MLX71122_16 datasheet(35/59 Pages) MELEXIS

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MLX71122

300 to 930MHz FSK/FM/ASK Receiver

Datasheet

REVISION 014 – AUGUST, 2016

Page 35 of 59

390 10 71122 01

3.4.4. Output Comparator

The output comparator or data slicer decides whether the incoming signal is a digital LOW or HIGH by using

the reference voltage at SLC (pin 32). If the internal voltage is larger than the reference then the output is

HIGH and vice versa. Nevertheless, the polarity of the output comparator can be inverted. The driving

capability of the comparator output is ±2mA and in standby mode the tri-state output is at high

impedance.

Pin DTAO must not be connected by a low impedance to a fixed voltage supply or a stronger driver output!

We recommend using a series resistance of 10kΩ to connect DTAO.

3.5. Frequency Acceptance Range

The frequency acceptance range is defined as

the bandwidth where the input sensitivity can

be degraded by 3dB at a maximum, compared

to the sensitivity at the center frequency of

the channel.

Typically, the frequency acceptance range of

the MLX71122 is about 180kHz, see Fig.16.

The frequency acceptance range is mainly

depending on the frequency deviation, and

slightly on the modulation frequency. The

larger the frequency deviation the smaller the

acceptance range.

Fig. 16: Measured sensitivity characteristic

(BER=3·10

-3, 4kbps, Δf=±20kHz)

3.6. Biasing System

The biasing system needs an external 30kΏ resistor that is connected between RBIAS (pin 10) and the PCB

ground. The band-gap voltage at RBIAS causes a reference current flow of about 42µA through this

reference current resistor. The accuracy of the external resistor should be within ±2%. To minimize the

temperature dependency it is recommended to use a metal film resistor.

3.7. Operating Modes

The MLX71122 has four operating modes having an impact on the receiver’s current consumption. The

OPMODE bits in register R0 (see 4.1.1) determine the operating mode. Selections are:

 00 – Shutdown

all blocks deactivated, only SPI active (default)

 01 – Receive

receiving data from LNAI at selected frequency

 10 – RO and bias only only biasing system and reference oscillator are working

 11 – Synthesizer only

only biasing system, reference oscillator and PLL are working

The first operating mode consumes virtually no current. The circuit is dead except of the SPI that can listen

to commands. In Receive mode all necessary blocks are turned on in order to receive data at the

programmed frequency.

frequency / MHz

-110

433.7

433.8

433.9

434.0

434.1

-107

-104

-101

-95

-98

-92

180kHz

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