MLX71122_16 datasheet(30/59 Pages) MELEXIS

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MLX71122

300 to 930MHz FSK/FM/ASK Receiver

Datasheet

REVISION 014 – AUGUST, 2016

Page 30 of 59

390 10 71122 01

3.3.4. FSK Demodulator

FSK reception is turned on if bit MODSEL in register R5 is set to LOW (default). The demodulator is

completely internally implemented, so no external, expensive discriminator device is needed. The used FSK

demodulator is based on a phase shifter and a mixer as depicted in Fig.14.

Fig. 14:

Block diagram of FSK demodulator

The phase shifter provides a phase shift of 90 degrees to the original IF signal at exactly 2MHz. There is no

AFC feature integrated since the frequency acceptance range of the demodulator is wide enough with

about ±150kHz. The phase shift is regulated by the same control loop that controls the center frequency of

the IF-filter. Tuning of the IF-filter will also change the DC value of the demodulator output. We

recommend turning off the tuning during receive mode using IFFHLT (see 4.1.7) if small frequency

deviations below ±20kHz have to be detected. The gain of the demodulator can be changed with bit

DEMGAIN (see 4.1.1). It can be set to 12mV/kHz (default) or to 14.5mV/kHz.

3.3.5. Autotuning Circuit

An auto-tuning mechanism is implemented that permanently adjusts the bias current of the

transconductance cells of the IF filter and the FSK demodulator in order to eliminate process, temperature

and supply voltage variations. For this purpose a matched master bi-quad is used as oscillator in a current

controlled oscillator (CCO) at 3MHz embedded in a PLL structure. A more detailed view on the tuning

circuit is shown in Fig.15.

CCO

digital

dead band

control

~2.9094MHz

IFFVAL

tuning signal

proportional

to frequency

Filter

fcenter = 2.0000MHz

IFAIN

MIX2OUT

RIFF

IFFSTATE[1:0]

IFFVAL [7:0]

IFFTUNE

IFFHLT

IFFPRES[7: 0]

tune

count CCO cylces

in Ttune/2 => N

N < 394 => IFFVAL + 1

N > 407 => IFFVAL - 1

default = 684

frequency

matching

once per Ttune:

typ.

10MHz

Fig.15:

block diagram of digital tuning circuitry

limiter

LP 1st order

buffer

OUT

phase shifter

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