MAX2741
Integrated L1-Band GPS Receiver
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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 29Ω + 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
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.
