Integrated L1-Band GPS Receiver

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9

Applications Information

Fundamentally, the only application areas that require

careful consideration are the LNA input match and the

1st IF filter. Of course, proper supply bypassing,

grounding, and layout is required for reliable perfor-

mance from any RF circuit.

LNA Input Matching

Input matching is critical for optimum noise figure and

system sensitivity. Optimum source impedance (as seen

from the LNA input) for lowest noise figure is 29

Ω + j47Ω.

Remember that optimum noise match and optimum gain

match (return loss) do not occur simultaneously, so a

good application circuit will sacrifice gain slightly in favor

of reduced noise figure. Gain and noise circles are pro-

vided in Figure 3; S11 tabular data is provided in Table 8.

1st IF Interface and Filtering

The typical application uses a 37.38MHz 1st IF, and

employs an IF filter. The order of the filter should be tai-

lored to suit the application—stand-alone GPS

receivers will not require the channel-selection and

stopband attenuation of GPS receivers that are inte-

grated into other wireless handsets. Be sure that the fil-

ter topology provides DC-blocking for the IF I/O ports.

Table 7. Detailed Register Definition for Read Address 0111: STATUS

DATA BIT

DEFAULT

DESCRIPTION

D15–D10

XXXXXX

Reserved

D9

X

Out-of-Lock (High Frequency): 1 = PLL is out of lock, VCO free-running at its highest frequency,

0 = Locked.

D8

X

Out-of-Lock (Low Frequency): 1 = PLL is out of lock, VCO free-running at its lowest frequency,

0 = Locked high.

D7

1

XTAL Clock Selected: 1 = Synthesized 16.8MHz reference, 0 = External clock.

D6

X

Parity: 1 = Even, 0 = Odd.

D5

X

Reserved

D4

X

LPF Autotune End: 0 = Autotune run ended; 1 = Calibrating or manual tuning.

D3–D0

XXXX

LPF Autotune: 0000 ~ 7.7MHz; 1111 = 2.9MHz.

Table 8. MAX2741 S11

G = Gmax - 1dB

NF = NFmin + 0.2dB

Figure 3. Gain and Noise Circles for MAX2741 LNA Input

FREQ (MHz)

S11 (MAG)

S11 (°)

1100

0.874

-50.9

1200

0.867

-56.1

1300

0.859

-61.5

1400

0.842

-66.9

1500

0.821

-72.3

1550

0.809

-74.9

1560

0.806

-75.4

1570

0.804

-75.9

1575

0.803

-76.2

1580

0.801

-76.4

1590

0.799

-77.0

1600

0.796

-77.5

1650

0.783

-80.0

1700

0.768

-82.5

1800

0.739

-87.4

1900

0.708

-92.3

2000

0.677

-96.8