MLX71122_16 datasheet(16/59 Pages) MELEXIS

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MLX71122

300 to 930MHz FSK/FM/ASK Receiver

Datasheet

REVISION 014 – AUGUST, 2016

Page 16 of 59

390 10 71122 01

3. Functional Description

3.1. 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 input frequencies could produce the same second IF (IF2). This may seem like a

problem, but being able to select high or low side injection makes it possible to avoid interference from

undesired signals. This can not be done with the more common receivers which are single conversion with

a low IF frequency. It is also often possible with the MLX71122 to use a simple RF filter to get better image

rejection than low IF receivers which have an image reject mixer.

Referring to the block diagram in fig.1, the following equations apply:

MHz

2 



LO2DIV = 4 or 8



desired RF signal frequency

LO1 on high side:

VCO





LO2 on high side:

MHz

VCO





LO1 on low side:

VCO





LO2 on low side:

MHz

VCO





From these seven equations, we get:

LO1 high side and LO2 high side:

)

(





VCO

MHz

(1)

LO1 high side and LO2 low side:

)

(







VCO

MHz

(2)

LO1 low side and LO2 high side:

)

(





VCO

MHz

(3)

LO1 low side and LO2 low side:

)

(







VCO

MHz

(4)

Fig. 3 on the next page shows the 4 possible RF frequencies when receiving at fRF High-High is desired.

Example:

Let

MHz

315



and

2 

. From (1) we get

MHz

VCO

417

)

315

(







and further

MHz

102

1 

MHz

104

2 

and the 2

nd IF frequency is 2.0MHz.

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