RF69 datasheet(26/81 Pages) ETC2

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RF69

Page 26

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3.4.5. Quadrature Mixer - ADCs - Decimators

The mixer is inserted between output of the RF buffer stage and the input of the analog to digital converter (ADC) of the

receiver section. This block is designed to translate the spectrum of the input RF signal to base-band, and offer both high

IIP2 and IIP3 responses.

In the lower bands of operation (290 to 510 MHz), the multi-phase mixing architecture with weighted phases improves the

rejection of the LO harmonics in receiver mode, hence increasing the receiver immunity to out-of-band interferers.

The I and Q digitalization is made by two 5th order continuous-time Sigma-Delta Analog to Digital Converters (ADC). Their

gain is not constant over temperature, but the whole receiver is calibrated before reception, so that this inaccuracy has no

impact on the RSSI precision. The ADC output is one bit per channel. It needs to be decimated and filtered afterwards. This

ADC can also be used for temperature measurement, please refer to section 3.4.17 for more details.

The decimators decrease the sample rate of the incoming signal in order to optimize the area and power consumption of

the following receiver blocks.

3.4.6. Channel Filter

The role of the channel filter is to filter out the noise and interferers outside of the channel. Channel filtering on the RF69 is

implemented with a 16-tap Finite Impulse Response (FIR) filter, providing an outstanding Adjacent Channel Rejection

performance, even for narrowband applications.

Note

to respect oversampling rules in the decimation chain of the receiver, the Bit Rate cannot be set at a higher value

than 2 times the single-side receiver bandwidth (BitRate < 2 x RxBw)

The single-side channel filter bandwidth RxBw is controlled by the parameters RxBwMant and RxBwExp in RegRxBw:

 When FSK modulation is enabled:

 When OOK modulation is enabled:

RxBw = -----------------------

FXOSC

---------------------

RxBwMant  2

R x B w E x p + 2

RxBw = -----------------------

FXOSC

---------------------

RxBwMant  2

R x B w E x p + 3

The following channel filter bandwidths are accessible (oscillator is mandated at 32 MHz):

Table 14 Available RxBw Settings

RxBwMant

RxBwExp

RxBw (kHz)

(binary/value)

(decimal)

FSK

ModulationType=00

OOK

ModulationType=01

10b / 24

2.6

1.3

01b / 20

3.1

1.6

00b / 16

3.9

2.0

10b / 24

5.2

2.6

01b / 20

6.3

3.1

00b / 16

7.8

3.9

10b / 24

10.4

5.2

01b / 20

12.5

6.3

00b / 16

15.6

7.8

10b / 24

20.8

10.4

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