LMX9814 National Semiconductor, LMX9814 Datasheet

LMX9814

Manufacturer Part Number

LMX9814

Description

Manufacturer

National Semiconductor

Datasheet

1.LMX9814.pdf (4 pages)

Specifications of LMX9814

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LMX9814, LMX9820, and

LMX5251: Crystal Tuning

Guide

1.0

The RF local oscillator and internal digital clocks for the

National

LMX9820 Serial Port module, and LMX5251 CMOS Radio

chip are derived from the reference clock at the CLK+

input. This reference may either come from an external

clock or a dedicated crystal oscillator. The crystal oscillator

connections require an Xtal and two grounded capacitors.

This docment describes how to set up the crystal driver cir-

cuit.

2.0

The LMX9814, LMX9820, and LMX5251 each contain a

crystal driver circuit. This circuit operates with an external

crystal and capacitors to form an oscillator. (See Figure 2-

1.)

The crystal appears inductive near its resonant frequency.

It forms a resonant circuit with its load capacitors. The res-

onant frequency may be trimmed with the crystal load

capacitance.

2.1

For resonance at the correct frequency, the crystal should

be loaded with its specified load capacitance, which is the

value of capacitance used in conjunction with the crystal

unit. Load capacitance is a parameter specified by the

crystal, typically expressed in pF. The crystal circuit shown

in Figure 2-2 is composed of:

• C1 (motional capacitance)

• R1 (motional resistance)

• L1 (motional inductance)

• C0 (static or shunt capacitance)

The LMX9814, LMX9820, and LMX5251 provide some of

the load with internal capacitors C

come from the external capacitors labeled Ct1 and Ct2 as

shown in Figure 2-1. Ct1 and Ct2 should have the same the

value for best noise performance. Crystal load capacitance

The C

capacitance C0 as shown in Figure 2-2, so the total capac-

itance is:

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total

= C

is calculated as the following:

LOAD CAPACITANCE

int

Scope

Xtal Mode

= C

above does not include the crystal internal self-

Semiconductor

+ Ct1//Ct2

+ C0

LMX9814

int

. The remainder must

WPAN

module,

National Semiconductor

Application Note, Confidential

July 2003

Revision 1.0

3.0

Pullability is another important parameter for a crystal,

which is the change in frequency of a crystal with units of

ppm/pF, either from the natural resonant frequency to a

load resonant frequency, or from one load resonant fre-

quency to another. The frequency can be pulled in a paral-

lel resonant circuit by changing the value of load

capacitance. A decrease in load capacitance causes an

increase in frequency, and an increase in load capacitance

causes a decrease in frequency.

Figure 2-1. LMX5251 Crystal Recommended

Figure 2-2. Crystal Equivalent Circuit

Clk+

Crystal Pullability

Ct1

Crystal

Circuit

Clk-

www.national.com

Ct2

Related parts for LMX9814

LMX9814 Summary of contents

Page 1

... C1 (motional capacitance) • R1 (motional resistance) • L1 (motional inductance) • C0 (static or shunt capacitance) The LMX9814, LMX9820, and LMX5251 provide some of the load with internal capacitors C int come from the external capacitors labeled Ct1 and Ct2 as shown in Figure 2-1. Ct1 and Ct2 should have the same the value for best noise performance ...

Page 2

... So a crystal with an aging and temperature drift specifica- tion of better than ±20 ppm is required. Here is an example: The VXE4-1055 MHz SMT crystal from Vite. National is using this crystal with the LMX5251, LMX9814, and LMX9820. Table 4-1 shows the specification of VXE4- 1055. ...

Page 3

Figure 4-2. Frequency Offset with 9 pF//10 pF Capacitors Revision 1.0 (Confidential) 3 www.national.com ...

Page 4

... NATIONAL’S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT AND GENERAL COUNSEL OF NATIONAL SEMICONDUCTOR CORPORATION. As used herein: 1. Life support devices or systems are devices or systems which, (a) are intended for surgical implant into the ...

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