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利用集成CMOS RF前端GPS接收機MAX2742，附加極少的外部元件，即可構成一個完整的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片內集成了低噪聲放大器(LNA)、混頻器、BPF、自動增益控制放大器(AGC)、本振合成器、時鐘緩沖器和內部數字采樣器。

該器件能夠與許多商用GPS基帶IC 接口，適合多種應用，其中包括：汽車導航、遠程信息處理、自動安全監控、資產跟蹤、定位服務(LBS)及其它消費類電子產品。用MAX2742構成一個完整的GPS RF解決方案只需少量的外部元件。以下給出了系統框圖。

MAX2742工作于18.414MHz*晶體或TXCO，提供差分或單端** IF輸出(1.023MHz)?？偟?a target="_blank">信號變換增益為120dB，噪聲系數4.5dB。IF信號以18.414MHz的參考時鐘頻率進行采樣。

完備的GPS RF前端方案

圖1給示了Maxim完整的GPS RF方案的重要結構框圖和特點。表1給出了框圖說明，表2給出了級聯特性。圖2所示電路是用MAX2742構成的GPS接收機的RF電路。

圖1. GPS RF前端框圖

表1. RF部分系統框圖

Block	Description	Note
GPS antenna	Ac tive 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. 系統級聯特性

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構成的GPS RF部分原理圖

完備的GPS接收機方案

構建一個完整的GPS接收機，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、復位電路、延時電路等都是必需的。表3給出了框圖說明；表4給出了級聯特性。

圖3. 完整的GPS接收機框圖

表3. 系統各部分電路

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

給出了參考設計的實際PCB布線圖。這個完整的解決方案僅占用25mm x 25mm的PCB空間。

圖4. GPS接收機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	Sensitivity	Sens			-138	-138	dBm
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