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RF65
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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
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
-
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:
The receiver stays in WAIT mode, until
RssiValue
exceeds
RssiThreshold
for two consecutive samples. Its power
consumption is the receiver power consumption.
When this condition is satisfied, the receiver automatically selects the most suitable LNA gain, optimizing the sensitivity/
linearity trade-off.
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:
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.
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
