LMX9830 datasheet(21/46 Pages) NSC

21 / 46 page

www.national.com

9.0 Digital Smart Radio (Continued)

The post-detection filter also helps for attenuating the first

adjacent channel signal. The equalizer improves the eye-

opening for 101010 patterns. The post-detection filter is a

third order Butterworth filter.

9.4 AUTOTUNING CIRCUITRY

The autotuning circuitry is used for tuning the bandpass fil-

ter, the detector, the post-detection filter, the equalizer, and

the transmit filters for process and temperature variations.

The circuit also includes an offset compensation for the FM

detector.

9.5 SYNTHESIZER

The synthesizer consists of a phase-frequency detector, a

charge pump, a low-pass loop filter, a programmable fre-

quency divider, a voltage-controlled oscillator (VCO), a del-

ta-sigma modulator, and a lookup table.

The frequency divider consists of a divide-by-2 circuit (di-

vides the 5 GHz signal from the VCO down to 2.5 GHz), a

divide-by-8-or-9 divider, and a digital modulus control. The

delta-sigma modulator controls the division ratio and also

generates an input channel value to the lookup table.

9.5.1 Phase-Frequency Detector

The phase-frequency detector is a 5-state phase-detector.

It responds only to transitions, hence phase-error is inde-

pendent of input waveform duty cycle or amplitude varia-

tions. Loop lockup occurs when all the negative transitions

on the inputs, F_REF and F_MOD, coincide. Both outputs

(i.e., Up and Down) then remain high. This is equal to the

zero error mode. The phase-frequency detector input fre-

quency range operates at 12MHz.

9.6 TRANSMITTER CIRCUITRY

The transmitter consists of ROM tables, two Digital to Ana-

log (DA) converters, two low-pass filters, IQ mixers, and a

power amplifier (PA).

The ROM tables generate a digital IQ signal based on the

transmit data. The output of the ROM tables is inserted into

IQ-DA converters and filtered through two low-pass filters.

The two signal components are mixed up to 2.5 GHz by the

TX mixers and added together before being inserted into

the transmit PA.

9.6.1 IQ-DA Converters and TX Mixers

The ROM output signals drive an I- and a Q-DA converter.

Two Butterworth low-pass filters filter the DA output signals.

The 6 MHz clock for the DA converters and the logic circuit-

ry around the ROM tables are derived from the autotuner.

The TX mixers mix the balanced I- and Q-signals up to 2.4-

2.5 GHz. The output signals of the I- and Q-mixers are

summed.

9.7 CRYSTAL REQUIREMENTS

The LMX9830 contains a crystal driver circuit. This circuit

operates with an external crystal and capacitors to form an

oscillator. Figure 12 on page 22 shows the recommended

crystal circuit. Table 22 on page 24 specifies system clock

requirements.

The RF local oscillator and internal digital clocks for the

LMX9830 is 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.

It is also important to consider board and design depen-

dant capacitance in tuning crystal circuit. Equations that

follow allow a close approximation of crystal tuning capaci-

tance required, but actual values on board will vary with

capacitive properties of the board. As a result, there is

some fine tuning of crystal circuit that has to be done that

can not be calculated, must be tuned by testing different

values of load capacitance.

Many different crystals can be used with the LMX9830. Key

requirements from Bluetooth specification is + 20ppm.

Additionally, ESR (Equivalent Series Resistance) must be

carefully considered. LMX9830 can support maximum of

230ohm ESR, but it is recommended to stay <100ohms

ESR for best performance over voltage and temperature.

Reference Figure 17 on page 25 for ESR as part of crystal

circuit for more information.

9.7.1 Crystal

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.

1. Load Capacitance

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 13 on page 22 is composed of:

■ C1 (motional capacitance)

■ R1 (motional resistance)

■ L1 (motional inductance)

■ C0 (static or shunt capacitance)

The LMX9830 provides some of the load with internal

capacitors Cint. The remainder must come from the exter-

nal capacitors and tuning capacitors labeled Ct1 and Ct2

as shown in Figure 12 on page 22. Ct1 and Ct2 should

have the same the value for best noise performance.

The LMX9830 has an additional internal capacitance

CTUNE of 2.6pF. Crystal load capacitance (CL) is calculated

as the following:

CL = Cint + CTUNE + Ct1//Ct2

The CL above does not include the crystal internal self-

capacitance C0 as shown in Figure 13 on page 22, so the

total capacitance is:

Ctotal = CL + C0

LMX9830 Datasheet(PDF) 21 Page - National Semiconductor (TI)

LMX9830 Datasheet(HTML) 21 Page - National Semiconductor (TI)

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46

Datasheet Download

Link URL