AN 074 Rev E

3 / 24

19-November-2002

Applications Note No. 074

Silicon Discretes

of the composite signal – a vertically polarized

component,

and

a

horizontally

polarized

component. The two polarizations enable more

efficient use of the available bandwidth and

power.

(The isolation between “vertical” and

“horizontal” radio signals permits a greater

number of channels to be simultaneously

broadcast within the available bandwidth than

would otherwise be expected.) The satellite’s

transmitted signal is received by an earth-based

antenna like that shown in the photo on page 1.

The signal is focused by the parabolic “dish”

antenna onto a waveguide integrated into the

LNB.

The received signal travels a short

distance down the waveguide until reaching a

waveguide-to-microstripline

transition

that

carries the signal onto the LNB circuit board

assembly.

The LNB must be able to receive channels on

both the vertically and horizontally polarized

signals – and one way to do this is to have

essentially two different receiver front-ends as

shown in Figure 3 below.

Various approaches and switching schemes are

employed in different LNB designs to enable the

end user(s) to select between channels riding on

either the horizontally or vertically polarized

signals. Each approach has its own unique cost

and performance trade-offs.

Optimizing LNB

architectures

to

achieve

performance

requirements while continuously reducing cost

as new, higher-performance and lower cost

semiconductor devices become available is a

challenging task.

A

further

complication

to

the

switching

requirements is added if one wishes to have a

“dual output LNB” – e.g. an LNB that can drive

two different set-top boxes and television sets

simultaneously, allowing each TV to display a

different channel. (“Quad output” LNBs are also

available). One possible switching scheme for a

“single output LNB” is shown in Figure 3. Note

that the vertical / horizontal switching is done at

the I.F. Amplifier block.

The BGA430 and

BGB540 are shown in the shaded I.F. Amplifier

section.

Figure 3. Generic Block Diagram, "Single Output" Direct

Broadcast Satellite Television Block Downconverter (DBS LNB)

LNA

LNA

LNA

LNA

Horizontal

Polarization

Vertical

Polarization

BPF

BPF

Switch

BPF

Mixer

(Passive or Active)

Intermediate Frequency ("I.F.")

950 - 1450 MHz (North America)

950 - 2150 MHz (Europe, Asia)

IF Amp

IF Amp

75 ohm Coaxial Cable

To Set-Top Box

(75 ohm system)

RG-6, RG-6/U

BGA430 + BGB540

I.F. Amplifier

"Set Top Box"

(Channel Selector,

Demodulator, etc.)

Audio + Video

to TV set

LNB

(Components in Shaded Area)

Mixer

(Passive or Active)

4

Approximate Gain, dB =>

Approximate Noise Figure, dB =>

10

0.5

12

0.7

-1.5

1.5

-10 to +8

7 to 12

- 4

30 to 45

3 to 7

U.S., Japan, Korea,

Latin America

11.45 - 12.75 GHz

(depending on region)

Europe

10.7 - 12.75 GHz

(Two L.O.'s required for

full coverage)

U.S., Japan, Korea,

Latin America:

in 10.5 - 11.25 GHz range

Europe:

10.6 GHz or

9.75 + 10.6 GHz

(dual DRO)

Local Oscillator

(DRO)

Cascaded Gain: approximately 55 - 60 dB; Cascaded Noise Figure: approximately 1 dB