MLX71122_16 Datasheet, PDF (30/59 Pages) Melexis Microelectronic Systems – 300 to 930MHz FSK/FM/ASK Receiver

MLX71122
300 to 930MHz FSK/FM/ASK Receiver
Datasheet
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.
IN
limiter
phase shifter
LP 1st order LP 1st order
buffer
OUT
limiter
limiter
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.
MIX2OUT
default = 684
f_RO
typ.
10MHz
RIFF
IF
fcenter = 2.0000MHz _Filter
IFAIN
IFFHLT
IFFTUNE IFFSTATE[1:0]
IFFPRES[7: 0] IFFVAL [7:0]
10
10
tuning signal
proportional
to frequency
frequency
matching
digital
dead band
8
D
control
4
T_tune
IFFVAL
A
once per Ttune:
count CCO cylces
in Ttune/2 => N
N < 394 => IFFVAL + 1
N > 407 => IFFVAL - 1
CCO ^~2.9094MHz
Fig.15: block diagram of digital tuning circuitry
REVISION 014 – AUGUST, 2016
390 10 71122 01
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