概述
MAX2742是CMOS、单芯片、GPS前端下变频器。这款性能优异的器件只需消耗极低的功率(32mW,2.4V),并且不需要昂贵的IF SAW滤波器和体积庞大的分立IF SAW滤波器。MAX2742片内集成了
低噪声放大器(LNA)、混频器、BPF、自动增益控制放大器(AGC)、本振合成器、时钟缓冲器和内部数字采样器。
该器件能够与许多商用GPS基带IC接口,适合多种应用,其中包括:汽车导航、远程信息处理、自动安全监控、资产跟踪、定位服务(LBS)及其它消费类电子产品。用MAX2742构成一个完整的GPS RF解决方案只需少量的外部元件。以下给出了系统框图。
MAX2742工作于18.414MHz*晶体或TXCO,提供差分或单端** IF输出(1.023MHz)。总的信号变换增益为120dB,噪声系数4.5dB。IF信号以18.414MHz的参考时钟频率进行采样。
详细信息请参考
MAX2742数据资料。
* 注:18.414MHz是1.023MHz的18倍,1.023MHz是GPS基频10.23MHz的十分之一。
** 注:有三个IF输出引脚—一对儿差分输出和一个单端输出,由第10引脚控制选择。
完备的GPS RF前端方案
图1给示了Maxim完整的GPS RF方案的重要结构框图和特点。
表1给出了框图说明,
表2给出了级联特性。
图2所示电路是用MAX2742构成的GPS接收机的RF电路。
图1. GPS RF前端框图
表1. RF部分系统框图
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 |
|
表2. 系统级联特性
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 |

放大图形图2. 用MAX2742构成的GPS RF部分原理图
完备的GPS接收机方案
构建一个完整的GPS接收机,除了RF和基带IC外,还需要其它电路。如图3所示,天线电源控制电路、复位电路、延时电路等都是必需的。
表3给出了框图说明;
表4给出了级联特性。
图3. 完整的GPS接收机框图
表3. 系统各部分电路
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. |
图4给出了参考设计的实际PCB布线图。这个完整的解决方案仅占用25mm x 25mm的PCB空间。

放大图形
图4. GPS接收机PCB布线
表4. 模块性能(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 |
注1:T.C. = 测试条件,N.M. = 没有测试。
© , Maxim Integrated Products, Inc.
|
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APP 3447:
应用笔记
3447,
AN3447,
AN 3447,
APP3447,
Appnote3447,
Appnote 3447
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