Abstract: This reference design is a complete RF front-end solution for a GPS receiver using Maxim's MAX2769 GPS receiver chip. The MAX2769 is a low cost, single conversion, low IF GPS receiver that offers two integrated LNAs with different specifications, I/Q channel filters with variable bandwidth and order, and a digital IF output that supports 1-bit and 2-bit operation. This reference design offers high performance, small size, and low BOM cost.
Figure 1. GPS reference design features the MAX2769.
Important Design Features
Dual-Input Uncommitted LNA for Separate Passive and Active Antenna Inputs
Integrated Active Antenna Sensor Which Can Autoswitch the Antenna Input
The I and Q Channel-Select Filters Can Be Selected as Either Third or Fifth Order
Provides 110dB Cascaded Gain and a 1.5dB Cascaded NF in LNA1 Mode
The IF Output Is Adjustable in 63 Steps Between 0MHz and 12.5MHz
Can Be Used in Preconfigured States that Do not Require Programming Through the 3-wire SPI™ Interface
Figure 2. Block diagram for the GPS reference design.
Lab Measurements
Supply Current Summary
VCC = +3.0V, TA = +25°C
Parameter
Test Conditions
Meas.
Unit
Operating Supply Current
Using LNA1 in default and low-current modes
18
mA
Receive Summary
Operating conditions: VCC = 3V; TA = +25°C, Operating in default mode, PCB trace losses are included.
Parameter
Test Condition
Meas.
Unit
LNA1 Power Gain
Default and low-current modes
18.6
dB
LNA1 Noise Figure
Default and low-current modes
1.05
dB
LNA1 Third-Order Input Intercept Point
Default and low-current modes; LNA input to output. P1 = P2 = -30dBm; f1 = 1575MHz, f2 = 1576MHz
-1
dBm
LNA2 Power Gain
Default mode
12.9
dB
LNA2 Noise Figure
Default mode
1.35
dB
LNA2 Third-Order Input Intercept Point
Default mode
-2
dBm
Overall System NF
Using LNA1; PGA gain is maximum (CONF3[27:22] is 111111)
1.6
dB
Using LNA2; PGA gain is maximum (CONF3[27:22] is 111111)
3.1
IF Out Freq/Gain Range Through System Using LNA1
1575.42MHz in, 4.092MHz out; analog output mode
59 -117
dB
4MHz Offset Rejection
1579.42MHz in, third-order BPF; PGA gain is maximum (CONF3[27:22] is 111111)
32
dB
1579.42MHz in, fifth-order BPF; PGA gain is maximum (CONF3[27:22] is 111111)
51
IF Passband 3dB Bandwidth
IF filter center bandwidth 2.6MHz; set bit 4:3 of CONF1 register to 00
2.9
MHz
Operating Characteristics
Figure 3. Input S11 of the MAX2769 LNA1. Measured by network analyzer at 1575.42MHz. The S11 is -10.4dB.
Detailed Description
This reference design is a low-cost, single-conversion, low-IF GPS receiver. The design offers more flexibility and performance than its predecessors for a wide range of GPS applications such as mobile handsets, PDA's, embedded PCs, and automotive applications.
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