th71111 Melexis Company, th71111 Datasheet - Page 5

• Manufacturer Part Number: th71111
• Description: Receiver - 868/915mhz Fsk/ask, Single-conversion Superhet Lqfp32
• Manufacturer: Melexis Company

By using the internal PLL synthesizer of the TH71111 with the fixed feedback divider ratio of N = 32
(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.
1.6.1
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.
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
39010 71111
Rev. 014
RF image / MHz
Injection type
Signal type
REF / MHz
LO / MHz
Selected Frequency Plans
Maximum Frequency Coverage
Injection type
Injection type
Parameter
REF / MHz
RF image
RF / MHz
LO / MHz
IF/ MHz
REF
LO
IF
RF = 868.3 MHz
26.80000
857.6
846.9
low
RF = 868.3 MHz
(RF – IF)/32
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32 • REF
27.46875
RF – 2IF
RF – LO
879.0
889.7
789.3
high
25.0
10.7
high
low
800
f
min
RF = 915 MHz
28.25938
904.3
893.6
low
FSK/ASK Receiver
(RF + IF)/32
32 • REF
RF + 2IF
LO – RF
29.0625
TH71111
940.7
high
10.7
f
930
low
max
RF = 915 MHz
868/915MHz
28.92813
925.7
936.4
high
Data Sheet
April/08