EVB71112 Datasheet, PDF (5/14 Pages) Melexis Microelectronic Systems – 868/915MHz Receiver Evaluation Board Description

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.
Signal type
RF =
868.3
MHz
RF =
868.3
MHz
Injection type high-high low-low
REF / MHz 28.58667 25.22353
LO1 / MHz 914.77333 807.15294
IF1 / MHz 46.47333 61.14706
LO2 / MHz 57.17333 50.44706
RF image/MHz 961.24667 746.00588
IF1 image/MHz 67.87333 39.74706
RF =
868.3
MHz
high-low
29.3
937.6
69.3
58.6
1006.9
47.9
RF =
868.3
MHz
RF =
915
MHz
RF =
915
MHz
RF =
915
MHz
RF =
915
MHz
low-high high-high low-low high-low low-high
25.85294 30.14333 26.59706 30.85667 27.22647
827.29412 964.58667 851.10588 987.41333 871.24706
41.00588 49.58667 63.89412 72.41333 43.75294
51.70588 60.28667 53.19412 61.71333 54.45294
786.28824 1014.17 787.21176 1059.83 827.49412
62.40588 70.98667 42.49412 51.01333 65.15294
1.6.2 Maximum Frequency Coverage
Parameter
Injection type
RF / MHz
REF / MHz
LO1 / MHz
IF1 / MHz
LO2 / MHz
IF2/ MHz
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.
39012 71112 01
Rev. 011
Page 5 of 14
EVB Description
June/07