MRF39RA datasheet(7/72 Pages) MICROCHIP

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DS40001778B-page 7

MRF39RA

2.4

Receiver Description

The MRF39RA features a digital receiver with the

Analog-to-Digital conversion process being performed

directly following the LNA-mixers block. The zero-IF

receiver is able to handle (G)FSK and (G)MSK

modulation. ASK and OOK modulation is, however,

demodulated by a low-IF architecture. All the filtering,

demodulation, gain control, synchronization and

packet handling is performed digitally, which enables a

very wide range of bit rates and frequency deviations

to be selected. The receiver is also capable of

automatic gain calibration to improve precision on

RSSI measurements.

FIGURE 2-2:

RECEIVER BLOCK DIAGRAM

Figure 2-2 shows the receiver block diagram, and the

following sections provides a brief description of each

of the receiver blocks.

2.4.1

LNA – SINGLE-TO-DIFFERENTIAL

BUFFER

The LNA uses a common-gate topology, which enables

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

canceled

with

the

external

choke.

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.

Table 2-1 shows the LNA Gain settings.

LNA

Single to

Differential

Mixers



Modulators

RSSI

AFC

RFIN

FSK

Demodulator

Local

Oscillator

Channel

Filter

Cancellation

Rx Calibration

Reference

Bypassed

in FSK

Phase

Output

Module

Output

Complex

Filter

CORDIC

OOK

Demodulator

AGC

Note:

In the specific case where the LNA gain is

manually set by the user, the receiver is

unable to properly handle FSK signals

with a modulation index smaller than 2 at

an input power greater than the 1 dB

compression

point,

tabulated

Section 2.4.2

“Automatic

Gain

Control”.

TABLE 2-1:

LNA GAIN SETTINGS

LnaGainSelect

LNA Gain

Gain

Setting

000

Any of the below, set

by the AGC loop

—

001

Max gain

010

Max gain – 6 dB

011

Max gain – 12 dB

100

Max gain – 24 dB

101

Max gain – 36 dB

110

Max gain – 48 dB

111

Reserved

—

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