EVB71111
868/915MHz Receiver
Evaluation Board Description
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
low
high
REF
LO
(RF – IF)/32
32 • REF
RF – LO
RF – 2IF
(RF + IF)/32
32 • REF
LO – RF
IF
RF image
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
RF = 868.3 MHz
low
RF = 868.3 MHz
high
RF = 915 MHz
low
RF = 915 MHz
high
27.46875
879.0
28.92813
925.7
26.80000
857.6
28.25938
904.3
LO / MHz
RF image / MHz
889.7
936.4
846.9
893.6
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
fmin
high
789.3
25.0
800
fmax
low
940.7
29.0625
930
REF / MHz
LO / MHz
IF/ MHz
10.7
10.7
39012 71111 01
Rev. 011
Page 5 of 14
EVB Description
June/07
MELEXIS [ Melexis Microelectronic Systems ]
