APPLICATION NOTE 3447

Complete Stand-Alone GPS Receiver Solution with MAX2742


Abstract: A complete GPS solution is achieved with minimal external components, using the MAX2742 integrated CMOS RF front-end GPS receiver.

Introduction

The MAX2742 is a CMOS, single-chip, GPS front-end downconverter. This state-of-the-art device consumes very low power (32mW at 2.4V), and eliminates the need for a costly IF SAW filter or bulky discrete IF bandpass filters. The MAX2742 integrates a low-noise amplifier (LNA), mixer, BPF, automatic-gain-control amplifier (AGC), local oscillator synthesizer, clock buffer, and internal digital sampler.

This device can interface with many commercially available GPS baseband ICs. It is suitable for many applications, including vehicle navigation, telematics, automatic security, asset tracking, location-based service (LBS), and consumer electronics. The external components required around the device are minimal for a complete GPS RF solution. The system block diagram can be found in the following section.

The MAX2742 works with an 18.414MHz* crystal or TXCO, and offers differential or single-ended** IF output at 1.023MHz. Total signal conversion gain is 120dB, with a noise figure of 4.5dB. The IF signal is sampled at the reference clock rate of 18.414MHz.

For more details, please refer to the MAX2742 data sheet.

* Note: 18.414MHz is 18 times 1.023MHz, which is one-tenth of the GPS fundamental frequency 10.23MHz.
** Note: There are three IF output pins—one pair of differential outputs and one single-ended output. Selection is controlled by pin 10.


Complete GPS RF Front-End Solution

Figure 1 shows the important building blocks and features of Maxim's complete GPS RF solution. Description of the building blocks can be found in Table 1; cascaded performances are shown in Table 2. Figure 2 shows the application circuit of the GPS receiver RF portion with the MAX2742.

Figure 1. GPS RF front-end block diagram.
Figure 1. GPS RF front-end block diagram.

Table 1. RF Portion System Building Block
Block Description Note
GPS antenna Active GPS antenna, commonly has ~1.5dB NF and ~20dB gain  
LNA MAX2641/MAX2654/MAX2655 LNA External LNA is required when active GPS antenna may not be present.
RF SAW RF bandpass filter centered at 1575.42MHz RF BPF is required for jammer immunity
Power management MAX8510 low-noise LDO  
RF downconverter MAX2742  
Reference clock 18.414MHz crystal/TCXO  
DSP Baseband processor  

Table 2. System Cascaded Performance
Specification Cascaded Performance Note
Gain 131dB Without active antenna, assuming 15dB external LNA gain, 3dB BPF loss, and 1dB matching network loss
Noise Figure 1.9dB Without active antenna, assuming 1.5dB external LNA NF
Power Consumption 20mA MAX2744+MAX2654 as an example

Figure 2. GPS RF portion schematic for MAX2742.
For Larger Image

Figure 2. GPS RF portion schematic for MAX2742.

Complete Stand-Alone GPS Receiver Solution

There are more building blocks to construct a complete GPS receiver, other than just the RF plus baseband IC. As shown in Figure 3, an extra antenna supply-control block, a RESET circuit, and a delay circuit are needed. Description of the building blocks can be found in Table 3; cascaded performances are shown in Table 4.

Figure 3. Complete GPS receiver solution block diagram.
Figure 3. Complete GPS receiver solution block diagram.

Table 3. System Building Block
Block Description Note
GPS Antenna GPS antenna Refer to the RF portion for details.
LNA MAX2654 LNA Refer to the RF portion for details.
RF SAW RF bandpass Refer to the RF portion for details.
Power management MAX8510 low-noise LDO  
RF downconverter MAX2742  
Reference clock 18.414MHz TCXO The TCXO also supplies the baseband clock. In this reference design, GPS clock output from the MAX2742 is not used.
GPS baseband Sony CXD2932 The CXD2932 also controls active antenna supply, RESET function. See details below.
Antenna control block Antenna supply control with p-MOSFET and current-sense resistor The sense resistor feeds the antenna supply information back to CXD2932. There are three possible conditions: normal, short, and open.
RESET/delay circuitry Different levels of RESET, and delay circuit required by CXD2932 To ensure successful startup, CXD2932 requires two RESET signals 100ms apart.
Baseband interface TX: external control command  
RX: GPS data out Data-rate programmable
1PPS: one pulse-per-second signal This signal is aligned with GPS atomic clock.

Figure 4 shows the real PCB layout of the reference design. The complete solution only occupies 25mm x 25mm of PC board space.



Figure 4. GPS Receiver PCB layout.
For Larger Image

Figure 4. GPS Receiver PCB layout.

Table 4. Module Performance (LNA + MAX2742 + SONY CXD2932)
No. Parameter Symbol Conditions System Specifications Maxim Target Measured Units
1 Power supply VCC (Note 1) 2.7 ~ 3.6   T.C. V
2 Current consumption ICC VCC = 3.0V   91 88 mA
3.1 Sensitivity Sens Acquisition Tracking -145 -145 dBm
3.2     -138 -138
4 TTFF TFC Cold start   58 (typ) 60 (max) s
5 TFW Warm start   45 30 (max) s
6 TFH Hot start   17 6 (max) s
7 Position drift range PR 95% possibility   100 90 ft
8 1PPS output accuracy 1PPS     1 N.M. ns

Note 1: T.C. = test condition, N.M. = not measured

Related Parts
MAX2641 300MHz to 2500MHz SiGe Ultra-Low-Noise Amplifiers Free Samples  
MAX2654 1575MHz/1900MHz Variable-IP3 Low-Noise Amplifiers Free Samples  
MAX2655 1575MHz/1900MHz Variable-IP3 Low-Noise Amplifiers Free Samples  
MAX2742 Single-Chip Global Positioning System Receiver Front-End  
MAX8510 Ultra-Low-Noise, High PSRR, Low-Dropout, 120mA Linear Regulators Free Samples  


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© Feb 21, 2005, Maxim Integrated Products, Inc.
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APP 3447: Feb 21, 2005
APPLICATION NOTE 3447, AN3447, AN 3447, APP3447, Appnote3447, Appnote 3447