BGA430 datasheet(10/24 Pages) INFINEON

10 / 24 page

AN 074 Rev E

10 / 24

19-November-2002

Applications Note No. 074

Silicon Discretes

Figure 15. Stand-Alone BGA430 Amp, Output Return Loss, Smith Chart.

Reference Plane = PCB Output SMA Connector.

CH1

S 22

1 U FS

START 900.000 000 MHz

STOP 2 200.000 000 MHz

Cor

Avg

Smo

Del

PRm

31 Jul 2002 00:57:34

2_: 58.355

-37.225

2.9486 pF

1 450.000 000 MHz

1_: 38.189

14.771

950 MHz

3_: 25.271

-1.2881

2.15 GHz

Why a Positive Gain Slope LNB I.F.

Amplifier?

The question arises: why would one want an

I.F. amplifier with a positive gain slope? The

reader is referred to Figures 16 – 20 on pages

11 – 13. Two points are worth noting:

The return loss of this 75 ohm cable

appears to be quite reasonable in a

50 ohm measurement system (partly

due to the cable’s insertion loss

characteristic)

II.

The insertion loss of the 15.2 meter

length

cable

increases

with

increasing frequency. (Figures 18 &

20.)

A 15 meter length of coaxial cable has a

negative gain slope of approximately 2 dB

across the 950 – 2150 MHz range.

additional 15 meter section of lower-cost coax

cable (RG-6) was attached via a “barrel”

connector to the first section, making a 30.4

meter cable run, which was then also tested.

For a 30.4 meter / 100 foot length of cable, there

is a delta of approximately 4 dB in insertion loss

as we move from 950 to 2150 MHz.

If an LNB I.F. Amplifier were made with a

positive gain slope, the net gain across the

I.F. bandwidth could be ‘flattened out’

somewhat, and the range of input power that

the set-top box demodulators need to work

over could be reduced.

Furthermore, this

description of coaxial cable negative gain

slope only considers gain decrease with

increasing frequency or “gain roll-off” in the

cable itself. In all likelihood, the LNB’s 12

GHz LNA block and mixer stage will have a

gain roll-off of their own, further worsening

the overall net negative gain slope of the

entire LNB and coax cable assembly taken

together.

Table 3 on page 13 summarizes coaxial cable

insertion loss and return loss for both 15.2 meter

and 30.4 meter cable runs. The next section

describes the complete BGA430 + BGB540 LNB

I.F. Amp with positive gain slope.

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