BGA430 datasheet(4/24 Pages) INFINEON

4 / 24 page

AN 074 Rev E

4 / 24

19-November-2002

Applications Note No. 074

Silicon Discretes

After the waveguide to microstrip transition, the

signals enter a PC Board assembly. The signal

is amplified in two or more low noise amplifier

(LNA) stages and then hits a band pass filter.

The LNAs provide enough gain to boost the

level of the received signal such that the overall

receiver noise figure is dominated by the LNA

block itself. The LNA stages must have enough

gain and a sufficiently low noise figure to

minimize the noise floor for the entire receive

chain. Achieving enough gain and a low enough

noise figure at 12 GHz is costly, and anything

that can reasonably be done to relax the

requirements on the LNA section will reduce

cost.

The LNA is then followed by a band pass filter

(BPF) which provides for some rejection of out –

of-band signals and noise, as well as image

rejection. The amplified and filtered signal then

enters the mixer stage.

The types of simple, inexpensive mixers likely to

be used in an LNB will usually convert both the

desired input signal (12 GHz in this case) and an

undesired “image” frequency (10 GHz) to the

intermediate frequency (1 GHz for this example).

The band pass filter in front of the mixer stage

can attenuate any undesired signals or noise

present at the 10 GHz image frequency before it

hits the mixer stage, preventing the undesired

image from being down-converted on top of the

desired, down-converted 12 GHz input signal.

At present, most LNB manufacturers use one of

three main types of mixers:

1. GaAs FET used as a simple active mixer

2. GaAs FET with no DC bias applied ( “FET

resistive mixer”)

3. Schottky Diode based mixer

Some references for mixers are given in [1] and

[2] at the end of this applications note.

The FET active mixer will usually have

“conversion gain” while the FET resistive mixer

or Schottky diode mixers have “conversion loss”.

Conversion gain or loss is simply the ratio of the

amplitudes of the down-converted output I.F.

signal to the RF input signal. A poor noise figure

in the mixer stage, as well as high conversion

loss, places additional demands (and cost) on

both the LNA block up front, as well as the I.F.

amplifier which follows.

The down-converted I.F. signal undergoes

further band pass filtering and then is amplified

in the I.F. amplifier block. The I.F. amplifier is

the focus of this applications note, and is the

primary point of discussion regarding the

BGA430 and BGB540 Silicon MMICs. The I.F.

amplifier boosts the signal up to a reasonable

input level for the set top box. It is worth noting

that the system impedance in this area is 75

ohms, not 50 ohms, and that the coaxial cable

typically used (RG-6, RG-6/U or sometimes RG-

59) is very low cost, and has a relatively high

attenuation per unit length a the intermediate

frequency. Furthermore, the attenuation of the

cable increases with increasing frequency –

coaxial cable loss at 2150 MHz is higher than

cable loss at 950 MHz. Herein lies the reason

for designing an I.F. amplifier with a gain

slope

that

increases

with

increasing

frequency – this positive gain slope in the

I.F. amp will help to compensate out the

negative gain slope of the coaxial cable and

other RF front-end blocks.

4. Information on Printed Circuit Board

The PC board used in this applications note was

simulated within and generated from the

Eagleware GENESYS® [3] software package.

After simulations, CAD files required for PCB

fabrication, including Gerber 274X and Drill files,

were created within and output from GENESYS.

Photos of the PC board are provided in Figures

4, 5 and 6. A cross-sectional diagram is given

in Figure 7. A schematic diagram and a Bill Of

Material (BOM) for the complete BGA430 +

BGB540 I.F. Amplifier are given in Figures 8

and 9, respectively.

The PC Board material

used is standard FR4. Note that each MMIC

may be tested individually; capacitor C3 (see

schematic) may be positioned to “steer” the RF

from the BGA430 output to the SMA connector

on the bottom of the PCB, or, C3 may be used

to link the track from this same RF connector to

the input of the BGB540.

When testing the

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