參數(shù)資料
型號(hào)： TH71112
廠商： Electronic Theatre Controls, Inc.
英文描述： 868/915MHz FSK/FM/ASK Receiver
中文描述： 868/915MHz FSK信號(hào)/調(diào)頻/ ASK接收機(jī)
文件頁(yè)數(shù)： 4/20頁(yè)
文件大小： 668K
代理商： TH71112
TH71112
868/915MHz
FSK/FM/ASK Receiver
TH71112 Data Sheet
3901071112
Page 4 of 20
Nov. 2001
Rev. 005
Frequenc y Planning
Frequency planning is straightforward for single-conversion applications because there is only one IF that
might be chosen, and then the only possible choice is low-side or high-side injection of the LO1 signal (which
is now the one and only LO signal in the receiver).
The receiver’s double-conversion architecture requires careful frequency planning. Besides the desired RF
input signal, there are a number of spurious signals that may cause an undesired response at the output.
Among them are the image of the RF signal (that must be suppressed by the RF front-end filter), spurious
signals injected to the first IF (IF1) and their images which could be mixed down to the same second IF (IF2)
as the desired RF signal (they must be suppressed by the LC filter at IF1 and/or by low-crosstalk design).
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
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
IF1
LO2
IF2
The following table depicts generated, desired, possible images and some undesired signals considering the
examples of 868.3 MHz and 915 MHz RF reception at IF2 = 10.7 MHz.
Signal type
RF =
868.3 MHz
RF =
868.3 MHz
RF =
868.3 MHz
RF =
868.3 MHz
RF = 915
MHz
RF = 915
MHz
RF = 915
MHz
RF = 915
MHz
Injection type
high-high
low-low
high-low
low-high
high-high
low-low
high-low
low-high
REF / MHz
28.58667
25.22353
29.3
25.85294
30.14333
26.59706
30.85667
27.22647
LO1 / MHz
914.77333
807.15294
937.6
827.29412
964.58667
851.10588
987.41333
871.24706
IF1 / MHz
46.47333
61.14706
69.3
41.00588
49.58667
63.89412
72.41333
43.75294
LO2 / MHz
57.17333
50.44706
58.6
51.70588
60.28667
53.19412
61.71333
54.45294
RF image/MHz
961.24667
746.00588
1006.9
786.28824
1014.17
787.21176
1059.83
827.49412
IF1 image/MHz
67.87333
39.74706
47.9
62.40588
70.98667
42.49412
51.01333
65.15294
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 915 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. The columns in bold depict the selected frequency
plans to receive at 868.3 MHz and 915 MHz, respectively.
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