MRF39RA Datasheet, PDF (7/72 Pages) Microchip Technology – Low-Power, Integrated UHF Receiver

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
Rx Calibration
Reference
LNA
RFIN
Single to
Differential

Mixers Modulators
Channel
Filter
DC
Cancellation
Complex
Filter
Local
Oscillator
AFC
AGC
Bypassed
in FSK
CORDIC
Phase
Output
Module
Output
FSK
Demodulator
RSSI
OOK
Demodulator
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 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 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 in
Section 2.4.2 “Automatic Gain
Control”.
 2015 Microchip Technology Inc.
Table 2-1 shows the LNA Gain settings.
TABLE 2-1: LNA GAIN SETTINGS
LnaGainSelect
LNA Gain
Gain
Setting
000
Any of the below, set
by the AGC loop
—
001
Max gain
G1
010
Max gain – 6 dB
G2
011
Max gain – 12 dB
G3
100
Max gain – 24 dB
G4
101
Max gain – 36 dB
G5
110
Max gain – 48 dB
G6
111
Reserved
—
DS40001778B-page 7