MLX71121 datasheet(13/28 Pages) MELEXIS

13 / 28 page

MLX71121

300 to 930MHz

FSK/FM/ASK Receiver

39010 71121

Page 13 of 28

Data Sheet

Rev. 007

Feb/08

Functional Description

2.1

Frequency Planning

Because of the double conversion architecture that employs two mixers and two IF signals, there are four

different combinations for injecting the LO1 and LO2 signals:

•

LO1 high side and LO2 high side:

receiving at fRF(high-high)

•

LO1 high side and LO2 low side:

receiving at fRF(high-low)

•

LO1 low side and LO2 high side:

receiving at fRF(low-high)

•

LO1 low side and LO2 low side:

receiving at fRF(low-low)

As a result, four different radio frequencies (RFs) could yield one and the same second IF (IF2). Fig. 10

shows this for the case of receiving at fRF(high-high). In the example of Fig. 10, the image signals at fRF(low-

high) and fRF(low-low) 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 fRF(high-high) and fRF(high-low) are entering 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. 10, LO2 high-side injection

has been chosen to select the IF2 signal resulting from fRF(high-high).

fLO2

fLO1

f RF

Fig. 10: The four receiving frequencies in a double conversion superhet receiver

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

quencies (fLO1 , fLO2) and the reference oscillator frequency fRO.

LO2

LO1

⋅

LO2

⋅

The operating frequency of the internal IF filter (IFF) and FSK demodulator (FSK DEMOD) are 1.8MHz.

Therefore the second IF (IF2) is set to 1.8MHz as well.

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