MLX71122_16 Datasheet, PDF (6/59 Pages) Melexis Microelectronic Systems – 300 to 930MHz FSK/FM/ASK Receiver

MLX71122
300 to 930MHz FSK/FM/ASK Receiver
Datasheet
1. Theory of Operation
1.1. General
The MLX71122 receiver architecture is based on a double-conversion super-heterodyne approach. The two
LO signals are derived from an on-chip integer-N PLL frequency synthesizer. The PLL reference frequency is
derived from a crystal (XTAL). The PLL synthesizer consists of an integrated voltage-controlled oscillator
with external inductor, a programmable feedback divider chain, a programmable reference divider, a
phase-frequency detector with a charge pump and an external loop filter.
In the receiver’s down-conversion chain, two mixers MIX1 and MIX2 are driven by the internal local
oscillator signals LO1 and LO2, respectively. The second mixer MIX2 is an image-reject mixer. As the first
intermediate frequency (IF1) is very high (typically above 100 MHz), a reasonably high degree of image
rejection is provided even without using an RF front-end filter. At applications asking for very high image
rejections, cost-efficient RF front-end filtering can be realized by using a SAW filter in front of the LNA.
The receiver signal chain is set up by a low noise amplifier (LNA), two down-conversion mixers (MIX1 and
MIX2), an on-chip IF filter (IFF) as well as an IF amplifier (IFA). By choosing the required modulation via an
FSK/ASK switch (at pin MODSEL), either the on-chip FSK demodulator (FSK DEMOD) or the RSSI-based ASK
detector is selected. A second order data filter (OA1) and a data slicer (OA2) follow the demodulator. The
data slicer threshold can be generated from the mean-value of the data stream or by means of the positive
and negative peak detectors (PKDET+/-).
In general the MLX71122 can be set to shut-down mode, where all receiver functions are completely
turned off, and to several other operating modes. There are two global operating modes that are
selectable via the logic level at pin SPISEL:
 8-channel pre-configured mode (ABC mode)
 fully programmable mode (SPI mode).
In ABC mode the number of frequency channels is limited to eight but no microcontroller programming is
required. In this case the three lines of the serial programming interface (SPI) are used to select one of the
eight predefined frequency channels via simple 3-bit parallel programming. Pins ENRX and MODSEL are
used to enable/disable the receiver and to select FSK or ASK demodulation, respectively.
SPI mode is recommended for full programming flexibility. In this case the three lines of the SPI are
configured as a standard 3-wire bus (SDEN, SDTA and SCLK). This allows changing many parameters of the
receiver, for example more operating modes, channels, frequency resolutions, gains, demodulation types,
data slicer settings and more. The pin MODSEL has no effect in this mode.
REVISION 014 – AUGUST, 2016
390 10 71122 01
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