SX1239 데이터 시트보기 (PDF) - Semtech Corporation

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SX1239

Low Power Integrated UHF Receiver

Semtech Corporation 

SX1239

ADVANCED COMMUNICATIONS & SENSING

DATASHEET

3.4.2. LNA - Single to Differential Buffer

The LNA uses a common-gate topology, which allows for a flat characteristic over the whole frequency range. It is

designed to have an input impedance of 50 Ohms or 200 Ohms (as selected with bit LnaZin in RegLna), and the parasitic

capacitance at the LNA input port is cancelled with the external RF choke. A single to differential buffer is implemented to

improve the second order linearity of the receiver.

The LNA gain, including the single-to-differential buffer, is programmable over a 48 dB dynamic range, and control is either

manual or automatic with the embedded AGC function.

Note

In the specific case where the LNA gain is manually set by the user, the receiver will not be able to properly handle

FSK signals with a modulation index smaller than 2 at an input power greater than the 1dB compression point,

tabulated in section 3.4.3.

Table 8

LNA Gain Settings

LnaGainSelect

000

001

010

011

100

101

110

111

LNA Gain

Any of the below, set by the AGC loop

Max gain

Max gain - 6 dB

Max gain - 12 dB

Max gain - 24 dB

Max gain - 36 dB

Max gain - 48 dB

Reserved

Gain Setting

-

G1

G2

G3

G4

G5

G6

-

3.4.3. Automatic Gain Control

By default (LnaGainSelect = 000), the LNA gain is controlled by a digital AGC loop in order to obtain the optimal sensitivity/

linearity trade-off.

Regardless of the data transfer mode (Packet or Continuous), the following series of events takes place when the receiver

is enabled:

Š The receiver stays in WAIT mode, until RssiValue exceeds RssiThreshold for two consecutive samples. Its power

consumption is the receiver power consumption.

Š When this condition is satisfied, the receiver automatically selects the most suitable LNA gain, optimizing the sensitivity/

linearity trade-off.

Š The programmed LNA gain, read-accessible with LnaCurrentGain in RegLna, is carried on for the whole duration of the

packet, until one of the following conditions is fulfilled:

Š Packet mode: if AutoRxRestartOn = 0, the LNA gain will remain the same for the reception of the following packet. If

AutoRxRestartOn = 1, after the controller has emptied the FIFO the receiver will re-enter the WAIT mode described

above, after a delay of InterPacketRxDelay, allowing for the distant transmitter to ramp down, hence avoiding a false

RSSI detection. In both cases (AutoRxRestartOn=0 or AutoRxRestartOn=1), the receiver can also re-enter the WAIT

mode by setting RestartRx bit to 1. The user can decide to do so, to manually launch a new AGC procedure.

Š Continuous mode: upon reception of valid data, the user can decide to either leave the receiver enabled with the same

LNA gain, or to restart the procedure, by setting RestartRx bit to 1, resuming the WAIT mode of the receiver, described

above.

Notes - the AGC procedure must be performed while receiving preamble in FSK mode

- in OOK mode, the AGC will give better results if performed while receiving a constant “1” sequence

Rev 3 - Jan 2011

Page 18

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