure 1 below shows two hybrid couplersinterconnected with two phase shifters (P1 andP2), illustrating the basic operation of a TVswitchless combiner. High power applicationsgenerally dictate waveguide construction (for theUHF band) while lower power applicationsallow for the use of coaxial construction. Thefirst coupler (H1) is a 90° hybrid coupler or'short slot' hybrid, while the second (H2) is a180° hybrid coupler or 'magic tee' hybrid . For

Introduction Basic Operation


high power UHF, the phase shifters employedare the conventional waveguide phase shifterswith a movable interior dielectric slab that canbe translated from the sidewall (minimum phaseshift) to near the center of the guide (maximumphase shift). Note that H2 could be a 90° hybridas well, in which case a corresponding additional90° delay line would need to be employed in oneof the transmission lines connecting the hybridsto ensure successful operation of the system.

The mode chart in Figure 1 indicates thattwo phase values for each of the phase shiftersare required to accomplish all of the necessarymodes for parallel transmitter or single transmit-ter configurations.

90°. Therefore, an input signal at port 1 willresult in two half power signals exiting ports 3and 4 (where the signal at port 3 leads the signalat port 4 by 90°in phase). Port 2 is isolated.

Since the hybrid is a reciprocal device, twoequal magnitude signals in phase quadrature willcombine at one of the opposing ports dependingon the relative phasing of the signals. This isshown in Figure 2b.

The operation of the magic tee hybrid issimilar to that of the short slot hybrid except thatthe critical phasing is 180° instead of 90°. This isillustrated in the right half of Figure 2b where itis shown that two signals entering ports C and Dwill combine at the sum port (port A) if thesignals have a phase difference of 0° uponentrance, or they will combine at the differenceport (port D) if their initial relative phase differ-ence is 180°.

During the operation of the SwitchlessCombiner, it is desired that unwanted signals bedirected to the difference port of the hybrid towhich is normally attached an RF absorbingload. Therefore, some means of instituting a180° phase shift to these unwanted signals mustbe accomplished. Future paragraphs will bededicated to this situation.

A

D

C

B

difference port

sum port

180° 'Magic Tee' Folded Hybrid

Waveguide Hybrid ConstructionFigure 2a

1

2

3

4

90° Riblet 'Short Slot' Hybrid

Hybrid Operation

Figure 2a below shows the typical construc-tions for the hybrids used in the switchlesscombiner. Figure 2b shows the phasing proper-ties of each of the waveguide hybrids for differ-ent situations that occur during operation of theswitchless combiner.

The Short Slot hybrid is a quadrature hybridin that an input signal at any port will be dividedbetween the two opposing ports equally inmagnitude but with a relative phase difference of


Phasing - Waveguide Hybrid OperationFigure 2b

Figures 3 a,b & c show RF schematics of thethree major modes of a Switchless Combinerwhere Figure 3a illustrates the parallel transmittermode (Both TxA and TxB combined to antenna),Figure 3b shows the TxA to antenna mode andFigure 3c shows the TxB to antenna mode.

Refer to Figure 3b. Signal A gets split by theinput hybrid such that the signal opposite the inputfor signal B receives a relative delay of 90° ascompared to the exiting signal opposite the inputfor signal A. The phase shifter opposite the signalA input then brings both half signals back into

phase by delaying the signal opposite input A by90° so that the half signals will combine at thesum port of the hybrid tee to which is attachedthe antenna feedline.

For signal B the phase difference of the twohalf signals will be 180° since the phase shifteropposite the signal A input will give the [90°shifted] half signal B an additional 90° shift tothat already imposed by the input hybrid. There-fore the full power of TxB will be directed to thedifference port to which is typically attached areject load.

A

BΦ=0°

Φ=-90° A+B

no signal

to load

to antennaA

B Φ=0°

Φ=-180°

A+B

no signal

A

BΦ=0°

Φ=0° A/2+B/2

A/2+B/2to load

to antennaA

B Φ=0°

Φ=0°

A+B

no signal

180° MAGIC TEE90° RIBLET HYBRID

Switchless Combiner Phasing


B to Antenna ModeFigure 3c

INPUTB

INPUTA

A+B

phaseshifters

H1 H2

0°

to load

0°P1

P2

Figure 3aA+B to Antenna Mode

INPUTB

INPUTA

B

phaseshifters

H1 H2

-90°

to load

0°P1

P2

A

INPUTB

INPUTA

A

phaseshifters

H1 H2

0°

to load

-90°P1

P2

B

Figure 3bA to Antenna Mode

Figure 4a-b below illustrate how theswitchless combiner can be configured to elimi-nate the phenomenon of ghosting which occurswhen a portion of the output signal reflects at theantenna (labelled 'R' in Figure 4a) and headsback toward the RF system. Since there isnothing to impede the progress of this reflectedsignal, it is directed right back toward the inputhybrid where the two in-phase half-signals areeach divided and added to one another in afashion as shown in the top left corner of Figure2b.

These signals then are reflected at thetransmitters and are directed through the systemin exactly the same fashion (see Figure 4a) as theoriginal signal except that the new signal is timedelayed with reference to the original signalultimately resulting in a ghost image at thereceiving end.

The key, then, to ghost signal elimination(ghost cancellation) is the implementation of the90° section between TxA and the input hybrid asshown in Figure 4b so that these reflected signalspossess a relative phase difference of 180° suchthat the signals will combine through the magictee at the difference port and will summarily beabsorbed in the attached reject load.

Note that, in order to institute the extra 90°section, the transmitters must be fed in quadra-ture so that the main input signals A & B are inphase when they are directed into the inputhybrid.

Ghost Signals and Cancellation

Similar considerations pertain to the othertwo modes of operation where either A is di-rected to the load and B to the antenna or both Aand B are directed to the antenna.


output

phaseshifters

H1 H2

0°

to load

0°

P1

P2

Tx1

Tx2[Φ = -90°]

-90°

[Φ = 0°]

[Tx's are fed in quadrature]

The 90° section between the first hybrid (H1) and Tx1 allows for reflections that originate at the antenna and travel back through the system to reflect again off the transmitters

(thus creating a ghost signal) to have a relative phase difference of 180° and to be subsequently directed to the load through the magic tee (H2).

E1

E2

to antenna

Origination of Ghost SignalsFigure 4a

Cancellation of Ghost SignalsFigure 4b

outputphaseshifters

H1

H2

to load

Tx1

Tx2

to antenna

reflectionat antenna

Reflected signal at the antenna will be split by H2 and directed to the input hybrid. Since the signals are in phase, they will be directed to the transmitters

which will reflect both half signals back toward the input hybrid. These signals will then be directed through the system to the antenna along with the

main signals but will be time delayed with reference to the main signals A & B.

'R'

R1

R2R1/2 + R2/2

R1/2 + R2/2


In typical television applications the powerreflected at the antenna system is usually notmore than about 1-2% of the transmitter power.(which corresponds to a VSWR at the antenna ofabout 1.2-1.3:1, worst case). So for a combinedpower of 120kW (peak of sync), the averagesignal level of a ghosting signal would amount tonear 1kW of power. Depending on other param-eters for the RF system, such as antenna HAATand proximity considerations, this level could besignificant.

The TV Switchless Combining system, aspart of a larger RF system designed for parallelamplification of equal frequency transmitters,can be configured to successfully implementghost signal cancellation by feeding the transmit-ters in phase quadrature.

The construction of typical switchlesscombiner systems has been in waveguide [witheither a 180° output magic tee or a 90° outputhybrid] or in coaxial form. High power UHFsystems dictate the use of waveguide.

Concluding Remarks

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