Rainbow-electronics AT86RF211 User Manual download pdf (Page 16)

AT86RF211

1942C–WIRE–06/02

Figure 19. ADC Converter Input Selection

Note: For voltage measurement, the LSB weighs 85 mV and the reference voltage is 1.25V.

The ADC measuring the RSSI can be turned into voltage or discriminator output DC

level measurement.

FSK Demodulator Its structure is based on an oscillator:

Figure 20. Schematic of the FSK demodulator

The oscillator’s natural frequency is F

and it actually oscillates at the Fin frequency.

The signal at the output of the oscillator (point A) is proportional to the frequency differ-

ence between Fin and F

. The XOR function translates the difference into a pulse duty

cycle (point B).Thereafter by low-pass filtering of the signal is obtained a mean voltage

of the signal (point C).

The architecture of this demodulation is thereby analog and allows the transmission of

continuous data stream of the same value as the output voltage is proportional to the

input frequency. Thus it is not mandatory to use Manchester encoding and the first bit is

correctly demodulated.

The oscillator feedback resistor controls the center frequency F

.Itisadjustedaccord-

ing to the output of a dummy FSK demodulator driven by a 455 kHz internal reference

frequency which is a division of the reference crystal. The discrete components con-

nected to pin 32 DISCFILT are the loop filter of the PLL stabilizing the 455 kHz signal.

Voltage

RSSI

ADC

MRSSI

MVCC

STATUS register

CTRL1[24]

CTRL1[1]

Vcc supply

DISCOUT

(MOFFSET)

oscillator

Fin

RBW

1 2 ... 11 12 13 14 15 16 17 18 19 20 21 ... 47 48

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Rainbow-electronics AT86RF211 User Manual Page 16