MLX71122_16 datasheet(17/59 Pages) MELEXIS

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MLX71122

300 to 930MHz FSK/FM/ASK Receiver

Datasheet

REVISION 014 – AUGUST, 2016

Page 17 of 59

390 10 71122 01

The image frequencies of the two mixers are now:

MHz

IMAGE

MIX

519

102

417







, RF response, suppressed by the RF bandpass filter,

MHz

IMAGE

MIX

106

104







, suppressed by the image rejection of mixer 2.

IMAGE

MIX

leads to two further RF response frequencies:

MHz

311

106

417





suppressed by 30dB of the image rejection of mixer 2

MHz

523

106

417





suppressed by 30dB of mixer 2 plus the RF bandpass filter

In the example of Fig. 3, the image signals at 519.66 and 523.66 are suppressed by the bandpass

characteristic provided by the RF front-end. The bandpass shape can be achieved either with a SAW filter

(featuring just a couple of MHz bandwidth), or by the tank circuits at the LNA input and output (this

typically yields 30 to 60MHz bandwidth). In any case, the high value of the first IF (IF1) helps to suppress

the image signals at fRF Low-High and fRF Low-Low.

The two remaining signals at IF1 resulting from 102.33 and 106.33 enter the second mixer MIX2. This mixer

features image rejection with so-called single-sideband (SSB) selection. This means either the upper or

lower sideband of IF1 can be selected. In the example of Fig. 3, LO2 high-side injection has been chosen to

select the IF2 signal resulting from fRF High-High.

Fig. 3: The four receiving frequencies in a double conversion superhet receiver

It should be mentioned that each high-side injection mixing mirrors the frequency spectrum of the input

signal. Only Low-Low and High-High injection mixing preserve the spectrum or in other words a higher

frequency at RF remains a higher frequency at IF2. The polarity of the data slicer can be switched in order

to compensate this for FSK reception of digital data.

It can be seen from the block diagram of Fig. 1 that there is a fixed relationship between the LO1 signal

frequency fLO1 and the LO2 signal frequency fLO2.

LO2

LO1

LO2

LO2DIV



(5)

The LO1 signal frequency fLO1 is directly synthesized from the crystal reference oscillator frequency fRO by

means of an integer-N PLL synthesizer. The PLL consists of a dual-modulus prescaler (P/P+1) with P=32,

a program counter N and a swallow counter A.

fLO2

fLO1

f RF

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