EVB71112
868/915MHz Receiver
Evaluation Board Description
By configuring the TH71112 for double conversion and using its internal PLL synthesizer with fixed feedback
divider ratios of N1 = 16 (DIV_16) and N2 = 2 (DIV_2), four types of down-conversion are possible: low-side
injection of LO1 and LO2 (low-low), LO1 low-side and LO2 high-side (low-high), LO1 high-side and LO2
low-side (high-low) or LO1 and LO2 high-side (high-high). The following table summarizes some equations
that are useful to calculate the crystal reference frequency (REF), the first IF (IF1) and the VCO1 or first LO
frequency (LO1), respectively, for a given RF and second IF (IF2).
Injection type
REF
LO1
IF1
LO2
IF2
high-high
(RF – IF2)/30
32•REF
LO1 – RF
2•REF
LO2 – IF1
low-low
(RF – IF2)/34
32•REF
RF – LO1
2•REF
IF1 – LO2
high-low
(RF + IF2)/30
32•REF
LO1 – RF
2•REF
IF1 – LO2
low-high
(RF + IF2)/34
32•REF
RF – LO1
2•REF
LO2 – IF1
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 IF2 = 10.7 MHz. The columns in bold depict the selected frequency plans to
receive at 868.3 MHz and 915 MHz, respectively.
RF =
868.3
MHz
28.58667
914.77333
46.47333
57.17333
Signal type
RF =
868.3
MHz
low-low
25.22353
807.15294
61.14706
50.44706
746.00588
39.74706
RF =
868.3
MHz
high-low
29.3
937.6
69.3
58.6
1006.9
47.9
RF =
868.3
MHz
low-high
25.85294
827.29412
41.00588
51.70588
786.28824
62.40588
RF =
915
MHz
high-high
30.14333
964.58667
49.58667
60.28667
1014.17
70.98667
RF =
915
MHz
low-low
26.59706
851.10588
63.89412
53.19412
787.21176
42.49412
RF =
915
MHz
high-low
30.85667
987.41333
72.41333
61.71333
1059.83
51.01333
RF =
915
MHz
low-high
27.22647
871.24706
43.75294
54.45294
827.49412
65.15294
Injection type high-high
REF / MHz
LO1 / MHz
IF1 / MHz
LO2 / MHz
RF image/MHz 961.24667
IF1 image/MHz
67.87333
1.6.2
Maximum Frequency Coverage
f
min
high-low
739.3
25.0
800
60.7
50.0
10.7
f
max
low-low
998.825
29.0625
930
68.825
58.125
10.7
The selection of the reference crystal
frequency is based on some assumptions. As
for example: the first IF and the image
frequencies should not be in a radio band
where strong interfering signals might occur
(because they could represent parasitic
receiving signals), the LO1 signal should be in
the range of 800 MHz to 930 MHz (because
this is the optimum frequency range of the
VCO1). Furthermore the first IF should be as
high as possible to achieve highest RF image
rejection.
Parameter
Injection type
RF / MHz
REF / MHz
LO1 / MHz
IF1 / MHz
LO2 / MHz
IF2/ MHz
39012 71112 01
Rev. 011
Page 5 of 14
EVB Description
June/07
MELEXIS [ Melexis Microelectronic Systems ]
