LMX9820SB/NOPB National Semiconductor, LMX9820SB/NOPB Datasheet - Page 16

• Manufacturer Part Number: LMX9820SB/NOPB
• Manufacturer: National Semiconductor

Specifications of LMX9820SB/NOPB

• Lead Free Status / Rohs Status: Compliant

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8.0 Digital Smart Radio
The LMX9820 Digital Smart Radio includes a high perfor-
mance, monolithic, radio transceiver optimized for Blue-
tooth communications systems.
The radio transceiver is a highly integrated design and
includes the Low Noise Amplifier (LNA), mixer, on-chip fil-
ters, 2.5 GHz
Amplifier (PA) driver, and modem functions. Digital modula-
tion and demodulation techniques are utilized for a robust
manufacturable design. Power management includes con-
trol over individual chip functions and internal voltage regu-
lation for optimum performance.
8.1 RADIO RECEIVER
The signal path of the RX architecture contains an internal
LNA and quadrature RF downconverting mixer at 2.5 GHz.
A low Intermediate Frequency (IF) receiver provides high
performance at low cost and low current consumption. The
IF demodulator is implemented digitally in combination with
a limiting amplifier.
8.2 LOW NOISE AMPLIFIER (LNA)
The on-board LNA is a single-ended structure designed
with a 50
ing. The LNA is closely integrated with the mixer providing
low noise and good immunity from blocking signals.
8.3 RX MIXER
The receive mixer is an image reject ring diode type mixer.
An internal low noise gain block is incorporated prior to the
mixer to achieve extremely low noise performance. A dif-
ferential IF output improves noise immunity while maintain-
ing a high intercept point.
8.4 CHANNEL SELECT FILTER
The IF circuitry is followed by an integrated complex active
bandpass filter that provides the required channel selectiv-
ity and image rejection. The I and Q outputs of the filter are
then converted to the digital domain using a limiter, discrim-
inator, and A/D converter.
8.5 LIMITER
The limiter circuit consists of I and Q limiting amplifiers that
provide the remaining gain in the receiver such that an
acceptable signal level exists at the frequency modulation
(FM) discriminator. Limiting amplification of the downcon-
verted wanted signal minimizes the input range require-
ments of the A/D converter.
8.6 FM DISCRIMINATOR
The limited signal is translated to digital format by using an
analog Frequency Shift Keying (FSK) demodulator and A/D
converter. The A/D converter extracts the RX signal at a
sample rate of 72.0 MHz.
8.7 RECEIVE SIGNAL STRENGTH INDICATOR
(RSSI)
The receive signal strength indicator (RSSI) signal is
derived from the input level to the limiter and covers a
range low detector level = -59dBm and high detector level
= -38dBm. The information is typically fed back to the
baseband controller via the serial interface.
input impedance for simple capacitive match-
PLL, voltage controlled oscillator, Power
16
8.8 RADIO TRANSMITTER
The signal path of the TX architecture contains an internal
modulator for 1 Mb/s GFSK (Gaussian Frequency Shift
Keying) modulation of the 2.5 GHz VCO. Closed loop
modulation is chosen since it is the most low power solu-
tion. The integrated pre-amplifier provides output levels
sufficient for Class 2 Bluetooth operation.
8.9 MODULATOR
An internal digital Gaussian filter provides the FSK modula-
tion waveform. A modulation input to the completely inte-
grated 2.5 GHz
deviation. This eliminates the risks of open loop modulation
such as frequency drift and frequency offset.
8.10 TRANSMIT FREQUENCY OUTPUT
The transmit RF output is differential, and is connected to
the antenna through an integrated passive balun.
8.11 FREQUENCY SYNTHESIZERS
The 2.4 to 2.5GHz RF range is provided by an on-chip volt-
age controlled oscillator (VCO). A programmable 2.5 GHz
oscillator can be configured with 12MHz crystal, or for
TCXO frequency input. Internal dividers provide internal
clocks and clock outputs to the baseband controller.
8.12 CRYSTAL CIRCUIT
Due to the need for clock accuracy, the LMX9820 has a
dedicated crystal oscillator. The LMX9820 uses the crystal
to supply a 12 MHz clock source to the baseband control-
ler. The 12 MHz is buffered, providing a receive data clock
to the baseband controller. It is also possible to configure
the crystal oscillator for input only when another high qual-
ity crystal oscillator is available in the system. The
LMX9820 can accommodate 12 MHz crystal.
8.13 EXTERNAL CRYSTAL OSCILLATORS
The LMX9820 contains a crystal driver circuit. This circuit
operates with an external crystal and capacitors to form an
oscillator. See Figure 10 on page 17 and Figure 11 on page
17. The LMX9820 also can operate with an external TCXO
(Temperature Compensated Crystal Oscillator).
8.13.1 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 11 is composed of:
• C1 (motional capacitance)
• R1 (motional resistance)
• L1 (motional inductance)
• C0 (static or shunt capacitance)
The LMX9820 provides some of the load with internal
capacitors C
nal capacitors labeled Ct1 and Ct2 as shown in Figure 10.
Ct1 and Ct2 should have the same value for best noise
PLL selects the channel frequency. An internal crystal
int
. The remainder must come from the exter-
PLL provides a consistent modulation