TH71111 Datasheet, PDF (5/22 Pages) Melexis Microelectronic Systems – 868/915MHz FSK/FM/ASK Receiver

TH71111
868/915MHz
FSK/FM/ASK Receiver
By using the internal PLL synthesizer of the TH71111 with the fixed feedback divider ratio of N = 16
(DIV_32), two types of down-conversion are possible: low-side injection of LO and high-side injection of LO.
The following table summarizes some equations that are useful to calculate the crystal reference frequency
(REF) and the LO frequency, for a given RF and IF.
Injection type
REF
LO
IF
RF image
low
(RF – IF)/32
32 • REF
RF – LO
RF – 2IF
high
(RF + IF)/32
32 • REF
LO – RF
RF + 2IF
1.6.1 Selected Frequency Plans
The following table depicts crystal, LO and image signals considering the examples of 868.3 MHz and
915 MHz RF reception at IF = 10.7 MHz.
Signal type
Injection type
REF / MHz
LO / MHz
RF image / MHz
RF = 868.3 MHz
low
26.80000
857.6
846.9
RF = 868.3 MHz
high
27.46875
879.0
889.7
RF = 915 MHz
low
28.25938
904.3
893.6
RF = 915 MHz
high
28.92813
925.7
936.4
The selection of the reference crystal frequency is based on some assumptions. As for example: the image
frequency should not be in a radio band where strong interfering signals might occur (because they could
represent parasitic receiving signals), the LO signal should be in the range of 800 MHz to 930 MHz (because
this is the optimum frequency range of the VCO1). Furthermore the IF should be as high as possible to
achieve highest RF image rejection. The columns in bold depict the selected frequency plans to receive at
868.3 MHz and 915 MHz, respectively.
1.6.2 Maximum Frequency Coverage
Parameter
Injection type
RF / MHz
REF / MHz
LO / MHz
IF/ MHz
f_min
high
789.3
25.0
800
10.7
f_max
low
940.7
29.0625
930
10.7
39010 71111
Rev. 013
Page 5 of 22
Data Sheet
June/07