用MAX2742構(gòu)建GPS接收機(jī)
利用集成CMOS RF前端GPS接收機(jī)MAX2742，附加極少的外部元件，即可構(gòu)成一個(gè)完整的GPS解決方案。
概述
MAX2742是CMOS、單芯片、GPS前端下變頻器。這款性能優(yōu)異的器件只需消耗極低的功率(32mW，2.4V)，并且不需要昂貴的IF SAW濾波器和體積龐大的分立IF SAW濾波器。MAX2742片內(nèi)集成了低噪聲放大器(LNA)、混頻器、BPF、自動(dòng)增益控制放大器(AGC)、本振合成器、時(shí)鐘緩沖器和內(nèi)部數(shù)字采樣器。
該器件能夠與許多商用GPS基帶IC接口，適合多種應(yīng)用，其中包括：汽車導(dǎo)航、遠(yuǎn)程信息處理、自動(dòng)安全監(jiān)控、資產(chǎn)跟蹤、定位服務(wù)(LBS)及其它消費(fèi)類電子產(chǎn)品。用MAX2742構(gòu)成一個(gè)完整的GPS RF解決方案只需少量的外部元件。以下給出了系統(tǒng)框圖。
MAX2742工作于18.414MHz*晶體或TXCO，提供差分或單端** IF輸出(1.023MHz)。總的信號變換增益為120dB，噪聲系數(shù)4.5dB。IF信號以18.414MHz的參考時(shí)鐘頻率進(jìn)行采樣。
完備的GPS RF前端方案
圖1給示了Maxim完整的GPS RF方案的重要結(jié)構(gòu)框圖和特點(diǎn)。表1給出了框圖說明，表2給出了級聯(lián)特性。圖2所示電路是用MAX2742構(gòu)成的GPS接收機(jī)的RF電路。
圖1. GPS RF前端框圖
表1. RF部分系統(tǒng)框圖
| 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. 系統(tǒng)級聯(lián)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 |
圖2. 用MAX2742構(gòu)成的GPS RF部分原理圖
完備的GPS接收機(jī)方案
構(gòu)建一個(gè)完整的GPS接收機(jī)，除了RF和基帶IC外，還需要其它電路。如圖3所示，天線電源控制電路、復(fù)位電路、延時(shí)電路等都是必需的。表3給出了框圖說明；表4給出了級聯(lián)特性。
圖3. 完整的GPS接收機(jī)框圖
表3. 系統(tǒng)各部分電路
| 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. |
圖4
給出了參考設(shè)計(jì)的實(shí)際PCB布線圖。這個(gè)完整的解決方案僅占用25mm x 25mm的PCB空間。
圖4. GPS接收機(jī)PCB布線
表4. 模塊性能(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. = 測試條件，N.M. = 沒有測試。
審核編輯：郭婷
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