MLX71121_15 Datasheet, PDF (7/29 Pages) Melexis Microelectronic Systems – FSK/OOK Receiver

MLX71121
300 to 930MHz
FSK/OOK Receiver
1.6 Mixer Section
The mixer section consists of two mixers. Both are double-balanced mixers. The second mixer is built as an
image rejection mixer. The first mixer’s inputs (MIXP and MIXN) are functionally the same. For single-ended
drive, the unused input has to be tied to ground via a capacitor. A soft band-pass filter is placed between the
mixers.
RFSEL
0
1
Description
Input frequency range 300 to 470MHz
Input frequency range 610 to 930MHz
Pin RFSEL is used to select the required RF band. The LO frequencies and the proper sidebands for image
suppression will be set accordingly.
1.7 IF Filter
The MLX71121 comprises an internal IF
filter with a -3dB bandwidth (B3dB) of
300kHz and a -40dB attenuation band-
width (B40dB) of 1.4MHz. This filter
contains three capacitively coupled bi-
quad stages that represent resonant
tanks at a filter center frequency (f_cent)
of 1.8MHz
gain/dB
0
-20
-40
Fig. 3: IF filter tolerance scheme
B_3dB
B_40dB
f _cent
f _IF2
1.8 IF Amplifier
After having passed the IF filter, the signal is amplified by a high-gain limiting amplifier. It consists of several
AC-coupled gain stages with a bandwidth of 400kHz to 11MHz. The overall small-signal pass-band gain is
about 80dB. A received-signal-strength indicator (RSSI) signal is generated within the IF amplifier and is
available at pin RSSI.
1.9 PLL Synthesizer
The PLL synthesizer consists of a fully integrated voltage-controlled oscillator that runs at 400MHz to
640MHz, a distributed feedback divider chain, an edge-triggered phase-frequency detector, a charge pump,
a loop filter and a crystal-based reference oscillator. The PLL is used for generating the LO signals. The LO1
is directly taken from the VCO output, and the LO2 is derived from the LO1 signal passing the N1 counter.
Another counter N2 follows N1 to provide the comparison frequency to the PFD. The overall feedback divider
ratio N_tot is 24. The values of N1 and N2 can be changed via pin RFSEL for selecting the LO1 and LO2 fre-
quencies. The table below shows the range of the LO frequencies.
RFSEL
f_LO1min
[MHz]
f_LO1max
[MHz]
f_LO2min
[MHz]
f_LO2max
[MHz]
N₁
0
400
640
100
160
4
1
400
640
200
320
2
N₂
N_tot
6
24
12
24
39010 71121
Rev. 012
Page 7 of 29
Data Sheet
Mar/15