MLX71122_16 Datasheet, PDF (16/59 Pages) Melexis Microelectronic Systems – 300 to 930MHz FSK/FM/ASK Receiver

MLX71122
300 to 930MHz FSK/FM/ASK Receiver
Datasheet
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:
LO1 high side and LO2 low side:
LO1 low side and LO2 high side:
LO1 low side and LO2 low side:
receiving at f_{RF High-High}
receiving at f_{RF High-Low}
receiving at f_{RF Low-High}
receiving at f_{RF 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:
f _{IF 2}  2.0MHz , N _LO2  LO2DIV = 4 or 8
f _IN  desired RF signal frequency
LO1 on high side: f_VCO  f _IN  f _IF1
LO2 on high side:
f_VCO
N _LO2

f _IF1
 2.0MHz
LO1 on low side: f _IN  f_VCO  f _IF1
From these seven equations, we get:
LO2 on low side:
f _IF1

f_VCO
N _LO2
 2.0MHz
LO1 high side and LO2 high side:
f _VCO
 ( f _IN
 2MHz)
N _LO2
N _LO2 1
(1)
LO1 high side and LO2 low side:
f _VCO
 ( f _IN
 2MHz)
N _LO2
N _LO2 1
(2)
LO1 low side and LO2 high side:
f _VCO
 ( f _IN
 2MHz)
N _LO2
N _LO2  1
(3)
LO1 low side and LO2 low side:
f _VCO
 ( f _IN
 2MHz)
N _LO2
N _LO2  1
(4)
Fig. 3 on the next page shows the 4 possible RF frequencies when receiving at f_{RF High-High} is desired.
Example:
Let
f _IN  315MHz and N _LO2  4 .
From (1) we get
4
f_VCO

(315MHz  2MHz)
4 1

417.33MHz
and further f_IF1  102.33MHz , f_LO2  104.33MHz and the 2^nd IF frequency is 2.0MHz.
REVISION 014 – AUGUST, 2016
390 10 71122 01
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