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參數(shù)資料
| 型號： | LMX2604LQ |
| 廠商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分類： | XO, clock |
| 英文描述： | Triple-band VCO for GSM900/DCS1800/PCS1900 |
| 中文描述： | VCO, 880 MHz - 915 MHz |
| 封裝： | LLP-20 |
| 文件頁數(shù)： | 11/14頁 |
| 文件大?。?/td> | 268K |
| 代理商： | LMX2604LQ |
Functional Description
PRODUCT DESCRIPTION
The LMX2604 IC has two VCOs, which are configured as an
LC resonant oscillator. The active components and tank
elements are all integrated on the IC. The oscillator core and
the tank circuit are designed to be immune to external noise
such as supply and load variation. The IC is easy-to-use and
occupies extremely small area in the board.
Two output amplifiers are also integrated to deliver high
output power of +6 dBm in GSM/DCS/PCS applications. The
amplifiers isolate the oscillator cores from the external load,
and drive the external 50 ohm load. The output driver is
designed to have low noise floor and to reduce pulling by
load variation.An additional amplifier to drive the mixer in the
transmit PLL is also provided. Only an attenuator composed
of a few passive elements is necessary to meet the power
level of the mixer input of the transmit PLL.
The frequency of the oscillator is controlled by the CPin pin,
which is internally connected to the varactor. This control pin
is connected to the loop filter of the modulation loop in the
transmit path. Any additional noise on this tuning input is
directly translated into FM noise, which can degrade the
phase noise characteristics of the oscillator. Typically, the
loop filter of the PLL provides an appropriately low imped-
ance source at its output and thus proper additional filtering
stage is often required to reduce the high frequency noise
and spurious signals.
FREQUENCY SELECTION AND POWER DOWN
CONTROL
The BSW pin selects the operating frequency band. The
table below shows the appropriate settings with the CE pin.
CE
Low
Low
High
High
BSW
Low
High
Low
High
Output
X
X
Low Band (GSM)
High Band (DCS/PCS)
In real implementation, the component values of the tank
circuit vary from the nominal value. The IC has built-in circuit
to track the variation and compensate that kind of the varia-
tion during normal operation. This self-correction algorithm
does not require any external control signal and elements.
The figure below shows an application circuit of the
LMX2604. The LMX2604 is developed for the use in a GSM
handset as part of the transmitter PLL. To complete the
offset PLL in the transmit path, several passive elements for
the external loop filter should be also provided. The charge
pump output port of the external transceiver IC is fed to the
CPin pin.
The locking process of the LMX2604 is composed of two
stages. First, the internal control circuitry of the LMX2604
controls the fast frequency acquisition behavior of the
LMX2604. After the frequency acquisition, the whole loop
enters in its normal operating mode.
Application Circuit of LMX2604
20066172
L
www.national.com
11
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