MLX71122_16 datasheet(28/59 Pages) MELEXIS

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MLX71122

300 to 930MHz FSK/FM/ASK Receiver

Datasheet

REVISION 014 – AUGUST, 2016

Page 28 of 59

390 10 71122 01

3.3. Receiver Front End

The radio frequency (RF) front-end of the receiver is a double-superheterodyne configuration that converts

the input RF signal via a first intermediate frequency (IF1) signal to a second intermediate frequency (IF2)

signal. While the range of IF1 can vary between 100 and 200MHz, IF2 is fixed to 2MHz. Both signals are

completely processed internally. According to the block diagram (see Fig. 1), the front-end consists of an

LNA, a first mixer (MIX1), a second mixer (MIX2), an internal IF filter (IFF) and an IF limiting amplifier (IFA)

with received signal strength indicator (RSSI). The local oscillator signal for mixer 1 (LO1) is directly

generated in the PLL frequency synthesizer. The LO2 signal for mixer 2 is derived from the LO1 signal via a

divider (see 4.1.4).

There is no inherent suppression of the first mixer’s image frequency. It depends on the particular

application and the system’s environmental conditions whether an RF front-end filter should be added or

not. If image rejection and/or good blocking immunity are relevant system parameters, a band-pass filter

must be placed either in front or after the LNA. This filter can be a SAW (surface acoustic wave) or LC-

based filter (e.g. helical type). Because mixer 2 is an image rejection mixer, the image frequencies of the

second mixing process are suppressed (see Fig. 2). The advantage of a two stage mixing receiver is the

higher gain that can be achieved in the front end.

3.3.1. Low Noise Amplifier (LNA) and Mixer 1 (MIX1)

The LNA is based on a cascode topology for low-noise, high gain and good reverse isolation. The open

collector output has to be connected to an external resonance circuit tuned to the receive frequency. The

gain of the LNA can be changed to achieve a high dynamic range. There are four gain settings selectable by

the control bits LNAGAIN (see 4.1.1). Default setting is the highest gain. The gain settings are automatically

set if the automatic gain control (AGC) feature is activated (see 4.1.4).

The first mixer is a double-balanced mixer which converts the receive frequency to IF1. The default LO

injection type for RF frequencies below 600MHz should be high side (fLO1 = fRX + fIF1). Low side injection (fLO1

= fRX - fIF1) is recommended for the higher frequency bands. Since the data polarity of an FSK modulated

signal will be inverted by changing the injection side it is possible to change the data polarity at the data

output (DTAO) via bit DTAPOL (see 4.1.1). Two gain settings of mixer 1 can be selected through MIX1GAIN

(see 4.1.1): 14dB as a default value or 0dB optionally.

Fig. 11: LNA and Mixer 1

LNAO

MIXP

MIXN

VCC

VCCLNA

LNAI

LO1

IF1

MIX1

VEELNA

LNA

Ibias

LNAGAIN [1:0]

MIX1GAIN

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