MLX71122_16 Datasheet, PDF (17/59 Pages) Melexis Microelectronic Systems – 300 to 930MHz FSK/FM/ASK Receiver

MLX71122
300 to 930MHz FSK/FM/ASK Receiver
Datasheet
The image frequencies of the two mixers are now:
f_MIX1IMAGE  417.33 102.33  519.66MHz , RF response, suppressed by the RF bandpass filter,
f_{MIX 2IMAGE}  104.33  2.0  106.33MHz , suppressed by the image rejection of mixer 2.
f _{MIX 2IMAGE} leads to two further RF response frequencies:
417.33 106.33  311.00MHz : suppressed by 30dB of the image rejection of mixer 2
417.33  106.33  523.66MHz : 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 f_{RF Low-High} and f_{RF 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 f_{RF High-High}.
f _LO2
f _LO2
f _RF
f _RF
f _LO1
f _RF
f _RF
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 f_LO1 and the LO2 signal frequency f_LO2.
LO2DIV 
N _LO2

f _LO1
f _LO2
(5)
The LO1 signal frequency f_LO1 is directly synthesized from the crystal reference oscillator frequency f_RO 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.
REVISION 014 – AUGUST, 2016
390 10 71122 01
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